Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to the selective absorption of
acoustic energy from kinetic energy under conditions of relative
motion between a surface and a fluid,
More particularly the invention provides means for
absorbing acoustic energy from a fluid stream, such as a gas,
which is a device including a surface past which surface the
fluid stream can flow, preferably with minimal or no loss of
kinetic energy due to surface resistance. ~he surface is pro-
vided with a multiplicity of fibers or filaments of a relatively
1~ small diameter, each fiber or filament being operatively
attached, either directly or indirectly, to the surface with
the ~ibers or filament ends extending freely into the fluid
stream.
In accordance with one aspect of the invention there
is provided a method for absorbing acoustic energy from a
fluid stream which comprises providing a surface past which
the fluid stream can flow, the surface having a multiplicity
of fibrous or filamentous members operatively attached to
and extending from the surface, flowing the fluid s-tream past
~0 the surface, and allowing the members to extend freely into
the 1uid stream parallel to the direction of flow of the
fluid stream and absorb acoustic energy.
In another aspect of the invention there is provided
a device for absorbing acoustic energy from a fluid stream
which device comprises a surface past which the stream can
flow, the surface having a multiplicity of fibrous or fila-
mentous members operatively attached to and extending from
the surface, the members being adapted to extend generally
parallel to and in the direction of fluid flow to provide
an unobstructed fluid 10w passage.
Particularly, the fibrous or filamentous members
G~ may comprise fibers or filaments in which each fiber or fila-
- 1 - ~
ment may be operatively attached to the surface at one end,
the other end extending freely into the fluid stream.
The fibrous ox filamentous members suitably define
a generally regular geometrical surface providing an unob-
structed fluid flow passa~e.
In particular the free ends of the fibers or fila-
ments may define such a surface.
In a particular embodiment the surface is an interior
sur~ace of a conduit.
In another aspect of the invention there is provided
a vehicle exhaust system comprising a device of the invention
as a silencer. Particularly there is provided in a vehicle
exhaust system comprislng a silencer, the improvement wherein
the silencer comprises a device of the invention in which the
support surface comprises an exhaust pipe, the fibers or fila-
ments extending inwardly of the pipe.
Thus in one embodiment of the method aspec-t, the
invention provides for a method of absorbing acoustic energy
2~ from a moving fluid stream, usually a gas, by passing the
~luid stream over and in contact with at least one surface
carrying a multiplicity of fibers or filaments or a relatively
small diameter, as described above. The moving fluid stream
impinges upon the free ends of the fibers or filaments and
the fibers absorb acoustic energy.
The invention is particularly useful, for example,
in providing an exhaust system, or portion of the exhaust
system of any desired shape or configuration, ~or an internal
combustion engine for automotive purposes, such as a motor
vehicle, marine craft or aircraft, or as a replacement for a
conventional automotive muffler.
The device may also be useful as an integument
applied to an external or internal wall face in order to
absorb acoustic radiation from the solid.
According to a feature of the invention the fibrous
or filamentous members comprise fibers or filaments which are
pre~erably ~lexible, and according to a further ~eature free
ends of the fibers or filaments usually extend parallel to
and trailing in the fluid with respect to the direction of
fluid or surface movement.
The fibers or filaments may be of inorganic, metal-
organic or organic material provided they have physical and
chemical properties appropriate for their integrity and survival
for an acceptable period of use in the environment in which
they are placed.
Thus, for example, for endurance in an engine exhaust
of a marine craft where the exhaust gas is cooled, such as by
water injection, the fibers or filaments may be of organic
origin. By contrast in the dry and uncooled conditions of the
engine exhaust of an automobile, aircraft or the like, the
fibers or filaments should preferably be of inorganic material
having a refractoriness and insolubility which are appropriate,
and in this case could be of siliceous, ceramic, carbon or
similar material. Conversely they may be metallic, or a
mixture of any number of them. Selection of an appropriate
fiber may be determined through preliminary experimentation by
one skilled in the art, It is preferred that the average dia-
meter of the fibers or filarnents employed will be in th~ range
oE about 1 to about 50 microns in size.
The configuration of the fibers may be such tha-t
they are suficiently close together so that their free ends
define a substantially regular geometrical surface or fairly
defined plane beyond which there is no obstruction, or
relatively little obstruction, to the passage of the fluid.
Furthermore, since this surface is flexible and of low
~a reflectivity, the acoustic energy present in the fluid is
readily transmitted throu~h it and absorbed by the mass of
fibers between it and the outer casing. The fibers at least
initially extend normal to the surface to which their fixed
ends are attached, although over a period of use may become
bent or curved in the direction of the fluid flow.
One embodiment of the invention will now be described,
by way of example, with reference to the accompanying drawing
which is an axial cross-section through a conduit lined with
an investment o~ fibers.
~0 T~le interior wall of conduit 10 is provided with an
investnlent of flexible uni-directional closely spaced fibers
11 analogous to certain types of animate fur, the majority of
which fibers after rising from the internal wall 10 to which
they are operatively attached, at least initially assume
positions parallel or substantially so, to the direction of
fluid flow.
