Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention relates to acoustic protection material and
to apparatus including such material.
The importance attached to reducing sound nuisance is well
known both in the home and in industry, and although various
means for combatting noise have been developed, the results
obtained are not always satisfactory or they can be made
satisfactory only at the cost of great difficulty. Thus,
besides an initial approach which consists in limiting as much
as possible the sound level emitted by a source e g. an
engine, a high speed flow of fluid, etc., proposals have been
made to interpose protective walls between the scurce of sound
and a region in which it is desired to reduce sound pressure,
with the effectiveness of the protective walls inoreasing with
increasing density of the material from which they are made.
~levertheless, good results can be obtained, e.g. in the
building industry, only by using wall thicknesses that are
technically and/or economically difficult to implement.
Another approach then consists in performing acoustic
correction by means of absorbent materials placed on a
partition delLmiting an enclosure to be protected so as to
reduce as nluch as possible the reverberation of soundwaves on
said partition. The sound pressure level reductions obtained
in this way are of the order of 4 dB to 6 dB, and that does not
make it possible to obtain an effect which is sufficient for
significantly protecting enclosures exposed to sources of
intense sound.
~ sing a different method, known as "active absorption",
proposals have also been made to detect and analyze the
soundwave emitted by a source of noise, and to cause the wave
to disappear completely or partially by means of loudspeakers
or analogous means disposed in the region to be protected and
generating a soundwave in phase opposition with the ir~cident
source wave. Such a method is both complex and expensive, and
consequently use thereof is limited to very specific cases
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where the regions to be protected are small in size and where
the sound frequency ranges are not too large. That is why use
is sometimes made of walls including air resonators of the
Helmholtz resonator type as described, for example, in British
Patent Specification No. GB-A 2 027 255 or composite wall~
made up of volume-o~cupying elements secured to a support
and that enter into resonance at predetermined frequ~ncies
as described, for example, in German
Patent Specification No. 2 834 823. In order to wor~ in
the low freouency range (lOG Hz to 300 HZ) walls of the first
type (including Helmholtz resonators) re~uire relatively large
resonator volurnes, while nevertheless limiting the acoustic
corrections obtained in this range to the (necessarily small)
ratio of the sum of the areas of the throats of the resonators
dis~osed in the wall to the total area of the wall. Although
the ~se of composite walls having volume-occupying elements
turns out to be effective when the materials associated with
the wall have large ~iscous friction ( e.g. elastomers), this
technique is nevertheless difficult to implement when a high
degree of acoustic protection is sought over a wide range of
sound frequencies.
The problem thus arises in the fields of acoustic
protection, oorrection, and conditioning, of supplying a
material and apparatus firstly enabling the drawbac~s of known
techniques to be mitigated, secondly being easy to implement
and providing results that are satisfactory including over a
wide audible frequency range, and finally having a cost that is
economically acceptable.
A general object of the invention is to provide a material
and apparatus, incorporating such a material that enable this
problem to be solved.
Another object of the invention is to provide a material
and apparatus incorporating such a material which obtain very
effective protect'on while using thin plates that are easy to
ma~e, that are suitable for being assembled, and for being
cleaned, and i~ general that are easily used by a non-
specialized user using means and tools that are simple and
commonly-avallable.
. .
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acoustic protection against a source of noise, the material
comprising a substrate and resonators, wherein said resonators
formed on ~he.substrate are constituted by threa~-like and/or
surface extending elements whose structural
characteristics (density, modulus of elasticity, shear modulus,
damping factor, piezoelectric factor, etc. ...) and whose shape
and/or size are selected to associate a predetermined resonant
frequency with each resonator, and also to absorb the sound
pressure energy from the noise source at said resonant
frequency and dissipate it in the form of mechanical heat
energy and/or electrical energy.
The substrate may be pierced bY orifices in which, or at the
edges of which, the elements constituting the resonators are
secured, the elements being in the form of vibrating members and/or
vibrating strings and/or vibrating blades.
