ACOUSTIC ATTENUATION DEVICE

Abstract

An acoustic attenuation device including an elongated hollow body having an open upstream end taking the form of a head for coupling to a compressed air outlet, and a closed downstream end, the hollow body having a sidewall with a plurality of air exhaust openings arranged in the downstream part, where the hollow body contains at least one upstream buffer lining forming a longitudinal filter and at least one downstream filtering lining forming a sheath.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No. 11/911,294 filed on 11 Oct. 2007 under 35 U.S.C. 371 with respect to International Application No. PCT/FR2006/000724 filed on 3 Apr. 2006 which claims priority to French Patent Application No. 05.03602 filed on 12 Apr. 2005, all of which said applications are herein incorporated by reference in their entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an acoustic attenuation device designed to be connected to a compressed air outlet.

BRIEF DESCRIPTION OF RELATED ART

The abrupt release of a large quantity of compressed air may generate a noise of very high sound intensity, which can even become dangerous for people situated close by. Nevertheless, it is occasionally necessary to purge the air from a compressed air reservoir in this way so that an operator can verify its state and make sure notably of its correct sealing, pressurization and air release. This is the case, for example, for the braking circuits of a train. Indeed, the braking of a train is carried out by decreasing the pressure in the braking circuit, the shoes then being forced against the wheels of the train in order to slow it down. The pressurization, the sealing and the continuity of the pressure within the whole circuit are, amongst others, important parameters which need to be monitored.

The verification of the braking circuit of a train involves several steps performed by at least one operator. First of all, the operator checks the correct operation of each brake shoe, then a complete purging of the air pipe of the braking circuit is carried out in order to verify its continuity. Given that the pressure of the air in these pipes can reach as much as 9 bar, the intensity of the noise generated reaches around 121 dB near to the compressed air outlet. This value is close to the pain threshold and repeated exposure could lead to a hearing handicap for the operator. It is not therefore acceptable to expose an operator performing this type of check to such a sound intensity.

Noise attenuation devices have been developed in order to solve this problem and to allow an operator to complete his work safely. An acoustic attenuation device is notably known comprising an elongated hollow body designed to be connected to a compressed air outlet via a first open end fitted with a coupling head and having air exhaust openings arranged radially within a wall of the tube close to a second closed end. The acoustic attenuation is obtained by disposing inside the hollow body a knitted stainless steel cloth lining rolled-up on itself and forming a buffer filtering the air. The effect of this is that the air flowing through the lining follows different paths, which slows the propagation and the development of low- and high-pressure regions that generate the noise.

Such a device allows the intensity of the noise to be reduced from 120 dB to about 78 dB at the outlet of the compressed air pipe for an air pipe under a pressure of around 5 bar. This value of 78 dB may still be judged to be too high by the operators and it is desirable to reduce it. It should however be noted that a check by an operator requires a minimum noise level to be conserved in order for the depressurization to remain audible. A standard dictates that the intensity of the noise of the air outlet from the pipe should not be below 70 dB. Furthermore, such a device reduces the speed of the air at the outlet and it is not desirable to prolong the time required for a total purging of the pipe too much. The same standard previously mentioned thus dictated that a reservoir of 600 L of air at 5 bar should be able to be purged until the final pressure reaches 0.4 bar in less than about one minute.

BRIEF SUMMARY OF THE INVENTION

The goal of the present invention is to provide an improved device meeting the requirements of the standard while, at the same time, providing an enhanced acoustic attenuation, and for that purpose it consists of an acoustic attenuation device comprising an elongated hollow body having an open upstream end taking the form of a head for coupling to a compressed air outlet, and a closed downstream end, said hollow body having a sidewall with a plurality of air exhaust openings arranged in the downstream part, characterized in that the hollow body contains at least one upstream buffer lining forming a longitudinal filter and at least one downstream filtering lining forming a sheath.

Thus, by providing at least one downstream lining functioning as radial filter, it has been observed that a device according to the invention allowed the noise generated to be attenuated more efficiently.

According to a first variant embodiment, the upstream lining is formed by rolling up on itself a filtering mesh formed of a suitable material.

According to a second variant embodiment, the upstream lining takes the form of a compartment containing lining elements formed from a loosely packed suitable material.

The materials used for the upstream lining could notably be based on stainless steel, synthetic materials or composites. Preferably, a knitted stainless steel cloth will be used, rolled up on itself or loosely packed, since this material is particularly satisfactorily resistant to the longitudinal compression that the flow of compressed air causes.

Advantageously, the downstream lining is disposed around an open mandrel designed to hold it. The open mandrel could, for example, be formed by a spring of round-section wire, thus allowing its resistance to the air intended to flow through the device according to the invention to be reduced.

Preferably, the downstream lining is formed by winding of at least one band of at least one suitable material around the mandrel.

