AU673598B2 - Acoustic energy attenuator - Google Patents

Description

Regulation 3.2

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

a a a Name of Applicant: WALKER AUSTRALIA PTY LTD Actual Inventor: JOHN SCHNEIDER Address for Service: R K MADDERN ASSOCIATES, 345 King William Street, Adelaide, South Australia, Australia Invention title:. ACOUSTIC ENERGY ATTENUATOR Details of Associated Provisional Application No: PL 9205 dated 7th June 1993 The following statement is a full description of this invention, including the best method of performing it known to us.

I

This invention relates generally to the attenuation of acoustic energy and in particular to an arrangement of acoustic energy conduits in a cavity to define a Helmholtz resonator, wherein the specific arrangement of conduits determines the quantity and relatively narrow frequency band of acoustic energy attenuated from a relatively broad acoustic energy spectrum passing into the arrangement.

BACKGROUND

Helmholtz attenuators are commonly used in gas flow arrangements to attenuate a selected band of acoustic energy.

The fields of application are many and varied and may make up a part of internal combustion (IC) engine exhaust systems, gas turbine engine exhaust systems, conditioned air reticulation systems, sound absorbing building constructions, etc.

Of particular relevance to the inventors, is the field of IC engine exhaust systems and within this field a particular arrangement known as side branch tuner which uses a Helmholtz attenuator within the muffler of an IC powered vehicle to attenuate unwanted acoustic energy. Typically, as depicted in Fig. 1 the muffler is constructed having a sealed cavity and an exhaust conduit passing through the cavity. Inside the cavity, and between the ends of the cavity is a further conduit branching off the side of the exhaust conduit which communicates the passing exhaust gas into the cavity. The length and cross-sectional area of the side-branch conduit and the volume of the cavity along with gas temperature, determines the degree of attenuation of a relatively narrow frequency band of exhaust gas acoustic energy passing through the muffler.

Exhausts using this particular arrangement are labour intensive to manufacture, involving at least one hole punching or nibbling process, a deburring process, a relatively complex forming process on one end of the short branch conduit, a deburring or flaring process, a fitting process, followed by a fixing process using a variety of known techniques which may included spot or fillet welds, riveting or friction engagement.

A further example of a Helmholtz attenuator arrangement includes the muffler configuration depicted in Fig. 2, which shows a sealed cavity having an exhaust conduit passing therethrough, wherein, a plurality of apertures is located in the conduit which is surrounded by a further conduit sealed at one end to the exterior of the exhaust conduit and having an open end communicating exhaust gases from the exhaust conduit into the cavity.

1 Not unlike the former muffler, this arrangement involves many manufacturing processes and is labour intensive to build.

Yet a further example of a Helmholtz attenuator arrangement includes the muffler depicted in Fig. 3, which shows, a housing having an internal wall forming two cavities within the housing and an exhaust conduit passing through both cavities within the confines of the housing. A portion of the exhaust conduit is perforated to communicate exhaust gases into a first cavity and further, a tuning conduit, which extends into the second cavity from an aperture in the ~internal wall, to communicate exhaust gases and form the attenuation cavity for a predetermined acoustic energy band dependent on the length and cross-sectional area of the further conduit and the volume of the second cavity.

The former muffler has many parts, involves many separate manufacturing processes and exhibits poor attenuation characteristics.

This invention relates in one embodiment to an exhaust arrangement which will reduce the number and complexity of parts and processes required to manufacture an exhaust acoustic energy attenuator of the type described in the embodiment of this invention.

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It is further apparent that the acoustic energy attenuator arrangement of the invention may be useful in other fields since the principles employed are equally applicable.

BRIEF DESCRIPTION OF THE INVENTION In a broad aspect of the invention, said acoustic energy attenuator comprises a sealed chamber having a predetermined volume, a gas flow inlet conduit a portion of which lies within said chamber and terminating within said chamber at an open end, a gas flow outlet conduit a portion of which lies within said chamber and terminating at an open end within said chamber, S said outlet open end being located a predetermined length within or overlapping said inlet conduit and said inlet conduit being spaced from said outlet conduit such that a predetermined radial cross-sectional area exists therebetween 24:' to form a communication path from said gas flow inlet conduit to said sealed chamber, such that when gas and acoustic energy flow into said inlet conduit and exit said outlet conduit a portion of said acoustic energy is attenuated.