When the construction of the invention is used in an
automobile enyine exhaust system, it has been found that the
acoustic energy present in the gas can be absorbed to a very
high degree without incurring any substantial decrease in the
kinetic energy of the gas.
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This device then provides an exhaust which can be
totally silent with minimal back pressure, or, at high
velocities of flow, pressure of such low value that the engine
maintains higher efficiency than is normally the case.
~ conventional automobile muf-fler reduces the noise
made by the auto engine using a series of baffle plates,
packings and walls inside of the muffler. In reducing noise
a substantial amount of back pressure is created which
d~cr~ases the efficiency of the engine. By reducing back
p~essure, the overall operating efficiency and economy of the
engine are improved.
While the theory of operation is not fully elaborated,
a reasonable explanation appears to be that alternating com-
pression/depression waves of acoustic energy of both longitudinal
and transverse propagation are absorbed by reason of the
multiplicity of phase changes and this energy is transformed
into heat. Also a high viscosity is provided in the closely
spaced fibers by the very great number of air columns of
minute diameters which interspace the fibers, such columns
~0 being a factor in the acoustic energy absorption.
To allow the exhaust gases to flow without resistance
the minimum cross-sectional area normally rQquired is main-
tained as an empty space 12 in the fiber field, and the annualar
space between the empty space and the roots at or near the
outer casing is occupied by the fibers as described.
The fibers or filaments may be mechanically or
adhesively attached to the conduit at their roots or the
fibers or fîlaments may be secured to a backing layer fi~ed
to the internal wall of the conduit.
When so attached an adhesive will be selected to be
compatible with the fibers and capable of maintaining its
integrity during conditions of operation.
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~ ccording to their composition they may, for
example, be deposited electrochemically, cataphoretically or
by precipitation directly on the conduit surface or on a support
which is attached to the conduit surface. They may be
supported, or additionally supported at intervals by a solid
keeper which exerts a retaining pressure on the investment
from the fluid face, such retainer having a small cross-
sectional dimension. An example of this is an arrangement of
small diameter rods or a splral of rod or wire so introduced
1~ that the rcd or wire becomes substantially buried in the
investment.
The keeper may even be, for example, a yauze or mesh
havin~ a very high percentage of open area. Clearly, the
greater the total area of such solid and rigid keeper material
which is not well buried into the investment, the less the
acoustic absorbent efficiency of the investment will be.
In the event the keeper or retainer is inadvertently
exposed to the sound waves present in the fluid, it is possible
to arrange, in the case of the spiral rod, for example, that
the pitch, or wave length is an aliquot of the total length
of uninterrupted pipe section. An aliquot part could conceivably
give rise to harmonics of certain frequencies.
According to the various embodiments of the devices
the fibers or filaments may extend perpendicularly from the
internal wall of the conduit and remain so over the whole of
their length. With this arrangement, in use, the fluid flow may
cause the fibers or filaments to bend over at some distance from
their roots. So as to provide the optirnum or most economical
use of materials, a ratio will be established involving several
factors, including the amount of incident energy and the
statistical data relating to the fibers or filaments, population
`:
per unit area, density or specific gravity, Youn~'s modulus,
diameter, and length, particularly that part of the fiber
or filament investment which is parallel to the direction of
fluid flow, the effective thickness or depth from the roots
when in use, environment humidity, and the length of axial
path invested with the fibers or filaments.
It is thus possible to provide a gas passage which
has a relatively smooth and flexible face having low acoustic
reflectivity, but high transmission, we have, according to an-
other aspect of our invention, found that the acoustic energy
present in a flowing yaseous stream can be absorbed effectively
without incurring any substantial decrease in the kinetic
energy of the gas by providing an investment to the interior
wall of a muffler or exhaust system silencer casin~ which
comprises a sound absorbing material of a mass of fibers,
either oriented or randomly orien-ted as in the case of a felt.
The fibers which extend to the surface, being fle~ible, are
readily bent over by the gas stream to become trailing, as
pre~iously described.
The ~nvention is further described and illustrated by
the ollowing examples of the use of various embodiments. In
the testing reported relative aspects of approximate noise
levels, temperatures and bac~ pressures were measured on a
comparative basis, absolute values not being required TWo
types of embodiment of the invention were tested, one in a wet
(marine) environment and the other in a dry environment.
EXAMPLE 1
-
An engine of 1100 cc displacement was randomly
selected for convenience as one commonly used for road vehicles
and also for small marine craft. A common practice in marine
engineering is to cool the exhaust gas, as ~y water injection,
and then discharge the cooled gas from the craft through an
essentially unsilenced pipe or conduit. This is in contrast
to an automobile exhaust system in which a muffler, silencer
or the like is employed.