In a first preferred embodiment, each resonator is made of a sheet
of material havin~ a higher modulus of elasticity and a low damping
factor, for instance, a metal or a metal alloy sheet, e.g.
aluminum.
In a first modification, each resonator is a composite membrane
comprising at least two sheet of material h~ving a igh modulus of
elasticity, and a low damping factor (tan ~),, and a sheet of
material having a low shear modulus and a high damping factor (tan
In one implementation, the composite membrane is of the
sandwich type with outer sheets of metal or metal alloy, such
as aluminumi of a thic'cness lying in the ~ge 10 mic-ons to
200 microns enclosing between them a sheet of elastomer
material selected to have a damping factor (tan ~) lying in the
range 10-2 to 50 10-2 and a thic~ness lying in the range 20
microns to 500 microns.
In a variant, the composite membrane is of the sandwich
type with outer sheets made of the elastomer material and a
core constltuted by a sheet of metal or metal alloy, e.g.
aluminum, the thic~nesses and the damping factors being the
same as those speclfied for the sandwich structure as defined
immediately above.
The membrane, whether composite or made of metal, may have a
circular outline, but it may also have an outline that is square,
rectangular, elliptical, crescent-shaped, etc., or -that has lobes.
2067480
If it has lobes, the invention provides for taking
advantage of the number of lobes used for securing the mernbrane
peripherally to the substrate to establish the value of the
predetermined resonant frequency.
This frequency may also be fixed or adjusted by forming
one or more arrays of corrugations on the membrane,
thereby reducing the bending stiffness of the membrane and thus
lowering its resonant frequency.
The resonant frequency may also be fixed to a
predetermined value (e.g. by calculation using the method of
finite elements) by ma~ing openings of various shapes and/or
dispositions in the surface of the membrane.
In another embodiment, each resonator secured in or at the
edges of orifices in the substrate is
constituted by a thread~ e composite element obtained by
twisting together fibers having a high modulus of elasticity
and impregnated ~ith an appropriate quantity of a material
having good damping characteristics.
In yet another embodiment, the resonators associated with
the orifices of the substrate are composite
vibrating blades each secured at one end, and constituted by a
metal and/or polymer material having a high modulus of
elasticity~
Whatever the embodiment ~membrane, string, or vibrating
blade)~ the inventio~ provides for the substrate being m,ade of
flexible material, e.g., an elastomeric or plastomeric type
material .
Although the above-defined embodiments and irnplementations
of the acoustic protection material of the invention dissipate
scund pressure energy directly in the form of heat, the
invention also provides for other embodiments in which the
acoustic protection material transforms the sound pressure
energy into electrical energy, -~hich electrical en2rgy is then
dissipated in the form of heat by means of the Joule effect.
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~ nder such circumstances, the substrate
is associated firstly with sheets of a crystalline or poly-
crystalline type material having piezoelectric properties such
that electric charges appear on the surfaces of the sheets in
response to a sound pressure wave, and secondly with very thin
conductive electrodes collecting the electric charges generated
to cause them to pass through electrical resistances or through
materials having analogous properties.
In one implementation, the sheets generating electric
charges in response to a sound pressure wave are constituted by
films of PVDF type polymer made semi-crystalline by an
appropriate thermomechanical treatment, the electric charge
collecting electrodes being constituted by fine metal films
obtained by vacuum metallization on the polymer sheets, or in a
variant by very thin sheets of metal or metal alloy, e.g. based
on aluminum, that are glued on said polymer film by means of an
intrinsically conductive adhesive.
The invention also provides acoustic conditioning and/or
protection apparatus constituted by a wall including at least
one layer of material as defined above.
In a preferred embodiment of such an apparatus, the wall
includes a plurality of layers of the material, the layers
being disposed in such a manner that the resonators of ad, acent
layers do not face one another.
