Note: Descriptions are shown in the official language in which they were submitted.
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ACOUSTIC/SHOCK WAVE ATTENUATING ASSEMBLY
Field of the Invention
This invention relates to pressure wave phenomena (acoustic and
shock waves) and more specifically to an assembly for providing attenuation of
pressure waves traveling generally at or above the speed of sound in ambient
conditions in order to mitigate undesirable effects of these waves (including
fragments and thermal energy release).
Background of the Invention
Acoustic and shock waves are traveling pressure fluctuations which
cause local compression of the material through which they move. Acoustic
waves cause disturbances whose gradients, or rates of displacement are small -
on the scale of the displacement itself. Acoustic waves travel at a speed
determined by and characteristic of a given medium; thus, one must speak of
the
speed of sound, or acoustic speed in that medium. An acoustic wave regardless
of
its frequency ,(pitch) or amplitude (loudness), will always travel at the same
speed
in a given substance.
Shock waves are distinguished from acoustic waves in two key
respects. First, shock waves travel faster than the speed of sound in any
medium.
Secondly, local displacements of atoms or molecules comprising a medium caused
by shock waves are much larger than for acoustic waves. Together, these two
factors produce gradients or rates of their displacement much larger than the
local
fluctuations themselves.
Energy is required to produce pressure waves. This is related to the
equation that states that energy equals force multiplied: by the displacement
caused
by the force. Once the driving source ceases to produce pressure disturbances,
the
waves decay. Attenuation involves acceleration of the natural damping process,
which therefore means removing energy from pressure waves.
All matter through which pressure waves travel naturally attenuates
these waves by virtue of their inherent mass. Materials possess different
acoustic
attenuating properties, strongly affected by density anti by the presence or
absence
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of phase boundaries and structural discontinuities. Porous solid materials,
thus,
are better attenuators of sound waves than perfect crystalline solids. Gases
are
inherently poor pressure wave attenuators.
All types of pressure waves can be reflected and diffracted by liquid
and gas media. They can also be deflected or, more generally, scattered and
dispersed by phase boundaries, such as liquid droplets or solid particulates
suspended in air. These deflections serve to increase the distance which the
wave
travels. Scattering and dispersion thus produce more attenuation because they
cause the transmitting pressure waves to displace more mass by virtue of the
longer path. Such deflections also reduce, or may altogether eliminate the
pressure waves originally traveling in a specific direction.
Acoustic Wave Attenuation
Documented efforts to reduce noise (attenuate acoustic waves) in
enclosed spaces extend to the early nineteenth century. Virtually all acoustic
wave
attenuation concepts have been based upon layers of solid materials with
significant sound absorbing properties serving as linings, coatings, or
loosely-packed fibrous or granular fillers between solid layers. These
sound-absorptive layers have been applied to or incorporated within structural
walls, floors, ceilings, and other types of panels and partitions when
acoustic
attenuation is required. Several dozen patents have been granted in the United
States alone which fall into this category.
In 1910, Mallock introduced the idea of using aqueous foams for
noise suppression, and conducted experimental evaluation of foams in this
role.
See Mallock, A., "The damping of sound by frothy liquids", Proc. Royal. Soc.
A84; pp. 391-5, 1910. Aqueous foams are agglomerations of bubbles, with the
gas phase within each bubble completely separated from that in adjacent
bubbles
by aqueous liquid film comprising the bubble walls. During the years following
Mallock's research, aqueous foams became widely used for fire suppression, in
numerous chemical processes, and for mineral ore separation.
Not until the 1960's did interest renew in using aqueous foams for
pressure wave attenuation. Research from that time and continuing to the
present
extended to their use for suppressing jet engine noise and acoustic
disturbances
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arising from artillery muzzle blast, ordnance disposal, and "sonic boom''
created
by supersonic aircraft flight. It was during this time that researchers
discovered
that aqueous foams dramatically attenuate impinging shock waves.
Shock Wave Attenuation
Much more energy is required to produce shock waves compared to
acoustic disturbances, which makes their attenuation more difficult. Shock
waves
decay to form acoustic waves when the source of the shock wave is removed or
suppressed.
When traveling through gases, shock waves produce increases in
pressure (often referred to as "overpressure") and temperature; they also
accelerate gas molecules and entrained particulates in t:he direction of shock
wave
travel. Shock waves produced by combustion processes, such as explosions and
deflagrations, release substantial amounts of thermal and radiant energy as
well.
For all shock waves, the shock wave speed, overpressure, and temperature
increase they induce in the local medium are mathematically linked.
Attenuation
of shock waves is thus achieved through directly suppressing one of these
three
parameters; if temperature is reduced, the overpressurf; and shock speed are
accordingly reduced, for example.
Mitigation of shock wave parameters has required different
approaches than those used for acoustic wave attenuation because of their
relatively large impulse and pressure magnitude. Mechanical mitigation methods
can be applied in many situations where barriers or confinement are allowable.
When shock waves are produced by explosions or deflagrations, chemical means
as well can often be used for suppression. None of the structures or materials
described in existing patents or in technical literature similar to the types
of solid
sandwich configurations discussed above for noise suppression can provide
significant attenuation of shock waves.
Two types of structures or mechanical arrangements have been
employed in reducing shock wave effects: solid barriers (including blast mats)
and
mechanical venting. Solid barriers and blast mats have been used to deflect
incident shock waves or remove energy from incident waves through momentum
transfer (to the high-inertia mats and barriers), and to provide protection
from
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fragments and thermal effects. Mechanical venting has been employed to keep
internal pressure below the level which would cause structural failure for
explosions in confined spaces. .
Solid barners for shock wave containment or protection suffer from
several shortcomings. Where protection of large areas from powerful shock
effects is necessary, concrete or earthen burners must be employed. These
structures must be massive and are thus inherently immobile and expensive and
time consuming to erect. They cannot, therefore, be used in the majority of
applications where explosion hazards are present: marine transport of liquid
and
liquefied hydrocarbons, petrochemical storage and processing facilities,
aboard
warships and munition-carrying vessels, or at hastily established munitions
transshipment points (which are common in military operations, for example).
They cannot be used within buildings or otherwise serve as partitions in
structures.
Similarly, large numbers of bulky and heavy blast mats are required
for blast overpressure exceeding a 1-meter scaled distance (the equivalent
blast
wave intensity of a 1-kilogram TNT detonation at a distance of 1 meter). When
not being used, these mats must be stored. Aboard ships, space is often
critically
limited, thus bulky items which provide no essential or alternate use cannot
be
justified. Furthermore, blast mats can at best provide only limited mitigation
of
blast effects in confined spaces and provide little acoustic damping. Their
bulk,
weight, and limited utility in confined spaces rule out their employment
aboard
aircraft. Blast mats cannot be easily or quickly moved from storage to
locations
where needed for blast wave attenuation due to their bulk and weight.
Mechanical venting is widely employed for mitigating blast
overpressure in containment structures (grain silos, explosive material
handling
rooms, etc.) These vents normally constitute part of the containment wall.
Besides reliability and response time problems, venting requires facilities to
be
designed such that overpressure release will not endanger personnel or nearby
structures. Venting cannot be employed where hazardous materials may be
released. Venting is also unacceptable aboard ships, where openings to the sea
and release of smoke and overpressure within the vessel are dangerous.
Mechanical venting cannot be used for noise attenuation.
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Chemical agents suppress shock waves by extinguishing or
interrupting the combustion process which generates them (along with their
thermal effects). Such agents include carbon dioxide and halogenated carbon
compounds ("halons"), which may either be gaseous or liquid initially at the
time
of application, and dry powders, most of which are sits of ammonium or alkali
metals such as sodium and potassium.
Gaseous combustion-extinguishing agents are generally effective in
confined spaces. A number of constraints limit their utility, however. No
gaseous
agent is effective in outdoor or well-ventilated areas. Within a confined
space,
effectiveness of gaseous agents is rapidly lost as these agents quickly escape
through leaks and penetrations (including those caused by projectiles or
weapons
fragments which generate the need for gas agent release). All of the gas and
liquid (which become gaseous in use) chemicals currently available for fire
and
explosion suppression have toxic effects upon humans at the concentrations
required to be effective.
The most effective and least toxic gaseous agents are halogenated
carbon compounds. However, these substances are quickly and irreversibly
broken down while performing their combustion-inhibiting function.
Furthermore,
these agents are being withdrawn from use by international government
agreements due to their profoundly adverse impacts upon upper-atmospheric
ozone.
Other considerations limit the capabilities of gas fire-extinguishing
agents. They cannot provide significant acoustic attenuation in and of
themselves.
