GB1602466A

GB1602466A - Sound absorbing device

Info

Publication number: GB1602466A
Application number: GB1779677A
Authority: GB
Original assignee: RECOURT MARTYN E DE
Current assignee: RECOURT MARTYN E DE
Priority date: 1977-04-28
Filing date: 1977-04-28
Publication date: 1981-11-11

Description

(54) SOUND ABSORBING DEVICE (71) We, CARL MATTHEWS, a British Subject, of 102 Gloucester Road, London, S.W.7 and ELIZABETH DE RECOURT MARTYN, a British Subject, of 37 Stanhope Gardens, London, S.W. 1, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to the selective absorption of acoustic energy from kinetic energy under conditions of relative motion between a surface and a fluid, and is more particularly but not exclusively applicable to exhaust silencers for internal combustion engines.

The invention provides an acoustic energy absorbing device, comprising a conduit having an interior surface, and means for absorbing acoustic energy from fluid flowing through said conduit, said means comprising a liner of an acoustically transmitting material defining a fluid passageway within said conduit and sound absorbing material occupying space between the liner and the interior surface of the conduit, the sound absorbing material comprising a plurality of fibres operatively secured at at least one end to the outer surface of the liner or the conduit interior surface, free portions of the fibres lying in contact with the other surface.

The flexible fibres may each be operatively attached at one end thereof to the conduit interior surface in which case the fibres extend generally inwardly from the conduit interior so that the free ends of the fibres contact the liner.

The fibres may comprise a fibre mat having a plurality of free ends extending generally inwardly from the conduit.

The fibres may be operatively secured at both ends thereof to the conduit interior surface to form loops, the looped fibres extending generally radially inwardly from the conduit interior surface so that free central portions of the fibres contact the liner.

The fibres may be fixed to a backing material secured to the conduit interior surface.

The fibres may be operatively secured at both ends to the outer surface of the liner to form loops, free central portions of the fibre loops laying in contact with the conduit interior surface.

The invention further provides an acoustic energy absorbing device comprising a con- duit having an interior surface and means for absorbing acoustic energy from fluid flowing through said conduit, said means comprising a liner of an acoustically transmitting material defining a fluid passageway within said conduit and sound absorbing material occupying space between the liner and the interior surface of the conduit, the sound absorbing material comprising a plurality of fibres operatively attached to the outer surface of the liner, the free ends of the fibres extending outwardly towards the conduit and lying in contact with the interior surface of the conduit.

The liner is preferably woven glass fibre, and the liner preferably has a thickness of at most 2 millimeters. Alternatively, the liner may be of a woven fabric, elastomeric sheeting, paper of a similar flexible material.

The conduit preferably has a circular cross-section and the liner is preferably also circular in cross-section and generally coaxial with the conduit.

The liner may be retained by a spiral wire coil.

The fibres preferably have an average diameter of between 1 and 50 microns.

By way of example, two embodiments of an acoustic energy absorbing device and modifications thereto according to the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a top view partial representation of a vehicle having an acoustic energy absorbing device according to the invention; Figure 2 is a partly cut away view of one embodiment of an acoustic energy absorbing device according to the invention; and Figure 3 is a partly cut away view of a second embodiment of an acoustic energy absorbing device according to the invention.

Figure 1 depicts an automobile showing the layout of a typical exhaust system. A silencer 10 (shown in more detail below) may be situated anywhere along exhaust line 11.

There may be more than one silencer 10.

Although this figure shows the silencer 10 as being of greater external diameter than exhaust pipe 11, the extension of this enlargement has been (for purposes of illustration) magnified. It is important that the internal space in silencer 10 should not be less than the average diameter of the exhaust line 11. In this way no constriction is built into the exhaust line and no additional back pressure is created.

The silencer 10 has a conduit, and as shown in Figure 2, the interior wall of conduit 13, shown in an axial cross-section, is provided with an investment of flexible, closely spaced fibres 12 which have a general appearance of animate fur.

An acoustically transmitting material provides a liner 14 and defines a gas passageway from inlet to outlet. As with the fibres 12 the liner material is selected to have both chemical and mechanical properties suitable for the environment in which it is used. A preferred material is a woven glass cloth which is capable of withstanding operational temperatures of up to at least 550"C. The material preferably has a thickness of less than 2 mm. It may be retained in place by suitable mechanical means such as by a network of rigid open work materials as an open wire mesh or a spiral coil of wire. The materials selected should effectively preclude the adjacent sound absorbing material from obstructing the passage of the fluid from the inlet to the outlet of the silencer and also prevent its loss into the atmosphere.

A silencer of the type described and illustrated is easily constructed. For example, a tightly woven glass cloth or mat 14 of the type described is wound onto a cylindrical mandrel, the diameter of which is equal to the diameter of the inlet tube of the muffler.

A spriral coil of wire 15 is then wound onto the mandrel and is secured in place. The sound absorbing material is positioned about the glass mat 14 and the spriral coil 15 and the whole is then inserted into the outer casing of the silencer, preferably of a cylindrical shape. The mandrel is then removed.