In addition, the invention provides for the layers of the
apparatus to be made of acoustic protection materials that
differ from one another in the resonators that they implement,
not only with respect to the shape, the disposition, and/or the
nature of the elements of the resonators, but also,
where appropriate, with respect to the substrates on which tAe
resonators are disposed.
When the absorbed sound pressure energy is transformed
into electrical energy, apparatus of the invention comprises a
multiplicity of layers of material as defined above, with the
electrodes of each layer being put into electrical continuity
via microperforations through the sheets generating the
electric charges in the layers.
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In the accompanying drawings:
Figures lA and lB are a section view and a plan view
respectively of a membrane resonator;
Figures 2A and 2s are highly diagrammatic section views of
composlte ~embranes suitable for use in maXing up a material of
the invention;
Figure 3 is a view analogous to Figures 2A and 2B for
another embodiment;
Figures 4 and s are plan views showing the shapes of
m~mbranes used in making up a material of the invention;
Figures 6A and 6B are respectively a section view and a
plan view of another eesonator suitable for use in making up a
malerial of the lnven~icn;
r igures 7A and 7B are views analogous to those of Figures
6A and 6B but for another embodiment;
rigure 8 is a plan ~iew of a layer of material of the
invention
Figure 8A is a view analogous to Figure 8 but showing a
varlant;
Figure g is a diagrammatic section vie-~ on line 9-9 of
Figure 8A;
Figure 10 is a very diagrammatic section view through
apparatus of the invention;
Figure 11 is a very diagrammatic sect on view through a
different embodiment of material of the invention;
Figures 12 and 13 are highly diagrammatic s~etches of
acoustic protection apparatus made using the material shown in
Figure 11; and
Figures 14 and 15 are graphs showing test results.
Reference is made initially to Figures lA and lB which
show an example of a vibrating membrane 1 of circular outline
secured via its edge 3 on a substrate 2 that has a
circular hole 4. When such a membrane 1 is subjected to a
pressure P that varies substantially sinusoidally with time and
2067480
that has a wide frequenc~ range, said membrane enters into
resonance at a frequency fO such that:
(I) f = f[R-2, E2, e, p-7
where:
P~ designates the radius of the membra~e;
E is the modulus of elasticity of the material from which
the membrane is made;
e is the thickness of the membrane; and
p is the density of the material constituting the
membrane.
If the shear modulus of said membrane is designated G and
if its damping factor is designated by tan ~, and given that
the membrane is subjected to the same number of alterations per
unit time as the exciting sound wave of frequency fO, the
energy absorbed per alternation is given by a formula of the
type:
(II) wJOule = f[R6, e~3, p2, G-l, (tan 6)-1~
where R and e have the same meanings as above, and where P
designates the pressure of the soundwave.
It thus appears from equations (I) and (II) that the
resonant frequency fO of the membrane is a function of the
modulus of the elasticity and of the shear modulus of the
material from ~Jhich it is made, and that for a given resonant
frequency and for a given membrane radius, there exists a
correlation domain between the modulus of elasticity, the shear
modulus, and the damping factor enabling a preferred value of
energy to ~e absorbed per alternation.
In a~cordance with this invention, and in a preferred
embodiment,the value i~ obtained by using membrane 1 whic is made
of metal, e.g., aluminum, or is made of a metallic alloy, or else
is a composite metallic plate.
In a modification, membrane 1 is made as a sandwich-type
composite
membrane, see Figure 2A, comprising a core 5 made of a material
having a low shear modulus (G2), a thickness e2, and a high
damping factor ~tan 62), disposed between sheets 6 and 7
respectively of thicknesses el and e2, each being made of a
material having a high modulus of elasticity (El and E3,
respecti.vely), and a low damping factor (tan 61 and tan 63,
respectively)~
2067~80
In another embodiment, see Figure 2B, the middle core 8 or
the composite membrane is made of a material similar to that of
the sheets 6 and 7 of the preceding embodiment, while the outer
sheets 9 and lO are made of a material similar to that of the
core 5 of the embodiment shown in Figure 2A.