Furthermore, gases cannot provide cooling or quenching of the area surrounding
a
fire or explosion due to their inherently low heat capacities, which enables
hot
surfaces to reignite combustible materials. Gas supplies must be adequate for
extinguishment and be capable of reaching all spaces within a compartment,
otherwise they have no effect. Gaseous explosion suppression systems are
totally
dependent upon sensors to initiate release (within 100 milliseconds), which
has
proven to be a problem because of false-alarm activation or failure to
activate, due
to the vulnerability of their sensors to dirt and contaminants. Sensors also
require
maintenance to ensure minimum reliability.
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Powdered fire fighting agents (chemical extinguishants) can be
effectively used in both confined and unconfined areas for fire suppression -
and
by virtue of their dissociation and combustion interrupting tendency - can ,
suppress some deflagrations which could produce shock waves. Again, however,
they cannot provide acoustic attenuation or fragment or missile-stopping
capability.
Furthermore, they require large quantities of agent (with consequent bulk and
weight) to provide significant extinguishing capability. Flooding a space with
powdered agents is blinding to personnel present during emergency operations.
Pressure Wave Attenuation Using Aqueous Foams
Aqueous foams have been proven to be capable of providing more
pressure wave attenuation than any other medium on a mass basis. As noted
above, initial research into the use of aqueous foams for pressure wave
damping
was entirely devoted to noise abatement. Subsequent research revealed that -
unlike any material used in acoustic attenuation structures developed to date -
aqueous foams provide shock wave attenuation, regardless of the origin of the
shock.
All applications to date of aqueous foams for pressure wave
attenuation have been in two basic forms: unconfined deluge or massive foam
flooding and employment of solid confining walls in which aqueous foam is
placed. Massive deluge or high-capacity foam generation systems have been used
for perimeter security and for flooding of buildings to provide explosion
protection
from bombs. Aqueous foam-filled containers have also been used for safe
removal
and disposal of explosives. Variants of the foam-filled container concept have
been developed as noise-attenuation devices ("silencers") for the muzzles of
firearms and large naval guns.
In spite of their successful application to date, current methods and
systems for using aqueous foams in pressure attenuating roles are inefficient
and
unnecessarily bulky. Furthermore, such methods and systems prevent the full '
capabilities offered by aqueous foams from being realized because they require
that the foam attenuate the incident shock or acoustic wave without mechanical
augmentation or assistance. Solid walls utilized in current approaches are
used
only for fluid confinement and stopping fragments. Such usage requires much
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larger volumes of foam (foam agent and water) along with larger pumps and foam
generating equipment than are necessary to provide a specified level of
pressure
wave attenuation.
Comparisons Between Solid and Aqueous Foams
Acoustic attenuation by both types of materials are comparable due
to the fact that both rely upon scattering and dispersion of sound waves at
bubble/cell walls. Solid foams are more compact, aqueous foams are more
efficient on a mass basis. Major differences appear in regard to shock wave
attenuation, however.
Solid materials, including solid foams, used as rigid panels are
unable to attenuate shock waves because of two factors: the large amplitude of
the
displacements of atoms or molecules during shock wave propagation and the
overpressure created in the surrounding fluid. Shock waves propagating through
aqueous foams create turbulent flow fields, which have been shown to dissipate
substantial amounts of energy, particularly when reflected waves travel
through the
turbulent medium See Khosla, A. "A study in shock wave attenuation", Ph.D.
thesis, pp. 229-30, U. of Calgary, 1974. Turbulent flow fields cannot be
generated within solid materials.
The relatively large displacement of the liquid mass contained within
aqueous foam bubbles is resisted by surface tension and viscous forces,
removing
considerable shock wave energy as well. Again, such displacements cannot occur
within solids, even elastomeric foams. Most shock wave energy encountering
solid layers of any kind - including solid foams - is reflected, which
produces
overpressures exceeding the incident level. Furthermore, shock wave
overpressures can knock down solid panels and walls without expending much
energy.
Significant dissipation of shock wave energy can be accomplished
with solid materials, according to the present invention as discussed further
below,
when the solid materials are used as loosely packed beads, in which form they
are
capable of relative displacement in the nature of a fluid. In such a form, the
beads
act similarly to bubbles in an aqueous foam. Specifically, transmitting shock
waves are scattered and dispersed at the bead surfaces, and the displacement
of the
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bead mass absorbs substantial energy. Substantially more shock wave energy can
be absorbed when the beads are made to resist displacement to a limited extend
(below the degree where the bead mass would act more as a rigid panel than a
fluid). This can be accomplished by means of an adhesive surface coating or by
a.
surface texture which promotes friction or adherence.
Experimental work has shown that volcanic foam glass (vermiculite;)
beads have been able to attenuate shock waves originating from small
explosives
comparable to the extent achieved by some aqueous foams. Vermiculite, however,
provides less acoustic attenuation than solid organic foam materials such as
natural)
rubber and polyurethane, which are normally used in this role. Furthermore,
neither vermiculite nor any solid material used to date for acoustic
attenuation has
combustion extinguishing properties in and of itself; indeed, most organic
solid
foam materials are serious contributors to fire and toxic smoke generation.
Aqueous foams have additional mechanisms for dissipating shock
energy which no solid bead material can provide: elastic bubble walls which
absorb energy when they are deformed or ruptured, by uniquely and dramatically
slowing shock waves propagating through, and - in 'the case of stronger shock
waves - by causing these shock waves to separate into two separate waves,
whichn
are then more easily attenuated.
summary of the Invention
In view of the shortcomings for existing apparatus and assemblies to
attenuate acoustic and/or shock waves as noted above, there has been found to
remain a need for an improved assembly for more effectively attenuating
acoustic
and/or shock waves. The present invention accordingly provides a means for
attenuating substantially all types of pressure waves, existing as either an
acoustic
or shock wave, in generally all gaseous environments, particularly in ambient
atmospheric conditions. More specifically, the invention provides a means or
assembly for substantial suppression or attenuation of blast effects from
either
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proximate or remote explosions as one of the more severe examples of pressure
wave or acoustic/shock wave conditions effectively dE:alt with by the
invention.
The term "acoustic level pressure condition" is employed herein to
include both acoustic waves at the acoustic speed of a. selected medium and/or
shock waves exceeding the acoustic speed of a selecte~.d medium. Accordingly,
that term is employed as a replacement for either or both conditions of
acoustic
waves and/or shock waves.
As discussed in greater detail elsewhere, the invention contemplates
sonic/shock wave pressure conditions preferably traveling at or above the
acoustic
speed for a given medium. However, it will be apparent that the invention is
also
effective for pressure conditions generally approaching acoustic speeds in a
given
medium and thus exhibiting pressure characteristics to be desirably attenuated
in
the same manner as acousticlshock wave configurations.
In view of the above summary, the invention has a number of
objects and advantages set forth as follows:
(a) to provide pressure wave attenuation capabilities in both
confined spaces and unconfined areas;
(b) to provide attenuation of all acoustic, frequencies regardless of
orientation with respect to the source;
(c) to provide shock wave attenuation in confined spaces without
requiring the space to be completely filled by aqueous foam or any other agent
or
medium;
(d) to provide attenuation of shock waves for both proximate and
remote explosions;
(e) to provide a specified level of pressure wave attenuation in less
volume and with lower weight than is possible through any other existing
means;
(f) to provide shock wave attenuation in confined spaces without
requiring the confining walls to be gas-tight (free from leaks or
penetrations);
(g) to provide pressure wave attenuation with a mechanical
configuration which can be quickly stowed or removed to provide passageway or
space when the system is not in use;
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(h) to provide a pressure wave attenuation structure to which other
means of augmenting specific attenuating capabilities or to provide additional
capabilities can be applied or installed within (such as adding insulation to
protect
the system from fire or radiation, providing intumescent coatings to provide
additional thermal energy absorption from proximate explosions, or to include
chemical fire-suppressing power or gaseous agents within); and
(i) to provide explosion protection using the same agent as employed
for fire fighting (aqueous foam fire suppressants).
More specifically, the present invention provides an acoustic/shock
wave attenuating assembly formed by a flowable attenuating medium exhibiting
aqueous foam characteristics and a confinement means for containing and
supporting the flowable attenuating medium, the confinement means being porous
with respect to the acoustic/shock wave for allowing the shock wave to
penetrate
the flowable attenuating medium. Porosity of the confinement means is more
specifically characterized as macroscopic or microscopic openings allowing the
shock wave to pass therethrough but, at the same time, absorbing considerable
energy from the shock wave and creating turbulent zones or large numbers of
miniature shock waves as energy from the shock wave passes into the flowable
attenuating medium. With such porous material being preferably arranged on
opposite sides of the attenuating medium, similar energy absorbing conditions
occur as the shock wave penetrates and passes through both sides of the
confinement means. In addition, substantial energy from the shock wave is
absorbed by the flowable attenuating medium, particularly because of its
containment and restriction by the confinement means.