According to one preferred embodiment, the downstream lining is composed of at least one sheath formed of a suitable filtering material surrounded by a retaining jacket under tension. Advantageously, the filtering sheath and the retaining jacket under tension are formed from bands of the same width wound one over the other around the mandrel. Again advantageously, the band designed to form the retaining jacket under tension has a greater length than the filtering band. Thus, the band designed to form the retaining jacket under tension is wound once around the mandrel before winding it with the filtering band.

Preferably, the retaining jacket under tension is made of stainless steel cloth.

In an advantageous manner, the downstream lining comprises at least one composite material. Preferably, the composite material is a non-woven fiber glass material. This material is particularly recognized for its acoustic insulation properties.

Advantageously, a multi-perforated washer is disposed at the inlet of the upstream lining. Thus, the flow of air passing through the device is more efficiently distributed over the whole cross-section of the upstream lining.

In an even more advantageous manner, a centrally-perforated washer is disposed between the upstream lining and the downstream lining. Thus, the flow of air is channeled at the outlet of the upstream lining and centered on the axis of the tube of the downstream lining.

It should be noted that the present invention is not limited to a device with two linings and that it is perfectly possible to dispose several linings forming a buffer and/or several linings forming a sheath.

BRIEF DESCRIPTION OF THE DRAWINGS

The implementation of the invention will be better understood with the aid of the detailed description which is presented hereinbelow with reference to the appended drawings, in which:

FIG. 1 is a schematic longitudinal cross-sectional representation of an acoustic device according to the invention.

FIG. 2 is a cross-sectional view along the line II-II in FIG. 1.

FIG. 3 is a cross-sectional view along the line III-III in FIG. 1.

FIG. 4 is a cross-sectional view along the line IV-IV in FIG. 1.

FIG. 5 is a cross-sectional view along the line V-V in FIG. 1.

FIG. 6 is a representation of the disposition of two layers of materials before winding around a mandrel.

DETAILED DESCRIPTION OF THE INVENTION

An acoustic attenuation device, such as is shown in cross section in FIG. 1, comprises an elongated hollow body 1 formed from a cylindrical tube 2, a coupling head 4 and a base 5.

The tube 2 has air exhaust openings 6 arranged radially in a sidewall 9 of the downstream part. Furthermore, the tube 2 is preferably made of a material that is light but capable of resisting high pressures. This material could, for example, be a composite material molded under vacuum then fired. Advantageously, the sidewall 9 of the tube 2 comprises on its upstream part an anti-slip coating so as to facilitate handling and maneuvering of the device.

The coupling head 4 is designed to allow the connection of the device onto a compressed air outlet (not shown) of a conduit of a braking circuit of a train, and has corresponding standard dimensions. Furthermore, the coupling head 4 comprises a fixing lug 10 allowing the device to be suspended from a belt for example. The coupling head 4, generally made from gray cast steel, could advantageously be made from a lighter material, such as cast aluminum. The coupling head 4 is rigidly fixed, for example by bonding, into the upstream end 7 of the tube 2.

With the coupling head 4 thus fixed, the lining of the tube 2 is installed. The latter firstly comprises a multi-perforated washer 11 having a diameter substantially equal to the diameter of the tube 2, such as is shown in FIG. 2, pushing up against the coupling head 4 and against which an upstream lining 12 forming a buffer presses, as shown in FIG. 3. This upstream lining 12 is formed, for example, from a knitted stainless steel cloth rolled up on itself.

It should be noted that other materials may be used for the upstream lining 12, for example, synthetic materials, composites or even natural materials. This upstream lining 12 may furthermore be formed using materials taken in the form of a filtering mesh or loosely packed in a compartment, one wall of which may be formed by the multi-perforated washer 11.

The upstream lining 12 is closed by a centrally-perforated washer 13 having a diameter substantially equal to the diameter of the tube 2, such as is shown in FIG. 4, and pushing against the upstream lining 12. This centrally-perforated washer 13 is followed by a filtering downstream lining 14 forming a sheath, shown in FIG. 5. It should be noted that the centrally-perforated washer 13 could form a second wall of the compartment in the case where the upstream lining 12 is formed from loosely packed elements.

The downstream lining 14 is disposed around an open mandrel 15 formed by a spring of round-section wire and designed to be used as a support for the materials forming the downstream lining 14. The use of a round-section wire spring allows a minimal resistance to be offered to the air passing through it. More precisely, the downstream lining 14 comprises, on the one hand, a retaining jacket under tension formed from a band of stainless steel cloth 16 and, on the other hand, a band of composite material 17, for example non-woven fiber glass. The band of stainless steel cloth 16 and the band of composite material 17 are wound together around the open mandrel 15. Such a winding process results in alternating layers of stainless steel cloth 16 and of composite material 17. In the case where the composite material 17 is a non-woven fiber glass for example, the fibers are thus held by the stainless steel cloth 16 and the composite material 17 will not be degraded by the effect of the passage of compressed air. Each layer of stainless steel cloth 16 thus forms a pressure-retaining sheath surrounding a layer of filtering material.