2 In a further aspect of the invention, said inlet conduit is expanded at least said predetermined length along said inlet conduit from said inlet conduit open end and thereby spaced from said inlet conduit to form said predetermined radial cross-sectional area between said inlet and outlet conduits.

In a further aspect of the invention, said inlet conduit is expanded about one portion of its outer form so as to be spaced from said outlet conduit to form said predetermined radial cross-sectional area.

In a further aspect of the invention said inlet conduit is expanded about two opposed portions of its outer form so as -4to be spaced from said outlet conduit to form said predetermined radial cross-sectional area.

In yet a further aspect of the invention said inlet and outlet conduits are co-axial.

In yet a further aspect of the invention said inlet and outlet conduits are pipes.

In a further aspect of the invention said inlet and outlet conduits abut about at least a portion of their respective inner and outer surfaces along said predetermined length.

Ve. S In a further aspect of the invention said inlet and outlet conduits are fixed together.

*.oS In yet a further aspect of the invention said sealed chamber is a muffler or a portion thereof and said inlet conduit receives exhaust gases and acoustic energy from an internal combustion engine.

BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention will now be described by way of example only using a muffler as illustrated in the accompanying drawings in which: Fig. 1 depicts a prior art side-branch resonator muffler; Fig. 2 depicts a second prior art side-branch resonator muffler; Fig. 3 depicts a third prior art side-branch resonator muffler; Fig. 4 is a schematic representation of a generic acoustic energy attenuator according to the invention; Fig. 5 depicts a breakaway perspective view of a co-axial tube Helmholtz resonator muffler according to the invention; Fig. 6 depicts the cross-section A-A of Fig. Fig. 7 depicts a further cross-sectional shape of a co-axial tube Helmholtz resonator muffler according to the invention; Fig. 8 depicts a breakaway perspective view of an alternative multi-cavity muffler according to the invention; Fig. 9 depicts the cross section B-B of Fig. 8; Fig. 10 depicts a schematic of a parallel acoustic energy attenuator arrangement in a common volume; and Fig. 11 depicts a schematic of a series acoustic energy attenuator arrangement in a common volume.

DETAILED DESCRIPTION OF THE INVENTION The muffler depicted in Fig. 5 is a specific example of the generic application of an acoustic energy attenuator as depicted schematically in Fig. 4. As distinct from prior art construction of side-branch tuners, the exhaust inlet conduit carrying exhaust gas and acoustic energy has been modified to accommodate the exhaust outlet conduit and to form thereby a tuning conduit of the required length and cross-sectional area, which opens into a housing of a required volume.

Q Fig. 4 depicts a schematic representation of a generic acoustic energy attenuator comprising in their broadest sense a housing 10 having a volume 28 and an inlet conduit 12 through which inlet gas and acoustic energy 14 flows. The inlet conduit may be of any desired cross-sectional shape however for the purposes of this description is circular.

A portion 12a of the inlet conduit 12 is located within the housing 10 and terminates at an inlet conduit end 18, which comprises, an expanded portion 12b of said inlet conduit along at least a length 26. Preferably an outlet conduit 16 is aligned co-axially with the inlet conduit 12. The outlet conduit 16 has an inner outlet conduit portion 16a within the housing 10. The inner outlet conduit portion is located within the inner inlet conduit portion 16 such that an overlap length 26 is created. It is possible for the inlet conduit to be located inside the outlet conduit however this arrangement is not depicted or used in the embodiments.