Preferably, the flowable attenuating medium is an aqueous foam
known to have substantial energy absorbing capabilities from the prior art as
discussed above. However, the flowable attenuating medium may also be formed,
for example, from solid particulate material preferably having bulk mechanical
properties and flow properties of a fluid, the solid particulates also
preferably
comprising means for resisting relative displacement of the particulates in
order to
better simulate characteristics of an aqueous foam. In this regard, the term
"flow
properties of a fluid" and more specifically the term "mechanical properties
and
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flow properties of a fluid" refer to the ability of the attenuating medium to
act in
the nature of a liquid mass to resist relative displacement by surface tension
and
viscous forces and the ability to substantially scatter and disperse pressure
conditions transmitting therethrough by virtue of multitudinous curved
surfaces
dividing gaseous and solid or liquid or solid phases, and enabling the
generation of
turbulent flow fields by transmitting pressure conditions. More briefly, these
terms may be taken as referring to the ability to resist applied shear forces
in the
nature of fluid viscosity. Finally, the above terms are also intended to refer
to a
tendency of the flowable attenuating medium to assurne the shape of the
confinement means while at the same time resisting applied shear forces in the
nature of viscosity.
Numerous configurations are possible for the attenuating assembly
of the invention. Preferably, the confinement means provides generally
parallel
side portions forming a panel in combination with the flowable attenuating
medium
supported therebetween for intercepting the acoustic/s;hock wave. More
preferably, both side portions of the confinement means are porous in order to
achieve maximum attenuation in the manner summarized above. It is even further
contemplated that a plurality of such panel formation:. can be arranged with
intervening gaps whereby the acoustic/shock wave may be effectively caused to
successively penetrate the plurality of panel formations and intervening gaps
in
order to even more effectively attenuate the acoustic/shock wave.
A further possible configuration of the invention provides for
placing the acoustic/shock wave attenuating panel combination between a
structure
and a surrounding liquid medium such as sea water far the purpose of
protecting
the structure from shock waves or other pressure wave phenomena arising from
underwater explosions or seismic activity. In this application, an
acoustic/shock
wave attenuating assembly of one of the above mentioned configurations employs
a
non-porous membrane or rigid shell confinement means to isolate the
surrounding
liquid from a liquid transmitting medium emplaced between the confinement
means
and the acoustic/shock wave attenuating assembly. Preferably the flowable
attenuating medium is an aqueous foam and the transmitting liquid medium being
a
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homogeneous liquid without macroscopic gas bubbles or solid particulates in
suspension. . .
It is also contemplated that the panel combination may be shaped to .
form a generally enclosed chamber. With both side portions of the confinement
means being porous to the acoustic/shock wave, such a configuration is
effective .
to attenuate the acoustic/shock wave passing in either direction through the
panels.
It is yet another object of the invention to provide such a flowable
attenuating medium in solid form, the attenuating medium being formed by solid
particulates which may be hollow or otherwise include a gaseous phase, the
particulates preferably being macroscopic and even more preferably have a
dimension of at least about one millimeter.
It is a related object of the invention to provide such a solid
attenuating medium wherein solid particulates are supported and more
preferably
also confined by a filamentary material forming a matrix. In such a
configuration,
the filamentary material preferably has mechanical integrity for providing
confinement of the solid particulates in the matrix of filamentary material
while
allowing the solid particulates to be relatively displaced by interaction with
pressure conditions so that the panel is capable of scattering and dispersing
the
pressure conditions passing therethrough. In such a configuration, the
attenuating
medium or panel further enables formation of turbulent flow fields from the
pressure conditions.
Within such a configuration, the attenuating medium may be in the
form of a flexible attenuating panel and may further comprise means
interacting
with the solid particulates and filamentary material in order to increase
resistance
of the solid particulates to relative displacement by the pressure conditions
in
addition to resistance attributable to inertia forces.
It is a still further object of the invention to provide a flowable
attenuating medium for the present invention in the form of an aerogel, a very
light weight material described in greater detail below.
It is also a further object of the invention to provide the attenuating
medium and related components for protective applications, particularly in
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connection with ammunitions or devices of a type generally referred to as
shaped-
charge or hollow-charge devices, as described in greater detail below.
It is a related object of the invention to also employ the attenuating
medium and related components of the invention for protecting explosive
charges
or devices themselves from interaction or detonation, this phenomenon being
commonly referred to as "sympathetic detonation", "frattacide", "propagation"
or
"chain reaction", as also described in greater detail below.
It is yet a further object of the invention to employ the attenuating
medium and associated components of the invention to provide a liner for
containers, either to protect the contents of the container from external
blasts or to
protect the exterior of the container from blasts within the container.
Accordingly,
the invention particularly contemplates use in connection with air cargo
containers
and the like.
It is also a further object of the invention to employ a shield of a
frangible material in combination with the attenuating medium for protecting
against weather and the like while still permitting the desired function of
the
attenuating medium as also described in greater detail below.
It is another object of the invention to employ the attenuating
medium in combination with honeycomb, the honeycomb preferably providing at
least pan of the support for the attenuating medium.
Additional objects and advantages of the invention are to provide
total reliability and effectiveness by using no moving or electrical
components, and
by not depending upon materials which must be without flaws, imperfections, or
other defects. Operation of the invention is possible using materials in
common
use for years, and is not dependent upon development of materials, means of
manufacture, or analytical methods not currently available. Most
significantly, the
invention provides substantial attenuation of all types of pressure waves on
the
source side as well as the remote side of the pressure wave attenuating
structure.
In the case of proximate explosions, substantial reduction of both
overpressure and
thermal effects have been experimentally verified on the blast side as well as
the
opposite side of the pressure wave attenuating structure.
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Thus, in a broad aspect the invention provides an
assembly for attenuating acoustic level pressure conditions,
comprising a flowable attenuating medium exhibiting aqueous
foam characteristics, namely the ability of acting in the
nature of a liquid mass to resist relative displacement by
surface tension and viscous forces and the ability to
substantially scatter and disperse pressure conditions
transmitting therethrough by virtue of multitudinous curved
surfaces dividing gaseous and solid or liquid and solid
phases, and enabling the generation of turbulent flow fields
by transmitting pressure conditions, and confinement means
for containing and supporting the flowable attenuating
medium, the combination of the confinement means and
flowable attenuating medium being arranged for intercepting
the pressure conditions to be attenuated, the confinement
means being porous with respect to the pressure conditions
for allowing the pressure conditions to penetrate the
flowable attenuating medium, the porous confinement means
also causing substantial pressure decrease of pressure
conditions penetrating the porous confinement means.
In another broad aspect the invention provides an
attenuating panel for attenuating acoustic level pressure
conditions, comprising multitudinous solid particulates
generally having a dimension of at least 1 millimeter, and
filamentary material forming a matrix for the solid
particulates, the filamentary material having mechanical
integrity for providing confinement of the solid
particulates in the matrix of filamentary material while
allowing the solid particulates to be relatively displaced
by interaction with the pressure conditions whereby the
panel is capable of scattering and dispersing pressure
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conditions passing therethrough and further enabling
formation of turbulent flow fields within the attenuating
panel from the pressure conditions.
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Further objects and advantages of the invention will become
apparent form a consideration of the drawings and ensuing description.
Brief Description of the Drawings
FIGURE 1 is a perspective view of a panel configuration for the
attenuating assembly of the invention. The panel assembly is preferably
contemplated for containing an aqueous foam as the flowable attenuating
medium.
Accordingly, the assembly of FIGURE 1 illustrates means for recycling and
regenerating the aqueous foam within the confinement means.
FIGURE 2 is a view taken along section lines II-II of FIGURE 1
and better illustrates the interaction of the confinement means with the
flowable
attenuating medium.
FIGURE 3 is a view similar to FIGURE 2 and illustrates yet
another embodiment of an acoustic/shock wave attenuating assembly according to
the present invention which is placed between a structure to be protected from
shock waves and other pressure wave phenomena transmitting in a surrounding
liquid medium.
FIGURE 4 illustrates a variation of the panel configuration wherein
the side portions of the confinement means are articulated or corrugated in
order
to provide increased surface area and generate greater turbulence in the
flowable
attenuating medium, thereby producing even more effective attenuation for the
acoustic/shock wave.