In both cases, good results have been obtained by making
the sheets 6, 7, and 8 from a metal or a metal alloy, e.g.
aluminum, copper, or steel alloy, having a thickness lying in
the range lO microns to 200 microns, while the sheets s, 9, and
lO are sheets of elastomer material having a thickness lying in
the range 20 microns to soo microns and having a damping factor
(tan ~) lying in the range lo-2 to 50.10-2. The sheets 5,
9, and lO can thus be selected from sheets based on rubber, on
thermoplastic polymer(s) e.g. polyethylenes, polyvinyl-
chlorides, or polyamides, or sheets made of thermosetting
polymer(s) based on epoxy resin(s), phenol resins, or
polyurethane, the elastomer or polymer sheets being
reinforced, where appropriate, by a woven or non-woven cloth of
glass fibers, polyester fibers, cotton fibers, polyaramide
fibers e.g. those ~nown by the T~ade-mark KEVLAR (trade-mark
of Dupont De Nemours), metal films, or the like.
In the embodiment shown diagrammatically in ~igure 3, the
composite membrane is of the type shown in Figure 2A, i.e. it
has a sandwich structure with the middle core 5' being
analogous to the core 5 and with outer facings 6' and 7'
analogous to the facings 6 and 7. However, in this embodiment
the bending stiffness of the membrane is reduced by one or more
arrays of corrugations ll, thereby enabling the resonant
frequency fo to be reduced while the other dimensional
characteristics (radius R, thickness e, characteristic
moduluses ...) remain fixed.
Given that the resonant frequency fO also de~ends on the
nature of the fixing 3, the invention also provides for fixing
the predetermined value of the ,-esonant frequency by giving
the membrane 12 a shape having a periphery with cutouts 131,
132, 133, etc. (Figure 4), with said membrane being fixed
on the substrate by securing the edges l4l, 142, 143,
20~7~0
etc. of its lobes, with the number, the shape, znd the
disposition of said lobes being advantageously obtained by
calculation, e.g. by application of the method of finite
elements.
This same calculation method can be imple~mented for fixing
or adjusting the resonant frequency fO of the membrane 15 by
modifying its mass per unit area, which is done most simply by
forming holes 161, 162, 163, ..., (Figure 5), with the
shape, number, and distribution of the holes being established
by calculation.
Although the membrar.es 1 12. and 15 of the embodiments
described have been described and shown with an outline that is
totally or partially substantially circular, the invention is
naturally not limited to such examples, and the apparent out-
line or the mem~ranes may be square, rectangular, elliptical,
crescent-shaped, etc. ..., and each membrane may also be
partially perforated, corrugated, cutout into lobes, etc.
In the embodiment shown in Figures 6A and 6B, the
resonators are constituted by vibrating strings 20 tensioned
across orifices 4 formed through a substrate
2, each strlng 20 which is advantageously constituted by
twisting together fibers having a high ~odulus of elasticity
being secured at its ends 3 tc the edges of the orificè 4.
In one embodiment, the strings are made metal or metal alloys.
in another embodiment, the strings are made of polyester,
polyamide, or polyaramide fibres and the impresnating material is
a butyl type elastomer, for example.
In the embodiment shown in Figures 7A and 7B, the
resonators of the protective material and/or of the acoustic
conditioning material of the invention are constituted by
vibrating blades 21 each secured at one end 22 in the
sub~trate 2, with each blade advantageously being formed in a metal
or metal alloy sheet, or in a modiflcation, by a composite, e.g.,
those described above with
reference to Figures 2 to 5, i.e. by an assembly of metal
and/or polymer type materials selected as a function of their
characteristics concerning modulus of elasticit;, shear
modulus, and damping factor.