-6- The expanded portion of the inlet conduit 12b is spaced a maximum radial separation 20 such that the total area between the inlet and outlet conduit inner portions is a predetermined area. In another arrangement it is possible for the inner and outer conduits to overlap but not be fixed to each other or alternatively be spaced by a spacing element to maintain the required area The expanded portion of the inlet may, as will be depicted in the following embodiment, be distributed about the outer shape of the inlet conduit end 18 and may be achieved by means of more than one such expanded portion distributed about the inlet conduit end, 18. Figs. 6 and 7 depict the two of the large variety of shapes that could be used.

Furthermore, although preferable, the cross-sectional areas of the gas flow path through the inlet and outlet conduits are equal. However dependent on the type of attenuator being constructed the cross-sectional areas may be different.

A Helmholtz resonator is an acoustic resonator in which the mass reactance of a short column of air neutralises a predetermined relatively narrow band of frequencies. The reactance comprises in part the stiffness of the volume contained in the enclosure, which communicates with its surrounding environment solely through a short column.

The length of the overlap 26, the effective area 30 and the volume 28 of the arrangement determine the centre frequency of the relatively narrow band of frequencies to be neutralised/attenuated, therefore, the design parameters most easily varied comprise the length of the overlap and the effective area provided by the expanded portion of the inlet conduit.

One embodiment of the invention is depicted in Fig. comprising a muffler 10 having an exhaust inlet pipe 12 adapted to receive exhaust from an engine. The exhaust gases take a flow path 14 through the exhaust inlet pipe 12 and -7exits the muffler 10 via an exhaust outlet pipe 16 shown as an exhaust flow path 38.

The inlet pipe is preferably a welded steel pipe adapted to be fitted to the manifold of an internal combustion engine not shown. The outlet pipe is also preferably welded steel pipe adapted to be fitted to an exhaust pipe typically leading to the rear of the vehicle. The materials of the elements of the muffler are used aj an example however any suitable material will suffice.

An outer casing 20 of the muffler is typically oval or oblate in cross-section comprising a steel sheet rolled and spot welded or lock seamed. The muffler 10 further comprises an inlet bulkhead 23 sometimes referred to as an end plate and an outlet bulkhead 25 further sometimes referred to as an end S plate adapted to seal the outer casing of the muffler to create a cavity 28 of fixed volume. One or both end plates are not sealed to the outer casing until the pipes 12 and 16 ~are fixed together.

A variety of muffler designs exist whereby a plurality of cavities is required within the outer casing of the muffler, in which case partitions are located along the length of the muffler, forming fixed volume sub-cavities. A schematic of an alternative arrangement is depicted in Fig. 8. Multiple cavity mufflers are sometimes used in conjunction with other acoustic attenuating arrangements (expansion cavities) to further attenuate specific low and high pitched portions of the acoustic energy spectrum emitted from the manifold of the internal combustion engine to which it is attached.

In the embodiment depicted in Fig. 5, the exhaust outlet pipe 16 is inserted into and overlapped by the exhaust inlet pipe 12 a tuning length 26 and the exhaust inlet pipe is adapted by expansion deformation to create a cross-sectional area between the inlet and outlet pipes. This cross-sectional area is provided at least continuously from the exhaust flow path 14 inside the inlet pipe 12 from the open end of the -8exhaust outlet pipe to the open end of the exhaust inlet pipe. Path 40 depicts the path of gases and acoustic energy from the inlet pipe into the fixed volume cavity 28. The arrangement described is easy to manufacture and fitting is particularly simple.

In this embodiment the cross-sectional area 34 of inlet pipe 12 is larger than the cross-sectional area 36 of the outlet pipe 16, however, these areas may be the same or vary according to other design criteria. Furthermore in this embodiment, both the exhaust inlet and outlet pipes have coincident axes. It is possible, however, for these pipes to :8 *to have non-aligning longitudinal axes.

In this embodiment the relative positions of the pipes should be maintained so as to maintain the cross-sectional area Otto This is likely to be difficult, especially when considering the rugged environment in which the product is used. It is therefore important to construct this portion of the muffler using reliable fixing techniques which are easily applied to the two overlapping pipes.