FIGURE 5 is a view similar to FIGURE 2 while illustrating
multiple panel assemblies of similar construction with intervening gaps in
order to
even more effectively attenuate the acousticlshock wave.
FIGURE 6 illustrates yet another embodiment of an acoustic/shock
wave attenuating assembly according to the present invention wherein the
confinement means and the flowable attenuating medium contained therein are
supported in common from a suitable structure.
FIGURE 7 is a fragmentary view in section of a flowable
attenuating medium for the assembly of the present invention formed from solid
particulates.
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FIGURE 8 illustrates the arrangement of a plurality of panel
assemblies each generally similar to that of FIGURE 1 to form a generally
enclosed prismatic chamber.
FIGURE 9 illustrates yet another embodiment of an acoustic/shock
wave attenuating assembly constructed according to the present invention
wherein
the panel combination of the confinement means and flowable attenuating medium
forms a generally enclosed chamber. More specifically, the panel combination
illustrated in FIGURE 9 forms a cylindrical portion open at both ends.
FIGURE 10 similarly illustrates such a panel combination formed
generally as a dome to completely enclose a chamber therebeneath, with a
section
removed to show its construction.
FIGURE 11 also similarly illustrates yea another configuration
wherein the panel combination is arranged with an irregular shape to also form
a
chamber therebeneath open at one end.
FIGURE 12 is a view of another embodiment of the acoustic/shock
wave attenuating assembly of the present invention wherein the attenuating
medium is formed as a flexible panel including solid particulates confined and
also
preferably supported by filamentary material.
FIGURE 13 is an enlarged fragmentary view of a portion of a
flexible panel similar to that of FIGURE 8 but wherein the solid particulates
are
integrally formed with the filamentary material.
FIGURE 14 illustrates a flexible panel formed from an attenuating
medium comprised of solid particulates and filamentary material in generally a
similar manner as in FIGURES 12 and 13, the flexible; panel being usable as
insulation, a cushioning component, curtain barrier or lining material for
example.
FIGURE 15 is a cross-sectional view of flexible panel as illustrated
in FIGURE 14 employed as a lining in a container.
FIGURE 16 is a cross-sectional representation of an embodiment of
the invention including a frangible element on an exposed surface of the
attenuating medium.
FIGURE 17 is a cross-sectional representation of still another
embodiment of the invention illustrating its use in combination with armor
plate or
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the like particularly for enhancing the ability of the armor plate to resist
explosive
devices such as shaped-charge devices.
FIGURE 18 is a perspective representation of another embodiment .
wherein the attenuating medium is arranged in the cells of a honeycomb
structure.
Description of the Preferred Embodiments '
The various drawing figures accordingly illustrate a number of
embodiments according to the present invention. Those embodiments are
summarized below followed by a more detailed description of the respective
figures.
FIGURE 1 is a perspective view of a basic version of the pressure
wave attenuation device. The device comprises two mesh or perforated solid
screens which are parallel or substantially parallel for planar configurations
and
concentric or substantially concentric for cylindrical, spherical or other
three
dimensional forms which can be generated by revolving a planar curve about an
axis, with a pressure wave- attenuating fluid, such as aqueous foam or
vermiculite
beads, emplaced and filling the space between the mesh or perforated sheet
screens. The screen elements may be flat or corrugated, or a combination
thereof.
The screen elements are either held in place by a rigid structural frame or by
otherwise suspending and securing the lower edges of the screens to prevent
their
displacement. The minimum spacing between screens is preferably the least
distance between perforations in perforated sheet screens or least dimension
of
mesh openings in mesh screens.
Additional embodiments of the invention are shown in FIGURES
2-15. As illustrated, the basic configuration can be modified with the
addition of
any combination of mesh screen, perforated solid, or solid materials
connecting to
the mesh or perforated sheet screens of the basic version of our invention, or
to
the frame members which comprise the edge supporting members of the screen
elements of the FIGURE 1 basic version, which would then form top, bottom, and
side surfaces as shown in FIGURE 2.
The invention may include one or more linings, as shown in
FIGURE 2. These linings may be connected or affixed to any of the mesh or
perforated sheet screen elements, or to the structural members holding the
screens
WO 95/0148~~
PCT/US93/06319
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in place, or may be suspended. Said linings may be in the form of a sealed
enclosure or bag emplaced between the screen elements of the basic version of
the
invention, into which the pressure wave attenuating medium may be introduced.
Additional mesh or perforated sheet materials in any number or
combination thereof between the screens comprise oui:er surfaces of the basic
version of the invention to form interior screen elements in a sandwich
configuration, thus forming a sandwich arrangement of a plurality of
acoustic/shock wave attenuating assemblies as shown in FIGURE S. Linings may
be emplaced between one or more of these interior screens and elements forming
the outer surfaces of the invention. The preferred embodiment of the invention
uses corrugated mesh screens to form the outer surfaces, flat mesh comprising
the
interior screen elements, waterproofed paper lining inside the screen elements
and
with aqueous foam filling the sandwich formed by the above elements.
The pressure wave attenuating fluid may be emplaced in the volume
formed between an interior screen element and an outer screen, or between any
two interior screen elements where a plurality of interior screen elements is
employed, or.in any combination of such spaces. This fluid may be aqueous
foam, a gas emulsion, (wherein a gas i~ entrained and dispersed through a
liquid
matrix in the form of bubbles, with the gas bubble diameters generally
commensurate with the thickness of the liquid bubble walls), a gel (preferably
with
entrained gas), or granular or other solid particulates having necessary flow
characteristics. Gas may be emplaced and confined by an enclosing element in
one or more of the gaps between each sandwich assembly, with the gas pressure
being equal to, greater than, or less than atmospheric or ambient pressure.
Vacuum conditions may be generated in one or more of the gaps between each
sandwich assembly.
The embodiments of the various figures are described in greater
detail below.
Referring initially to FIGURE l, an acoustic/shock wave attenuating
assembly is generally indicated at 10. Confinement means for the assembly
comprises a screen or grid 12 arranged on four sides of the assembly to
provide an
enclosure for the tlowable attenuating medium 14.
WO 95/01484 PCT/US93/06319
-18-
As illustrated in FIGURE 1, the bottom of the assembly 10 is
formed by a tray 16 while the top of the assembly is formed or enclosed by a
plate
18. The tray 16 and plate 18 function in combination with the screen 12 to
completely enclose the flowable attenuating, medium 14 within the assembly 10.
L '
The flowable attenuating medium 14 in the assembly of FIGURE 1 '
is preferably contemplated as an aqueous foam of the type noted above. Since
such aqueous foams are subject to deterioration wherein the foam degenerates
into
a gaseous phase and a liquid phase, the assembly 10 is adapted for recycling
and
regenerating the aqueous foam in order to assure that it fills the space
within the
assembly 10. The tray 16 serves to receive and collect the liquid phase from
such
deteriorated foam. The liquid is recycled through a line 20 by a pump 22 to a
manifold 24 having multiple connections 26 through the upper plate 18 for
returning regenerated foam to the assembly 10. Preferably, a source of gas 28
is
provided for regenerating the foam within the manifold 24 so that it can flow
downwardly into the assembly 10.
When aqueous foams are used as the flowable attenuating medium
14, they may be generated from any foamable agents, preferably those which are
normally used in fire suppression. Such agents include hydrolyzed protein
liquids,
proteinaceous liquids with fluoropolymeric additives, along with a large
number of
synthetic surfactant and stabilizing chemical combinations. The foaming gas
for
use in the gas source 28 may be of a similarly wide range so long as the gas
is not
chemically reactive in a destructive manner to the stabilizing components in
the
bubble wall liquids. Foaming gases would preferably include inert elements
such
as argon or fire extinguishing compounds such as carbon dioxide, sulfur
hexafluoride, or halogenated carbon agents (halons). Compressed air is also an
acceptable foaming gas.
Referring now to FIGURE 2, the screen 12 forming the confinement
means for the flowable attenuating medium may not be sufficient for
maintaining
an aqueous foam within the assembly 10. Accordingly, FIGURE 2 illustrates a
preferred embodiment wherein a liner 30 is arranged inside the screen 12. The
screen 12 formed from metal, plastic or the like thus remain very porous to
the
acoustic/shock wave. At the same time, the liner 30 serves to maintain the
WO 95/01484. ~ PCT/US93/06319
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aqueous foam within the interior 32 of the assembly 10. At the same time, the
liner 30 is also porous to the acoustic/shock wave as defined above.