The fibres may be mechanically or adhesively attached to the conduit interior surface at their roots or they may be secured to a backing layer and the backing layer fixed to the internal wall of the conduit. When so attached an adhesive will be selected to be compatible with the fibres and capable of maintaining its integrity during conditions of operation. Depending on 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 retained, or additionally retained, at intervals by a solid keeper which exerts a retaining pressure on the investment, such retainer having a small cross-sectional dimension.An example of this is an arrangement of small diameter rods or spiral of rod or wire so introduced that the rod or wire becomes substantially buried in the investment.

The keeper may even be, for example, a gauze or mesh having 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 aliquant of the total length of uninterrupted pipe section. An aliquot part could conceivably give rise to harmonics of certain frequencies. The leading edge of the investment may be protected from attack by the fluid or gas by insert of a short collett, usually of metal, having one end swaged.

In Figure 3, the interior wall 13 of conduit 10, shown in an axial cross-section, is contacted with the free ends of an investment of flexible, uni-directional closely spaced fibres 16 which have a general appearance of animate fur.

A flexible acoustically transmitting material 17 defines a passageway from the inlet to the outlet of the silencer and is generally in the form of a tube or similar configuration.

Preferably, the material has a thickness of less than 2 mm and may be retained in place by suitable mechanical means, as discussed below, or retained by a network of rigid openwork material such as an open wire mesh gauze or a spiral coil of wire. The material selected when a textile should be woven tightly enough to effectively preclude the adjacent sound absorbing material from obstructing the passageway of the fluid from the inlet to the outlet of the silencer and also prevent its loss into the atmosphere.

The fibres may be mechanically or adhesively attached to the backing surface at the fibre roots or they may be secured to an intermediate backing layer and the backing layer fixed to the backing surface. When so attached an adhesive will be selected to be compatible with the fibres and capable of maintaining its integrity during conditions of operation. Depending on their composition they may, for example, be deposited electrochemically, cataphoretically, or by precipitation directly on the surface or on a suitable support which is attached to the conduit surface.

According to the various embodiments of the devices of out invention the fibres may extend perpendicularly from the internal wall of the conduit in the case of the first embodiment, or perpendicularly from the internal tube, in the case of the second embodiment and remain so over the whole of their length. With this arrangement, in use, the fluid flow may cause the fibres to bend over at some distance from their roots.So as to provide the optimum or most economical use of materials, a ratio will be established involving several factors such as the amount of incident energy and the statistical data relating to the fibres, population per unit area, density or specific gravity, Young's modulus, diameter, and length, particularly that part of the fibre investment which is parallel to the direction of fluid flow, effective thickness or depth from the roots when in use, environment humidity, and the length of axial path invested with the fibres.

When the construction of the present invention is used in an automobile engine 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.

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. This is an important feature of the present invention. A conventional automobile silencer reduces the noise made by the auto engine using a series of baffle plates, packings and walls inside of the silencer. In reducing noise a substantial amount of back pressure is created which decreases the efficiency of the engine. By reducing back pressure the overall operating efficiency and economy of the engine are improved.

While we have not fully elaborated the theory of operation of our invention, a reasonable explanation appears to be that alternating compression/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 apparently transformed into heat. Also a high viscosity is provided in the closely spaced fibres by the very great number of air columns of minute diameters which interspace the filaments, such columns being a factor in the acoustic energy absorption.

Taking an automobile exhaust system as an example, it is preferred that the diameter of the flexible liner defines an opening of about the same cross-sectional ares as that of the bore of an incoming exhaust line. to allow the exhaust gases to flow without resistance, the minimum cross-sectional area normally required is maintained and defined by the flexible tube which is positioned in the fibre field.

We have also found that a gas may be released silently yet at high velocity from the end of a conduit such as a compressed air line. For related physical reasons, organ piping is eliminated without reduction of rheological efficiency.

Our 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.

In a further modification, the fibres may be arranged to form loops, and the fibre loops may extend either inwardly from the interior surface of the conduit, being equivalent to the embodiment of Figure 2; or outwardly from the liner, being equvalent to the embodiment of Figure 3.

WHAT WE CLAIM IS: 1. An acoustic energy absorbing device comprising a conduit having an interior surface, and means for absorbing acoustic energy from fluid flowing through said conduit, said means comprising a liner of an acoustically transmitting material defining a fluid passageway within said conduit and sound absorbing material occupying space between the liner and the interior surface of the conduit, the sound absorbing material comprising a plurality of fibres operatively secured at at least one end to the outer surface of the liner or the conduit interior surface, free portions of the fibres lying in contact with the other surface.

2. An acoustic energy absorbing device comprising a conduit having an interior surface and means for absorbing acoustic energy from fluid flowing through said conduit, said means comprising a liner of an acoustically transmitting material defining a fluid passageway within said conduit and sound absorbing material occupying space between the liner and the interior surface of the conduit, the sound absorbing material comprising a plurality of fibres operatively attached to the outer surface of the liner, the free ends of the fibres extending outwardly towards the conduit and lying in contact with the interior surface of the conduit.