206748~
~ egardless of the resonator embodiment used by the
material of the invention, the substrate 2 exerts an influence
on the resonant frequency of sald resonators and on the
corresponding energy absorption. This .influence is related to
the conditions under which the resonators are secured, and thus
makes it possible to select a material for the substrate 2 such
that it presents damping characteristics that increase the
qualities of the material of the invention,
A relatively large increase in effectiveness can ~e o~tained,
as shown in Figure 14, when a support plate pierced by circular
orifices having a diameter of 10 mm is provided with
membranes
, depending on whether the plate is a rigid support or
is a support made of elastomer material.
The preferred embodiment of a material according to the
invention is thus a support of elastomeric type material with
resonators made of metallic membranes, blades or strings.
As shown in Figure 8, the substrate extends over a surface
S and has a multiplicity of orifices 4 pierced therethrough,
with each orifice being provided with a vibrating string,
blade, or membrane resonator.
When all of the resonators in the surface ~ are identical
or nearly identical, they present resonant frequencies that are
coherent and the absorbed sound pressure energy then
corresponds to a well-determined frequency fO, as shown for
example in Figure 15 which shows the response curve to pink
noise excitation of a plate having circular holes with a
diameter of 10 mm, as defined above, and having a
resonant frequency fO of 2100 Hz.
In contrast, when the surface S' is as shown in Figures 8A
and 9, i.e. when it is pierced with orifices 4', 4", etc. of
different shapes and sizes, the resonators formed on said
surface then reso~ate at different excitation frequencies fl,
f2, f3, etc., thereby absorbing a portlon of the sound
press~re energy from a noise source in discrete ~ands
corresponding to each of said frequencies.
In such an embodiment, th~ ~coustic protection material of
the invention i~ obtained by fixing a metàilic sheet or a ~heet 25
of the t~pe described above with reference to Figures 2A or 2B onto a
2a67~80
substrate 2 pierced with orifices 4~, 4ll, etc., the sheet bein~
fixed by adhesion or by analogous means such that the regions
of the sheet 25 that overlie the orifices 4', 4~, etc. and
which are secured by their edges constitute resonators having
diffe~ent resonant frequencies fl, f2, f3, etc- To
manufacture surfaces e-g- S or S', the invention provides
for making the substrate 2 from a sheet of elastomer or
plastomer having a thickness lying in the range 0.1 mm to 1 mm,
e.g. a sheet that is calendered and then perforated to form_~he
orifices 4', 4", etc., and then to fix, to the substrate, a
metallic sheet or foil, or else in a modification, a sheet e.g. 25
prepared by
calendering or by extrusion blowing its polymer or elastomer
component, which component is then coated with a metal foil, or
in a variant is made to adhere to such a foil by gluing or the
like. To fix the substrate and the metallic or composite sheet
together the invention proposes using hot plate presses or systems usinc-
"rotoc~re" heating cylinders as used in the rubber industry, or
else to fix the parts together by cold gluing using structural
adhesives such as epoxy resins or the like, or by melting a
film of thermoplastic polymer, etc. ... .
To make apparatus for providing acoustic protection
against a source of noise SO (~igureslO) by means of an
acoustic protec~ion material as defined above, the invention
provides f~r superposing and sticking together a multiplexity
of layers Sl, S2, S3, etc. ..., each of which is of the
type shown in Figure 8 or 8A, i.e. each of which comprises a
substrate 2 pierced by orifices 4, 4', 4",
..,, in which or at the edges of which vibrating blade, string,
or membrane resonators are secured.
In the embodiment described and shown, the resonators of
each layer are constituted by a sheet 251 for the layer Sl,
252 for the layer S2, 253 for the layer S3, etc. ....
and in order to limit the transmission of sound energy by the
effect of continuity between the substrates of the stuck-
together layers, the various layers are offset relative to one
another so that the resonators of adjacent layers do not face
one another. Thls offset may be the result of calculation, or
in a variant it may be obtained by placing the layers Sl,
12 2~67~80
S2, S3, etc. randomly relat ve to one another, thereby
supplying apparatus having the capacity to absorb incident
sound energy at a multiplicity of frequencies.