The dry gas temperature of the above engine emergin~
from the manifold was in the range of 500-720C, according
to the gas velocity. Water was injected into the engine exhaust
gas system at a rate of 2-2.5 gallons/minute or 9-11 liters of
water/minute. A few inches downstream from the water injection
point, the station being as usual convenlently close to the
engine, the temperature of the engine exhaust gas mixed with
water had dropped to 40-60C~
The lower temperature level is well within the
temperature tolerance of many organic filaments. In these
tests an 8~/o chlorofibrous material, having a monofilament
diameter in the region of 8 microns, derived from polyvinyl-
chloride, mixed with an acrylonitrile was used for the
e~haust pipe investment~ The overall appearance was similar to
the investment described in relation to the drawing. The
2~ investment was a typical artificial fur and had a polyester
backing with a polyacrylate resinous reinforcement, for all
o which insolubility in boiling sea water is claimed by the
ma~ers (Borg Textiles Limited), and was secured to the internal
part of the exhaust conduit.
For the water cooled run, the invested pipe was 11 ft
or 330 cm long, and the filaments had a dormant dep-th of 3/4
inch or 20 mm.
The performance of the invested exhaust was compared
with that of a normal water injected commercial system of
equal bore, but having, as is also common, one silencer of
the reaction type in the line. This routine system is also a
standard installation in naval craft.
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sefore running the invested example precautions were
taken, after injection, to separate out the water again so
that the ~as, having been cooled, was now dry or at least
relatively dry. It was observed that the drier the filaments
of the investment, the higher the acoustic absorption, There
are many convenient methods of effecting separation of the
injected water from the cooled exhaust gases, of which one or
a combination thereof may be employed. Examples include the
centriEugal effect of a pipe bend, making use of the traditional
riser on board the craft, a short section containing trans-
verse angled louvers, or if considerable length is available,
separation by gravity into a collector trough or pipe.
During each test, the engine was run at engine speeds
of 2300, 3500 and 5500 rpm. The length of the exhaust pipe
was 120 inches measured from the engine manifold. The
traditional (unmodified) system produced noise levels rising
to about 65dBA and back pressures of 3, 10 and 18 inches of
water (75, 250 and 460 mm). With further engine acceleration
in excess of 5500 rpm back pressure level rose steeply to 30
~0 inches of water ~760 mm).
The invested plain pipe of the present invention with
no muffler was then compared. At 5S00 rpm and above the
effluent was silent and later the pipe had to be shortened
to 9 ~t. (270 cm) before exhaust became audible. At the quoted
rpm periods the back pressure figures were 0 inches, 0.5-1 inch
and 2 inches (0, 12, 25 and 50 mm), respectively.
Zero readings reflected the small inertia of the
manometer and the above figures are corrected readings, meaning
that the back pressure introduced b~ the bent downtake pipe
arrangement of the manifold, which was a common factor through-
o~t all observations was deducted.
f `~
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In general terms the invested exhaust system of the
invention produced about one eighth of the back pressure of the
standard marine exhaust system, The higher the velocity, the
larger the difference in comparative back pressure of the
standard e~haust system and that according to the inven~ion.
The dry tests were then done. To the engine was
fitted the standard exhaust pipe with silencer made and sold
by the engine maker for that engine.
Corrected readings showed 2 - 3 inches water (50 -
75 mm) BP at 2000 - 2500 rpm, 3 - ~ inches (75 - 100 mm) at
3300 - 40000, and 10 inches (250 mm) at 5500 rpm. Again the
noise level rose to a level of discomfort, as commonly
experienced in cities.
The invested pipe, 10 ft. 300 cms. long, with no
silencer box was run and produced no audible sound even at 12
inches, or 30 cms from the outlet. Back pressure readings
now showed 1, 1.5, and 2.5 inches (25, 37 and 63 mm water
column).
This invested pipe was then shortened progressively
~0 until at 24 inches or 60 cms len~th it produced at the outlet
an approximately similar noise level as the traditional, except
that again the higher fre~uencies have been removed, the re-
sulting being easier on the ear. At this much foreshortened
length the back pressures were 0, 0.5 - 1, and 2 inches water
(1, 12 - 25 and 50 mm).
The indication therefore is that in a ~ry system the
invested exhaust pipe produced only about one sixth the back
pressure induced by a traditional pipe with its silencer.
It has been found that a gas may be released silently
yet at high velocity from the end of a conduit such as a com-
pressed air line. Forrelated physical reasons, organ piping
is eliminated without reduction of rheological efficiency.
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The invention is not to be considered limited in any
way to the silencing of an internal combustion engine as there
are diverse areas in which the principles set forth above also
apply. Other areas in acoustics where our technique may be
applied are those where high noise level impulse waves are
produced; the absorbent effect of the investment considerably
chops down the initial oscilloscope deflection.
Throughout the above discussion, illustrations
and examples, the unidirection of the fibers and filaments, or
1~ at least a majority of them, and consequently minimal
reduction in kinetic energy, has been the theme~ Paradoxically,
the reverse may apply in certain cases. ~n example of this is
that in some marine engine exhausts it is not only preferable,
but vitally important to prevent any sea water returning under
the force of a heavy wave back up to the engine, Clearly, the
investment devices and procedures of our invention produce
a resistance to such return flowO
The preferred average diameters of the fibers or
filaments should be within the range 1 to 50 microns.
Various changes and modifications o~ the invention
can be made, and, to the extent that such variations incorporate
the spirit of this invention, they are intended to be included
within the scope o~ the appended claims,
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