It will be apparent to those skilled in the art that the Otto So.. expanded portion of the inlet pipe 12b may be created in a number of ways. An expansion process needs to be of a shape and position which maintains the required cross-sectional se ~area In one example of a manufacturing process suitable to create the expansion deformation, a mandrel may be forr-d into the end of the exhaust inlet pipe 12 having a shape over the required length of the intended overlap of the inlet and outlet pipes so as to deform the pipe and create the necessary cross-sectional tuning area A further manufacturing process may comprise the use of expanding fingers arranged segmentally about a tapered punch device which when operated expansively, deforms the required length of the end of the exhaust inlet pipe 12 either eccentrically as shown in Fig. 6 or evenly on opposed surfaces of the end of the exhaust inlet pipe as depicted in Fig. 7. Like numerals are used on like portions of Figs. 6 and 7.

Fig. 6 depicts a section A-A of the inlet and outlet pipes in the vicinity of the expanded portion 12b showing that the outer surface 42 of the exhaust outlet pipe 16 abuts the inner surface 44 of the inlet pipe 12 and allows fixing means 46 as depicted in Fig. 5 to maintain the spatial relationship between both the inlet and outlet pipes. Fixing means may comprise fillet or spot welding which may be achieved by hand or machine and/or suitable robust rivets (not shown) may be used. The manufacturing processes and others are simple, inexpensive and sufficiently robust to maintain the arrangement.

0ooo Fig. 7 depicts a cross-section of an alternative expanded portion of the inlet pipe having two expanded portions on the inlet pipe. The cross-sectional areas 48 and 50 combined provide the required tuning cross-sectional area 30. Such an arrangement allows for fixing means such as for example welding to be used along the abutted portions of the outlet 40*.

and inlet pipes while providing sufficient robustness to the finished product.

~Fig. 8 depicts a cross-section of a further alternative embodiment of a vehicle muffler which is characterised by the use of three sealed chambers, two of which contain the acoustic energy attenuator arrangement of the invention.

A muffler 10 preferably comprises an outer casing 20 which is closed off at a first end by an inlet bulkhead 23 and also closed off at a second end by an outlet bulkhead Intermediate these bulkheads are located a first baffle plate 52 and a second baffle plate 54 to form three sealed chambers 56,58 and A gas flow inlet conduit, in this embodiment a pipe 62 is located in an aperture in the inlet bulkhead 23 and extends in to the first chamber 56. In this embodiment the pipe 62 has a plurality of apertures 63 distributed over its surface to form a broad spectrum acoustic energy attenuator of known configuration. This particular arrangemen. operates independently of the further sealed chambers of the muffler and is described for completeness only. Pipe 62 terminates at an open end at a location which may be internal of said first chamber 56 but which is located internal of a gas flow pipe 64 which in this embodiment is located partially in chamber 56 and partially in chamber 58 by extending through an aperture in the first baffle plate 52. The pipe 64 is expanded about a portion of its open end 66 in a similar manner to that depicted in Fig. 7 allowing the open end of pipe 68 to be fixed therein by means previously described.

eeoo The Helmholtz formulae are used to determine the length of the overlap of the pipes 64 and 68, the cross-sectional area between the inner and outer pipe surfaces along the overlap and the volume of the chamber 58 using the centre frequency of the band that is desirably attenuated by this type of arrangement.

0555 However, it is not difficult to experiment with the arrangement until the desirable frequency band is attenuated by the maximum or acceptable amount.

The pipe 68 extends from inside the pipe 64 through an aperture in the baffle plate 54 and into the third chamber of the muffler terminating at an open end 70. The pipe 68 is expanded about a portion of its open end 70 in a similar manner to that depicted in Fig. 7 allowing the open end of pipe 72 to be fixed therein by means previously described.

The Helmholtz formulae are used to determine the length of the overlap of the pipes 68 and 70, the cross-sectional area between the inner and outer pipe surfaces along the overlap and the volume of the chamber 60 using the centre frequency -11of the band that is desirably attenuated by this type of arrangement.

As stated previously, it is not difficult to experiment with the arrangement until the desirable frequency band is attenuated by the maximum or acceptable amount.