Preferably,
the liner 30 is formed from paper or film which is resistant to wetting by the
aqueous foam. At the same time, the liner 30 tends t:o be readily ruptured by
the
shock wave so that it does not interfere with penetration of the shock wave
into
the attenuating medium 14 and thereby reduces the reflected overpressure that
inevitably develops when shock waves impinge upon a solid surface. The liner
30
thus serves to even further attenuate the acoustic/shock wave in combination
with
the screen 12 and the flowable attenuating medium 14..
Referring now to FIGURE 3, another embodiment of an
acoustic/shock wave attenuating assembly is generally indicated at 10', and is
placed in such an arrangement whereby the structure :34 is situated on the
side of
the assembly 10' opposite the liquid surrounding medium 36. A solid, non-
porous
membrane or rigid shell 37 provides confinement and isolation from the
surrounding liquid medium 36 for an acoustic/shock wave transmitting liquid
38.
FIGURE 4 illustrates yet another embodiment of the invention 10'
which is substantially similar to that illustrated in FIGURES 1 and 2.
However,
the screen 12' in FIGURE 4 is corrugated or articulatc~.,d or otherwise
configured to
have a substantially increased surface area in order to more effectively
attenuate
the acoustic/shock wave. Additionally, the corrugations or articulations serve
to
greatly increase turbulence and formation of miniature shock waves, and
thereby
specifically and even more effectively attenuating shock waves.
Referring now to FIGURE S, another embodiment of an
acoustic/shock wave attenuating assembly is generally indicated at 10' and
comprises panels 10A, lOB and lOC similar to the overall panel assembly of
FIGURES 1 and 2. The panels 10A, IOB, and lOC as illustrated in FIGURE 3
are spaced apart to form intervening gaps indicated at 40. Thus, an
acousticlshock
wave approaching the assembly of 10' of FIGURE 5 laterally would be caused to
sequentially penetrate the panels 10A, lOB and lOC as well as the intervening
gaps
in order to even more effectively attenuate the acoustic/shock wave.
Otherwise,
the various components for the multiple panels in the embodiment of FIGURE 5
are indicated by similar primed numerals in FIGURES 1 and 2.
WO 95/Ol~~ ~ ~ ~ PCT/US93/06319
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Referring now to FIGURE 6, yet another embodiment of an
acoustic/shock wave attenuating assembly is generally indicated at 50 and also
includes components generally similar to those described in FIGURES 1 and 2.
Accordingly, corresponding components,iwFIGURE 6 are indicated by similar
primed numerals. Generally, the screen or confinement means 12' in FIGURE 6
is in the configuration of one or more bags for containing the flowable
attenuating
medium 14'. At the same time, the bags or confinement means 12' is suspended
from a fabricated structure 52. The fabricated structure 52 thus tends to
provide a
panel configuration for the assembly even with the confinement means or bags
12'
being very flexible by themselves.
Referring now to FIGURE 7, another embodiment or variation of
the flowable attenuating medium 14' is illustrated. The flowable attenuating
medium 14' of FIGURE 7 is formed from solid particulates 62 preferably having
both mechanical properties and flow properties of a fluid. Also preferably,
the
solid particulates include means for resisting relative displacement of the
particulates in order to better simulate characteristics of an aqueous foam.
For
such a purpose, the particulates 62 may be provided with a coating 64 to
resist
relative motion between the particulates while permitting flow in accordance
with
the present invention. For example, the coating 64 may be a light adhesive or
may even comprise Velcro type hook and loop fasteners for resisting relative
movement between the particulates. It is noted that VELCRO is a trademark for
such a hook and loop type fastener.
Solid particulates 62 may be of any shape, including spherical and
irregular forms. The largest diameters or largest cross sectional dimensions
of
particulates used in this invention should be generally less than half the
distance
between the generally parallel screens 12. The solid particulates 62 should
generally be macroscopic. These particulates may be hollow with solid
surfaces,
solid shells with internal cavities containing liquid phases, or may be
comprised
entirely of solid materials. The solid material may be a solid foam, such as a
polyurethane or elastomeric compound, or otherwise be a sponge, whereby the
gas
and solid phases are both continuous, which thus distinguishes sponges from
foams, wherein the gas phase is entirely enclosed within a liquid or solid
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-21-
continuous phase. Alternatively, the solid particulates may be comprised of
entrapped gas phases, for example, in the nature of volcanic foam glasses,
periite,
pumice or the like.
Any of the solid particulates of the invention may be flexible or
elastic, or conversely may be rigid in their mechanical properties.
Referring now to FIGURE 8, multiple panels IOD, 10E, lOF and
lOG are formed in generally the same manner as the assembly 10 of FIGURE 1.
However, the panel assemblies lOD-lOG are suspended or otherwise supported to
enclose and define a chamber 60 which may also be used for a number of
applications as described below.
With any of the embodiments of FIGURES 1-8, either the
confinement means comprising the screen 12 and liner 30 and/or the flowable
attenuating medium 14 itself may be formed from materials absorbing
substantial
additional energy from the acoustic/shock wave. For example, intumescent and
ablative materials may be employed either as coatings, treatments for the
lining
30, or as comprising materials of solid particulates 62 or coatings for these
particulates 64. Alternatively, other materials which absorb thermal energy
through an endothermic chemical reaction may be used as linings 30 or as
treatments for these linings, or otherwise or in addition to coatings of the
screen
12 and solid particulates 62 where these are employed.
FIGURES 9, 10 and I 1 illustrate similar panel configurations,
preferably multiple panels with intervening gaps, formed as generally rigid
structures with enclosed shapes to substantially form a chamber therebeneath.
These structures of FIGURES 9-11 may be employed in a number of applications
as described in greater detail below,
Referring initially to FIGURE 9, multiple panels l0A', lOB', and
lOC' are commonly formed as a portion of a cylinder to define the chamber 70
therebeneath. The chamber is at the ends as illustrated.
FIGURE 10 illustrates yet another arrangement of .multiple panels,
l0A', 10B' and lOC' contigured as a dome configured as a dome forming a
chamber 80 which is completely enclosed therebeneath. FIGURE 10 provides a
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78228-1
-22-
fragmentary section of the multiple panel assemblies l0A', lOB' and lOC'
comprising the dome chamber 80.
FIGURE I 1 illustrates a relatively irregular configuration for similar
panels IOA', lOB' and lOC' to form a chamber 90 which is substantially
enclosr..d
therebeneath while being open at one end. Here again, such a configuration may
be used to advantage in particular applications.
FIGURES 12-14 illustrate another embodiment of the invention
wherein the attenuating medium 114 is formed by solid particulates 116
dispersed
in a matrix of filamentary fibers 118. The solid particulates 116 and the
filamentary fibers 118 together comprise a substantial portion of the solid
phase
for the attenuating medium 114. In this embodiment, the filaments serve to
entrap
the particulates while allowing them to experience limited displacement and
oscillations induced by pressure waves passing through the medium. The allowed
displacement of the solid particulates thus provides the ability for
transmitting
shock waves to generate turbulent flow fields among the solid particulates as
well
as for the filaments themselves to oscillate and further enhance turbulent
flow field
magnitude. Aerogel materials may be used to partially or entirely replace the
attenuating medium 114, as described in greater detail below.
Within the embodiment of FIGURE 12 and also in FIGURES 13
and 14, the filamentary material or fiber 118 also serves as a means for
confining
and preferably for supporting the solid particulates.
In this regard, FIGURE 14 illustrates a flexible panel 120 formed
from an attenuating medium 1 l4 substantially similar to that of FIGURE 12.
FIGURE 13 illustrates a fragmentary section of attenuating medium
114' including solid particulates 116' and filamentary material or fibers
118'. In
the embodiment of FIGURE 13, the solid particuiates 116' are formed as an
integral portion of the filaments or fibers 118' in a manufacturing process
described in greater detail below.
In the embodiment of FIGURE 13 or in the embodiments of
FIGURES 12 and 14, for example, the solid particulates and the filaments
themselves may be solid or hollow. For example, cavities may be created in the
solid particulates and/or in the filaments by the manufacturing process. The
WO 95/01484
PCT/IJS93/06319
-23-
cavities (not shown) may be filled by a liquid, gas or powdered solid. In the
case
of powdered solids, they would preferably have a mean diameter of less than
about
0.1 millimeters.
Referring also to FIGURE 15, the flexible panel 120 may be
employed as a liner 120' in a container 122. In this manner, the liner 120'
may
be employed for containing pressure conditions including acoustic and/or shock
waves as disclosed above, generated for example by means of an explosive
device
124.