3. An acoustic energy absorbing device as claimed in Claim 1 wherein the fibres are operatively attached at one end thereof to the conduit interior surface and extend generally inwardly from said conduit interior surface so that the free ends of said fibres lie in contact with the liner.

4. An acoustic energy absorbing device as claimed in Claim 3 wherein the fibres

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. extend perpendicularly from the internal wall of the conduit in the case of the first embodiment, or perpendicularly from the internal tube, in the case of the second embodiment and remain so over the whole of their length. With this arrangement, in use, the fluid flow may cause the fibres to bend over at some distance from their roots.So as to provide the optimum or most economical use of materials, a ratio will be established involving several factors such as the amount of incident energy and the statistical data relating to the fibres, population per unit area, density or specific gravity, Young's modulus, diameter, and length, particularly that part of the fibre investment which is parallel to the direction of fluid flow, effective thickness or depth from the roots when in use, environment humidity, and the length of axial path invested with the fibres. When the construction of the present invention is used in an automobile engine 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. 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. This is an important feature of the present invention. A conventional automobile silencer reduces the noise made by the auto engine using a series of baffle plates, packings and walls inside of the silencer. In reducing noise a substantial amount of back pressure is created which decreases the efficiency of the engine. By reducing back pressure the overall operating efficiency and economy of the engine are improved. While we have not fully elaborated the theory of operation of our invention, a reasonable explanation appears to be that alternating compression/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 apparently transformed into heat. Also a high viscosity is provided in the closely spaced fibres by the very great number of air columns of minute diameters which interspace the filaments, such columns being a factor in the acoustic energy absorption. Taking an automobile exhaust system as an example, it is preferred that the diameter of the flexible liner defines an opening of about the same cross-sectional ares as that of the bore of an incoming exhaust line. to allow the exhaust gases to flow without resistance, the minimum cross-sectional area normally required is maintained and defined by the flexible tube which is positioned in the fibre field. We have also found that a gas may be released silently yet at high velocity from the end of a conduit such as a compressed air line. For related physical reasons, organ piping is eliminated without reduction of rheological efficiency. Our 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. In a further modification, the fibres may be arranged to form loops, and the fibre loops may extend either inwardly from the interior surface of the conduit, being equivalent to the embodiment of Figure 2; or outwardly from the liner, being equvalent to the embodiment of Figure 3. WHAT WE CLAIM IS:

1. An acoustic energy absorbing device comprising a conduit having an interior surface, and means for absorbing acoustic energy from fluid flowing through said conduit, said means comprising a liner of an acoustically transmitting material defining a fluid passageway within said conduit and sound absorbing material occupying space between the liner and the interior surface of the conduit, the sound absorbing material comprising a plurality of fibres operatively secured at at least one end to the outer surface of the liner or the conduit interior surface, free portions of the fibres lying in contact with the other surface.

4. An acoustic energy absorbing device as claimed in Claim 3 wherein the fibres

comprises a fibre mat having a plurality of free ends extending generally inwardly from said conduit.

5. An acoustic energy absorbing device as claimed in Claim I wherein the fibres are operatively secured at both ends thereof to said conduit interior surface to form loops, the looped fibres extending generally radially inwardly from said conduit interior surface so that free central portions of the fibres contact the liner.

6. An acoustic energy absorbing device as claimed in Claim 5 wherein the fibres are fixed to a backing material secured to the conduit interior surface.

7. An acoustic energy absorbing device as claimed in Claim 1 wherein the fibres are operatively secured at both ends to the outer surface of the liner to form loops, free central portions of the fibre loops lying in contact with the conduit interior surface.

8. An acoustic energy absorbing device as claimed in any preceding claim wherein the liner is woven glass fibre.

9. An acoustic energy absorbing device as claimed in any one of Claim 1 to 7 wherein the liner is a sheet of elastomeric material.

10. An acoustic energy absorbing device as claimed in any preceding claim wherein the liner has a thickness of at most 2 millimeters.

11. An acoustic energy absorbing device as claimed in any preceding claim wherein the fibres have an average diameter of between 1 and 50 microns.

12. An acoustic energy absorbing device as claimed in any preceding claim wherein the conduit has a circular cross-section, and wherein the liner is also circular in crosssection and generally coaxial with and spaced from the conduit.

13. An acoustic energy absorbing device as claimed in any preceding claim comprising means for retaining the liner.

14. An acoustic energy absorbing device as claimed in claim 13 wherein the retaining means is a spiral wire coil.

15. An acoustic energy absorbing device as claimed in claim 13 wherein the retaining means is a wire mesh.

16. An acoustic absorbing energy device as claimed in Claim 13, Claim 14 or Claim 15 wherein the means for retaining the liner is located outside the inner surface of the liner.

17. An acoustic energy absorbing device substantially as hereinbefore described with reference to and as shown in Figures 1 and 2 or in Figures 1 and 3 of the accompanying drawings.

18. An acoustic energy absorbing device substantially as hereinbefore described in any of the foregoing examples.