Although the above-described embodiments and
implemen~ations of the acoustic protection material of the
invention dissipate sound pressure energy from the source So
directly in the form of heat, the invention also provides an
embodiment in which the acoustic protection material transforms
the sound pressure energy into electrical energy, with the
electrical energy then being dissipated in the form of heat by
the Joule effect.
For such a material, a composite element 30 (Figure 11) is
initially made from a sheet 31 of crystalline or poly-
crystalline type material that generates electric charges on
its faces 32 and 33 hy the piezoelectric effect in response to
the action on said sheet of sound pressure from a noise source.
Said sheet 31 is secured to conductive electrodes 34 and 35
which collect the electric charges, which electrodes disposed
on respective surfaces 32 and 33 convey the, charges and cause
them to pass through resistors elements for the purpose of
dissipating the electrical energy in the form of heat by the
Joule effect. The sheet 31 may be constituted by a PVDF type
polymer film which is made semi-crystalline by an appropriate
thermomechanical treatment, or by any other material having
analogous piezoelectric characteristics, and the electrodes 34
and 35 are advantageously formed by a very fine film of metal
or metal alloy obtained by vacuum metallization onto the sheet
31, or, in a variant, glued onto said sheet by means of an
adhesive that is intrinsically conductive.
To make acoustic protection apparatus using the above-
described material, the invention provides for disposing a
ccmposite element 30 on a substrate 2 pierced by orifices 4,
4', 4", ... (Figure 12), thereby constituting a first layer
C1, and then for superposing and gluing together layers Cl,
C2, etc. ... each formed by a substrate and a composite
element 30. In such an embodiment, each substrate 2 is then
made conductlve to have resistivity that may lie in the range
2~7~80
13
o.l ohm.cm to 100 ohm.cm, advantayeously by incorporating a
conductive filler e-g- Icarbon black or a metal powder in an
insulating matrix so as to confer a resistive effect on each of
the layers C, thereby ensuring that the electrical energy is
dissipated by the Joule effect.
Also in this embodiment, the invention provides for
ensuring electrical continuity between the electrodes 34 and 35
applied on the two opposite faces of the sheet 31 by forming
microperforations 40 through said sheet (Figure 13), such that
a conductive polymer material 41 provided for assembling
together the layers C also ensures electrical continuity
between the electrodes 3~ and 35 via small bridges 42 formed
through the microperforations 40.
Apparatus e.g. those described above have a very
large number of appiications both in the hcme and in industry.
Thus, and without these suggestions being limiting in any
way, they may be used for attenuating noise in the cab of a
vehicle by being interposed in the form of a plate between the
cab and a source of noise e.g. an engine, a transmission, or
an aerodynamic flow.
They may also be used for attenuating noise in aircraft
(by fixing plates on the inside walls of the cockpit) or else
in land vehicles, road vehicles or rail vehlcles and also in
river- or sea-going vehicles, being used as covers for sources
of noise e.g. engines, exhausts, etc.
Such plates may also be used for covering noisy machines,
for lining the walls of enclosures in factories or noisy
workshops, and in general in any building for dwelling or for
industrial purposes in which apparatus of the invention is
suitable for use not only for reducing noise coming from
adjacent premises, but also for protecting buildings against
noise from outside, with this function being particularly
advantageous for providing protection against roads or
motorways, particularly in an urban setting.
The materials and apparatus of the invention are also
advantageously applicable to providing acoustic correction
and/or conditioning for premises where they are used by being
2067~80
14
fixed on the walls of said premises to absorb soundwaves that
are reflected on the plates of the apparatus, thereby reducing
noise levels and increasing user comfort.