Referring in combination to FIGURES 14 and 15, the flexible panel
120, optionally employed as a liner 120' in FIGURE 15, consists of solid
particulates and filamentary fibers as disclosed above. The flexible panel may
be
used in order to mitigate deleterious effects produced by an explosion
resulting
from the device 124 in the container 122. Such a configuration might be
employed for example where the container 122 is a cargo carrying hold with the
explosive device 124 being a part of the cargo.
In such a configuration, the attenuating medium 120 can be made by
introducing substantial quantities of the solid particulates into a batch
process as is
typically used in the manufacture of glass fiber insulating batts (not
otherwise
shown). Uncured binder (also not shown) may be used to weakly attach solid
particulates to the glass filaments to the desired extent in this embodiment
of the
attenuating medium.
The attenuating medium of FIGURES 12 and 13 may be used as a
filler in assemblies such as illustrated in FIGURE l, for example, or may act
as
an attenuating assembly in and of itself wherein the attenuating assembly is
used as
a lining or otherwise suspended.
The attenuating medium of FIGURES 12 and 13, for example, may
be formed for example from conventional insulating materials, preferably a
variety
of minerals well known to those skilled in the art. For example, thermal
insulation of a type suitable for forming the attenuating medium 114 may be a
material available for example from Schueller International Corporation under
the
trademark MIN-K and in a variety of configurations. Such a material includes
both the solid particulates 116 and filamentary fibers or material 118 as
illustrated
WO 95101484 PCT/US93/06319
-24-
in FIGURE 12. Furthermore, such materials may be provided with a variety of
other characteristics adding superior performance in the attenuating medium of
the
invention. Such characteristics include low conductivity, reduced conductivity
at
high altitudes, low thermal diffusivity; 'flexibility, the capability of being
molded,
erc. These materials are also available in forms lending themselves to bonded
together or to other materials and may be obtained with special coatings such
as
silicones and the like.
As noted above, the attenuating medium 114 of FIGURE 12 may
include a variety of materials forming both the solid particulates and the
filamentary material. For example, the filamentary material may be fiberglass
or
a variety of other minerals or plastics for example. The solid particulates
may be
formed from the same material as the filamentary material or from other
materials
such as vermiculite, perlite, pumice, hollow glass beads, etc.
The solid particulates and/or the filamentary material may be more
densely distributed in selected regions of the attenuating panel in order to
achieve
focusing and/or diffraction of pressure conditions passing therethrough. The
solid
particulates and filamentary material may also preferably be formed from
materials
of high reflectivity in the infrared portion of the electromagnetic spectrum
or such
materials may be formed on surfaces of the solid particulates and/or
filamentary
material. Such a high reflectivity material may include titanium, for example
in
titanium dioxide. As noted elsewhere, materials in the solid particulates
and/or
filamentary material may also be selected with characteristics for
extinguishing
combustion reactions.
The invention may operate as a partition, lining, container, barrier
or barricade, wall element, or structure standing independent of any exterior
need
of support or attachment. The invention may operate as an acoustic or shock
wave barrier, simultaneously be employed for attenuation of all types of
pressure
waves, or for protection exterior to the invention or on either side of the
invention
when employed as a partition or wall structure. The invention may also operate
as
an acoustic wave absorber for protection of spaces either formed by the
invention
or in which partitions or lining elements of which variants of the invention
WO 95/0148a ~ ~ ~3 PCT/US93/06319
-25-
comprise a part are situated. The invention may seine a secondary purpose as
reservoir of fire fighting aqueous foam agents.
The basic version of the invention becomes operable when the
pressure wave attenuating fluid is emplaced between two adjacent screen
elements.
Pressure waves impinging on the invention from any angle are reflected when
they
encounter screen and solid elements of the invention, and are admitted into
the
flowable attenuating medium when the incident wave s encounter the porous
openings. Pressure waves transmitting through the outer screen element are
substantially slowed and scattered as they travel through the flowable
attenuating
medium, particularly where this medium is an aqueous foam.
Portions of the transmitting waves are reflected upon encountering
the second, or rear, screen of the acousticlshock wave attenuating assembly
and
the gas (or vacuum, as may be employed)/fluid interface, and remaining
portions
of transmitting pressure waves are dispersed as they encounter the interface
between the pressure wave attenuating fluid and contiguous gas or solid. A
substantial fraction of the initially incident pressure wave will thus undergo
multiple reflections within the fluid confined between screen elements, in
essence,
substantial portions of the incident pressure wave are trapped within the
screen/fluid sandwich. With a plurality of screen/fluid sandwich layers, this
effect
will be magnified.
When aqueous foams are used, substantial energy is removed from
the incident pressure wave by scattering at the multitudinous interfaces
presented
by bubble wall liquids and the gas entrapped which camprise the basic units of
aqueous foam structures, and through the displacement of the liquid in the
aqueous
foam. A similar effect is obtained when solid bead materials are employed --
particularly solids with entrained gas, such as vermiculite and organic solid
foams.
For the particular case of aqueous foams, substantial energy is also removed
from
pressure waves reflected back into the attenuating fluid from screen
components
due to turbulent flow fields established by passage of the initial pressure
wave.
This is impossible for solid foam materials.
Additional energy and thus attenuation of transmitting pressure
waves is accomplished by cancellation as scattered, slowed and reflected waves
WO 95/01484 . PCT/US93/06319
-26-
become coincident. A further contributor toward energy removal by the
invention
is that propagation paths of pressure waves through the foam are substantially
lengthened by their scattering and dispersion.
'v
Incident shock waves are attenuated by additional phenomena
generated by the invention. Shock and blast waves consist of an initial
overpressure, or positive pressure phase (in excess of the ambient initial
pressure)
followed by a negative, or rarefaction, phase. The rarefaction phase is
typically
longer in duration unless the shock wave undergoes reflections. Because shock
waves transmitting through aqueous foams are substantially slowed and thereby
further expanding the rarefaction wave duration relative to the overpressure
portion, and at different values due to random dispersion within the foam,
destructive interference by coincidence of positive and negative pressure
waves is
substantially increased with respect to unconfined aqueous foams or foams in
simple containers.
Another substantial factor related to destructive interference between
pressure wave components is that weaker (slower) shock waves have been shown
to separate into two components when transmitting through aqueous foams. The
precursor wave is lower in amplitude but propagates at a higher velocity. The
main wave follows, it is larger in magnitude but tends to lose velocity with
respect
to the precursor wave during passage through aqueous foam. The present
invention uniquely utilizes this phenomenon in two ways, by slowing strong
shock
wave propagation until the wave separates into precursor and main wave
components, then causing reflecting of the two components in such a manner as
to
promote destructive interference or cancellation.
Additionally, shock waves displace bubbles and accelerate liquids in
bubble walls of the aqueous foam, causing the bubbles to shrink and many to
collapse. This displacement of the liquid, the breaking of bubble walls
against the
cohesive force of their surface tension, and the acceleration of liquid
droplets
formed from shattered bubble walls all serve to absorb substantial energy from
the
transmitting shock wave. Substantial parts of the transmitting shock wave are
reflected back into the aqueous foam at the interface between the foam and
contiguous gas or solid, a process which is repeated numerous times by part of
the
WO 95/01484 ~ ~ ~ ~ ~~ PCTIUS93106319
-27-
original incident pressure wave, in essence trapping part of the original
incident
pressure wave.
Yet another substantial contributor to energy removal from the
incident shock wave, thus attenuating such waves, is that the incident wave
creates
choked flow conditions within the mesh or perforated sheet openings, which
serves
to reflect a portion of the incident shock wave. In this manner, only a
fraction of
the energy carried by the incident shock wave is allowed to pass through the
first
screen encountered. Where the transmitted shock encounters another screen,
another fraction of this shock wave is reflected back. When the reflected wave
must travel through aqueous foam dispersion and attenuation of the wave is
greatly
increased through the phenomena described in the prf:ceding paragraph.
Turbulent
flow fields are also established in the vicinity of screen elements by shock
wave
passage through screen openings, which significantly contribute to scattering
of
pressure waves within the foam and by transmitting pressure waves beyond.
Employment of an intervening evacuated space, a space filled by
gas, or a space filled with solid particulates in which a vacuum or gas is
present
between spaces filled with aqueous foam or other flowable attenuating media
will
greatly increase pressure wave attenuation. Evacuated or vacuum spaces will
not
transmit pressure waves. Incident pressure waves will reflect at the solid
surface
which confines the vacuum or gas unless sufficiently intense as to rupture the
confining surface. Upon rupture of the confining surface, the pressure wave
would be transmitted by the flowable attenuating medium accelerated through
the
rupture, and the ambient gas able to leak into the formerly evacuated space.