The band of stainless steel cloth 16 and the band of composite material 17 have identical widths and are wound together around the mandrel 15. However, the band of stainless steel cloth 16 is longer than the band of composite material 17, the difference in length corresponding to a first turn of the band of stainless steel cloth 16 around the mandrel 15. The disposition of the two bands before winding is shown in FIG. 6.

It should be noted that the exhaust openings 6 in the downstream part of the tube 2 are distributed along the downstream lining 14. More precisely, the tube 2 has three circular regions each comprising four exhaust openings 6 uniformly distributed around the circumference of the tube 2.

Once the lining has thus been disposed inside the tube 2, the base 5 is then thermo-bonded to the tube 2 at a downstream end 8 so as to close off said downstream end 8. The base 5 will advantageously be made of cast aluminum owing to the pressure forces to which it will be subjected. It should however be noted that the tube 2 and the base 5 may be formed as a single component, the lining then being introduced into the tube 2 via the upstream end that is subsequently closed off by the coupling head 4.

An operator performing a check of the train braking circuit will proceed in the following manner.

First of all, the operator connects the device to the air outlet that he wishes to monitor by means of the coupling head 4.

The stop-valve is then opened and the purging of the pipe started. The compressed air flows through the coupling head 4 then enters at high speed into the whole assembly of the hollow body 1.

This air firstly flows through the multi-perforated washer 11 which divides it into a plurality of flow streams distributed over the whole cross section of the tube 2 before penetrating into the inside of the upstream lining 12 which it crosses longitudinally. At the exit of the upstream lining 12, the air flow passes through the centrally-perforated washer 13 which channels this air flow inside the sheath formed by the downstream lining 14. Finally, the air flows radially through the open mandrel 15 and the downstream lining 14 before exiting from the device via the exhaust openings 6.

Thus, by flowing through the suitably-designed linings, the latter break up the regions of high and low pressure of the propagating air, which reduces the noise generated.

Experiments have demonstrated that the 120 dB produced by a depressurization without acoustic attenuation device of a compressed air pipe under 5 bar fall to 74 dB by using the attenuation device according to the invention. This represents a gain of 3 dB, or a sound intensity reduced by a factor of two with respect to the device known from the prior art, and adheres to the minimum audibility threshold of 70 dB fixed by the standard.

It should also be understood that the invention is not limited to a single lining forming a buffer and that it is of course possible to provide a plurality of stages of each type, buffer or sheath.

Although the invention has been described in association with particular exemplary embodiments, it will be clearly understood that it is not in any way limited and that it encompasses all the technical equivalents of the means described, including their combinations if these fall within the scope of the invention.

Claims

1. An acoustic attenuation device comprising an elongated hollow body having an open upstream end taking the form of a head for coupling to a compressed air outlet, and a closed downstream end, said hollow body having a sidewall with a plurality of air exhaust openings arranged in the downstream part, wherein the hollow body contains at least one upstream buffer lining forming a longitudinal filter and at least one downstream filtering lining forming a sheath.

2. The device as claimed in claim 1, wherein the upstream lining is formed by rolling up on itself a filtering mesh formed of a suitable material.

3. The device as claimed in claim 1, wherein the upstream lining takes the form of a compartment containing lining elements formed from a loosely packed suitable material.

4. The device as claimed in claim 1, wherein the downstream lining is disposed around an open mandrel designed to hold it.

5. The device as claimed in claim 4, wherein the downstream lining is formed by winding of at least one band of at least one suitable material (around the mandrel.

6. The device as claimed in claim 4, wherein the downstream lining is composed of at least one sheath formed of a suitable filtering material surrounded by a retaining jacket under tension.

7. The device as claimed in claim 6, wherein the filtering sheath and the retaining jacket under tension are formed from bands of the same width wound one over the other around the mandrel.

8. The device as claimed in claim 7, wherein the band designed to form the retaining jacket under tension has a greater length than the filtering band.

9. The device as claimed in claim 6, characterized in that the retaining jacket under tension is made of stainless steel cloth.

10. The device as claimed in claim 1, wherein the downstream lining comprises at least one composite material.

11. The device as claimed in claim 10, wherein the composite material is a non-woven fiber glass material.

12. The device as claimed in claim 1, wherein a multi-perforated washer is disposed at an inlet of the upstream lining.

13. The device as claimed in claim 1, wherein a centrally-perforated washer is disposed between the upstream lining and the downstream lining.