However, only a small portion of the incident pressure wave could be conveyed
in
this manner due to the small mass and irregular structure of accelerated,
unconfined flowable attenuating medium. Further reflection and scattering of
the
transmitted pressure wave occurs upon encountering successive screens,
linings,
and foam interfaces.
Employment of corrugated screens in any location of the invention
provides additional scattering and turbulence, which therefore further
increases
attenuation. Pressure waves impinging on the flowable attenuating medium from
a
gaseous medium arrive at the corrugated interface at differing times and at
WO 95/01484 ~ ~ ~ ~ PCT/US93/06319
-28-
different angles. Scattering and dispersion of the transmitting pressure waves
is
thus enhanced. Furthermore, the path through the flowable attenuating medium
is
thus greater for a fraction of the transmitting pressure wave from the instant
of
first encounter with the foam. Since aqueous foam is known to substantially
reduce the propagation velocity of pressure waves, further dispersion and .
destructive interference of transmitting wave components is accomplished when
they are.
Linings serve to provide confinement for aqueous foams, and for
solid particulate materials when these are employed. Some reflection of
incident
pressure waves will occur upon impingement, and such linings may provide
additional acoustic barrier capabilities. Where the invention is employed
primarily
for blast and shock wave attenuation, linings and any other materials used to
confine gases or maintain vacuum conditions must rupture or otherwise provide
openings upon the impingement of shock waves at a pressure substantially below
that of the impinging shock wave in order to avoid substantial pressure rise
as is
inevitably created by solid obstructions in these situations.
Coatings or chemical additions which serve to absorb thermal and
radiant energy may be used on any element or combination of elements
comprising
the invention. Such chemicals reduce the energy of incident blast waves due to
the mathematical linkage between blast wave temperature, overpressure, and
propagation velocity, which serves to enhance attenuation of the incident
blast
wave. The invention operates with or without the presence of an increase in
temperature, however, so that thermal energy absorbing materials only serve to
enhance capabilities in certain applications.
Accordingly, the pressure wave attenuating device can be used for
any type of pressure wave transmitted in a gaseous medium. The invention
requires no electric power source or sensor to operate since aqueous foam
generation and filling can be accomplished using only a compressed gas source
'
with which to create and mechanically place the foam within the desired space
or
spaces. There are no electronic or mechanically sensing components which can
prevent the invention from functioning. An additional advantage of the
pressure
wave attenuating device is that other energy absorbing or protective features
may
WO 95/01484.
PCT/US93/06319
-29-
be added to enhance its attenuating capabilities or to provide additional
capabilities, such as stopping fragments from explosions. Typical applications
would enable the same aqueous foam agents and generating equipment as are
commonly used in fighting fires to be employed in the invention.
Attenuation of acoustic waves is accornpIished without regard to
intensity, directionality, or frequency. This device operates regardless of
orienPation with respect to impinging pressure waves or, where present,
confining
walls defining an enclosure in which the invention is placed. Because of the
light
weight of aqueous foams and the structural elements required by the
attenuating
assembly described above, this invention is easily made portable in sizes
useful for
noise suppression around aircraft with jet or gas turbine engines. When
protected
from heat and sunlight, aqueous foams are stable for prolonged periods
enabling
the pressure wave attenuating device to be employed as acoustic walls in
anechoic
chambers or other applications requiring acoustic wave damping in enclosures.
Simultaneous attenuation of all types of pressure waves affords the
invention the capability to serve as means to dispose of explosives and
ordnance
near structures or inhabited areas. By mitigating blast energy, noise and
shock
waves, are attenuated. Bomb fragments are stopped by a combination of reducing
propelling energy and by multiple layers of high strength screen materials.
These
same capabilities enable this device to be employed to provide protection of
artillery crews exposed to enemy artillery and air dropped munitions from both
blast effect and from the noise produced by their own guns.
The ability of the pressure wave attenuating device to operate in a
variety of configurations enables it to be employed to provide blast
protection on
board aircraft which may carry explosive devices meant to destroy the
aircraft,
and for protecting personnel sent to remove or disarm such devices when
discovered. The invention can be configured to operate in curved spaces such
as
' missile launchers used aboard warships, around machinery in hazardous
environments such as in petrochemical refining and production facilities, or
as
protective barriers around rescue equipment. Our pressure wave attenuating
device is unique in its ability to operate effectively in unconfined
environments.
Furthermore, our invention operates effectively without a requirement to be
WO 95/01484 PCT/US93106319
-30-
located close to the source of the pressure wave, or without a specific
orientation
thereto.
Furthermore, the variety~of configurations allowed by this invention
enable the acoustic/shock attenuating assembly to be employed for protecting
ships
and offshore structures from shock effects arising from underwater explosions
when aqueous foams are employed as the flowable attenuating medium. The
invention can similarly be used for protecting offshore and coastal structures
from
seismic shock effects as well as aquatic life from any type of shock waves in
water. This can be accomplished by using a lining which confines a fluid which
serves to transmit the pressure wave between the outer screen and a lining
which
confines aqueous foam in the manner of sonar type acoustical detection devices
wherein a membrane is filled with water or other fluid to conduct acoustic
waves.
The invention preferably employs aqueous foam agents which have
neither toxic qualities nor produce toxic compounds as a result of operation.
It is
light in weight and may easily be stowed in most of its configurations when
not
needed or when being transported. When used in confined spaces, the invention
occupies a small fraction of the enclosed volume and does not involve
flooding.
The acousticlshock wave attenuating assembly enables personnel to occupy and
work in that space, which only explosion vents allow among all possible blast
pressure mitigating means in current use. Unlike explosion vents however, the
invention uniquely is usable in situations which proscribe opening confined
spaces
to adjoining spaces. This is critical aboard ships, which cannot be opened to
the
sea, and within any structure where smoke and combustion products must be
confined to avoid harm to trapped individuals and to facilitate emergency crew
operation.
In addition to the use of solid and liquid attenuating mediums as
described elsewhere, the invention further contemplates that substantially all
embodiments of the invention are adapted for use with a class of very light
weight
materials generally referred to as ''aerogels". These materials are similar in
structure, at least in certain aspects, to the filamentary materials described
in one
of the preceding embodiments of the invention. However, the aerogels differ in
their formation by the inclusion of multitudinous small cavities filled with a
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gaseous phase. Such aerogels can be manufactured with extremely low densities,
almost down to that of atmospheric air at sea level, and have long been know
to
those skilled in the art of low density structures, etc.
Rather than describing such aerogels in greater detail herein, it is
noted that the structure and typical compositions of such aerogels are
described in
a number of references, particularly an article by Jochen Fricke, entitled
"Aerogels", Scientific American, Vol. 258, No. 5, May 1988, pp. 92-97.
It is to be understood that such aerogels, because of their extremely
low density, are desirable for forming variations of substantially any of the
embodiments of the invention where minimum weight is important. It is further
noted that a flowable attenuating medium formed from aerogels may use
generally
the same support structures disclosed for the attenuating medium in different
embodiments of the invention. Accordingly, the attenuating medium described in
substantially all of the embodiments of the present invention may be replaced
partially or entirely by such aerogel materials.
A further embodiment of the invention is described immediately
below with reference to FIGURE 16 which illustrates the use of an attenuating
medium in combination with a frangible element or covering.
Referring to F1GURE 16, an attenuating assembly generally similar
to that indicated at 10 and 10' elsewhere, is indicated at 130. The assembly
130
includes a confinement means or support medium 12' in combination with a
flowable attenuating medium 14'. The attenuating medium may be any of the
forms described elsewhere herein.
A frangible element 132 is provided as a protective covering for the
assembly 130 and particularly for the attenuating medium 14'. The,frangible
element 132 is arranged on the blast side or exposed side 136 of the assembly
and
opposite the protected side 138 of the assembly.
A stiffener 134 is preferably associated with the frangible element
132 if necessary or desired for further support.
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-32-
This embodiment with the frangible element enables the attenuating
assembly to withstand severe outdoor weather environments and to otherwise
isolate the environment on one side of the attenuating assembly from the
other. ,
The frangible element 132 is preferably formed as a part of the
assembly 130 in order to prevent gas movement or diffusion through or across
the
assembly 130.
Frangible materials suitable for forming the element 132 are well
known to those skilled in the art and may be formed, for example, from scored
metal, composites of plastics and glass, plastics, glasses and other polymeric
materials. Alternatively, they may also be formed from agglomerations of
organic
and/or inorganic materials held together by binders and pressed or molded into
any
desired shape.
The frangible element 132 is preferably designed to withstand wind
loads or other common environmental conditions. At the same time, the
frangible
element 132 is adapted for shattering into small pieces when impinged upon by
acoustic level pressure conditions including acoustic waves and/or shock waves
as
discussed elsewhere herein. Accordingly, when such pressure conditions arise,
the
shattering of the frangible element 132 immediately exposes the attenuating
medium 14' to accomplish its function as described herein.
Still another embodiment of the invention contemplates its use for
protecting structures and/or people from the harmful effects of explosive
devices
including ammunitions, bombs and other types of explosive devices. A
particular
type of ammunition against which the present invention can provide protection
is
generally referred to as either a shaped-charge or hollow-charge device. By a
combination of substantially reducing the velocity of the shock wave created
by the
detonation of the explosive component and which forms the penetrating slug,
dispersing and scattering elements of the shock wave so as to disrupt the
uniform
shape of the shock front, suppressing the evolution of blast gases by the
explosive
charge which provides the motive force for the penetrating slug, and by
causing
entrainment of low mass components, with the possible inclusion of
volatilizing
components into the penetrating slug, the penetrating ability of the slug
formed by
the shaped-charge munition or device is greatly reduced.
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Such shaped-charge devices and their use are well known to those
skilled in the art and a detailed description of such devices is not believed
necessary within the scope of the present invention. It is noted that such
shaped-
charges and their functions are described by a large number of references,
such as
the publication Fundamenrals of Shaped-Charges by William P. Waiters,
published
1989 by John Wiley & Sons, lnc. (see pp. 13-14, et al.).
The use of the present invention in such applications is illustrated in
FIGURE 17. Referring to FIGURE 17, an attenuating assembly is generally
indicated at 140 including a confinement means or support medium 12' in
combination with a flowable attenuating medium 14' as described elsewhere
herein.
In the embodiment of FIGURE 17, the attenuating assembly 140' is
arranged upon an otherwise exposed surface 142 of typical armor plate
generally
indicated at 144.
The attenuating assembly 140 preferably and optionally includes an
actuating element or support surface 146 arranged either within or opposite
the
attenuating medium 14' from the exposed surface 142 of the armor plate.
When used to protect against the penetration and subsequent harmful
effects from shaped-charge munitions, the assembly 140 can be used as an
exterior
armor or barrier element for a wide variety of implements or structures (not
shown) including armor plate such as that indicated at 144 in FIGURE 1?. The
incorporation of combustion-extinguishing agents, particularly those with
rapid
reaction times, greatly interferes with the formation of an effective
penetrating
slug, as shaped-charge munitions are intended to produce, and further reduces
the
damage possible behind the armor or barrier should it be pierced, particularly
due
to hot spall and blast gases under pressure.
These mitigating events are produced by reducing the velocity of the
shock wave which forms the penetrating slug, dispersing and scattering
elements
of the shock wave so as to disrupt the uniform shape of the shock wave front,
to
suppress to a substantial degree the evolution of blast gases which constitute
the
motive force of the slug. and the subsequently formed jet to entrain materials
of
WO 95/01484 PC~'/iJS93/06319
-34-
low mass into the slug and thus accelerate the disruption of the jet's
mechanical
integrity, and to suppress to a substantial degree the thermal energy
component
which comprises a substantial portion of the deleterious effects generated by
the
shaped-charge munition. Incorporation of a low-boiling point or flashing
liquid
within the attenuating assembly is possible with the present invention, which
can
further accelerate disruption of the slug as vapor bubbles are entrained.
Accordingly, it is particularly contemplated that this embodiment of
the invention be employed in military or terrorist applications or the like.
Shaped-charges of the type referred to above generally fit the
definition of the present invention in exceeding sonic or shock wave speeds.
Generally, the present invention can be useful against shaped-
charges including only the attenuating medium 14' in combination with the
armor
plate as illustrated in FIGURE 17. In such a combination, the shaped-charge
may
or may not be actuated upon contact with the medium 14' itself. In any event,
the
shaped-charge device is detonated upon contact with the exposed surface 142 of
the armor plate 144. Assuming actuation upon contact with the armor plate, the
shaped-charge device is generally surrounded by the attenuating medium 14'
upon
detonation so that the attenuating medium interferes with proper operation of
the
shaped charge as described above.
It is also possible to employ the attenuating medium 14' together
with the actuating element or surface 146 which may form a portion of the
confinement means or support medium 12'. In this regard, the actuating element
146 is selected with sufficient mass or resistance in order to assure
actuation of the
shaped-charge device. Thus, with the actuating element 146 in place, actuation
of
the shaped-charge device is assured as it approaches the attenuating medium
14'
prior to engagement with the armor plate 144.
It is to be noted that shaped-charge devices of the type described
above may include sequential charges. The embodiment of FIGURE 17 with the
actuating element 146 may be desirable in connection with such devices since
an
initial smaller charge would be actuated by the actuating element 146.
Thereafter,
the main charge of the device would be actuated upon engagement with the armor
plate with the device being surrounded by the actuating medium as described
WO 95/0148~t ~ ~ ~ ~ ~ PCT/US93/06319
-35-
above. It is further contemplated that spaced apart layers of the attenuating
medium, either alone or in combination with suitable support structure (not
shown)
may be employed for even further protection against shaped-charge devices
including sequential charges.
Another embodiment or concept of the: invention is similarly
contemplated for protecting people and/or structures from the harmful effects
of
explosions where explosive ammunitions or devices, for example, may be
detonated due to shock wave over-pressure and/or thermal energy release from
the
detonation of another explosive charge or device. This phenomenon is commonly
referred to as "sympathetic detonation", "fratricide", "propagation" or "chain
reaction". Any of the embodiments of the present invention may be employed in
such applications as a barrier, buffer or ramp of one explosive munition or
device
in order to protect it from external overpressure and/or thermal energy
generated
by another explosive device arranged external of the :invention.
Accordingly, in this embodiment or concept, the attenuating medium
is used in generally the same manner described elsewhere herein. However, it
is
contemplated that one explosive element or device (not shown) be sheltered or
arranged within the attenuating medium assembly of the invention in order to
protect it from another explosive element or device (not shown) which is
external
to the attenuating medium assembly.
A still further embodiment or variation of the invention involves its
use in forming air cargo containers or the like. Here again, generally all of
the
above described embodiments of the invention are suitable for this application
with
the air cargo container forming the interior of the attenuating medium
assembly.
Preferably, the assembly is formed as a complete enclosure or lining for the
container. It is also particularly contemplated that the invention be used in
fabricated panels to form the lining for such containers.
In any event, a container including such a lining can provide
substantial protection against blasts occurring within the container, the
invention
thereby protecting the areas surrounding the container.. Similarly, the lining
could
also protect the interior of the container from external blasts.
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Referring particularly to FIGURE 18, the invention also
contemplates use of the attenuating medium of any of the preceding embodiments
in combination with conventional honeycomb which may preferably provide
support for the attenuating medium. A preferred configuration is indicated at
150
in FIGURE 18 wherein the attenuating medium 14' is preferably arranged in all
of
the cells 152 of a honeycomb structure 154. The attenuating medium is
illustrated
only in selected cells of the honeycomb structure in FIGURE 18 for simplicity.
The attenuating medium may be a solid or liquid as described
elsewhere above.
Preferably, the configuration 150 includes porous confinement
means 30' arranged on opposite sides of the honeycomb structure 154. The cells
152 of the honeycomb structure are arranged with their axes 156 intersecting
the
porous confinement means 30'. The configuration 150 is preferably arranged
with
one of the porous confinement means 30' facing a shock wave source as
indicated
by the arrow 158.
With this arrangement, the cells of the honeycomb are exposed Ito
the shock wave as it penetrates porous confinement means 30' Thus; the
attenuating medium is free to react in generally the same manner described
above.
Preferably, the cells of the honeycomb are sufficiently large to permit the
contemplated function of the attenuating medium.
More preferably, the honeycomb structure 154 is provided with a
large number of openings 160 in the walls 162 of the cells. The openings 160
permit lateral propagation of the shock wave between adjacent cells to further
facilitate the function of the attenuating medium as discussed elsewhere
herein.
There have accordingly been described a number of embodiments of
attenuating assemblies andlor mediums constructed according to the present
invention. Variations and modifications in addition to those described above
are
believed obvious from the description. Accordingly, the scope of the invention
is
defined only by the following appended claims which are also further exemplary
of
the invention.