The pipe 72 exits the chamber 60 through the outlet bulkhead and terminates at an open end which is external of the muffler Fig. 9 depicts a end cross-sectional view of the muffler along the lines B-B of Fig. 7 showing the position of the muffler housing relative to the pipes and the expanded o I' ~portions of the pipes.

Fig. 10 depicts a schematic of the use of parallel acoustic energy attenuator arrangements of the invention in a common sealed volume.

2.

n Fig. 11 depicts a schematic of the use of acoustic energy attenuator arrangements of the invention in series located in a common sealed volume.

r ogo A muffler arrangement of the type described in the embodiments above have a low number of manufacturing processes and may be made with a minimum of manual labour content.

Furthermore, it is found that such an arrangement has sufficient robustness to meet applicable standards and the requirements of vehicle manufacturers.

It will be appreciated that in this embodiment, the effective amount of attenuation will be dependent on the characteristics of the amount of overlap length, crosssectional area and the volume of the cavity. However, further characteristics such as the speed of sound which is proportional to the temperature are considered when designing -12such an arrangement, as are the further considerations including the number of cylinders used in the internal combustion engine, the vehicle geometry and the lengths of manifold pipes connecting the muffler to the exhaust system of the internal combustion engine. These further characteristics may be used to different degrees to determine the final dimensions of a muffler embodiment of the invention.

It will be further appreciated that an arrangement of substantially co-axial pipes, arranged in the manner disclosed may be usefully applied to the design of a narrow band acoustic energy attenuator, in one or more of the arts of gas turbine engines, conditioned air articulation systems o and sound absorbing building constructions and many other fields.

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Claims (8)

1. 4. An acoustic energy attenuator according to claim 1 wherein said inlet conduit is expanded along said inlet conduit from said inlet conduit open end at least said predetermined length and thereby spaced from said ouk\eL conduit to form said predetermined radial cross-sectional area between said inlet and outlet conduits. An acoustic energy attenuator according to claim 1 wherein said inlet and outlet conduits are coaxial. a: 6. An acoustic energy attenuator according to claim 1 0 wherein said inlet and outlet conduits are pipes. eoo 7 An acoustic energy attenuator according to claim 1 ••o wherein said inlet and outlet conduits abut about at least a portion of their respective inner and outer surfaces along O. said predetermined length. oaoe
2. 8. An acoustic energy attenuator according to claim 1 wherein said inlet and outlet conduits are fixed together. •too eeoc noee
3. 9. An acoustic energy attenuator according to claim 1 wherein said sealed chamber is a muffler and said inlet conduit receives exhaust gases and acoustic energy from an internal combustion engine.
4. 10. An acoustic energy attenuator comprising at least two sealed chambers within a housing of a vehicle muffler each said chamber having a predetermined volume, a gas flow inlet conduit arranged to enter into a first sealed chamber and terminating in an open end within said first sealed chamber, a first gas flow outlet conduit a portion of which lies within said first sealed chamber and terminating in an open end within said chamber, said outlet open end being located a predetermined length within or overlapping said inlet conduit and said inlet conduit being spaced from said outlet conduit such that a predetermined radial cross-sectional area exists therebetween to form a communication path from said gas flow inlet conduit to said first sealed chamber, such that when gas and acoustic energy flow into said inlet conduit and exit said first gas flow outlet conduit a portion of said acoustic energy is attenuated, a portion of said first gas flow outlet conduit passing into said second sealed chamber and terminating within said second sealed chamber at an open end, a second gas flow outlet conduit a portion of which lies within said second sealed chamber and terminating in an open *8*8 end within said second sealed chamber, said second gas flow oo outlet open end being located a predetermined length within or overlapping said first gas flow outlet conduit and said 2 first and second gas flow conduits being spaced from each oae0 other such that a predetermined radial cross-sectional area exists therebetween to form a communication path from said first gas flow outlet conduit to said second sealed chamber and said second gas flow outlet conduit terminating at an open end external of said second chamber, such that when gas and acoustic energy flow into said second gas flow outlet conduit and exit said external end of said gas flow outlet conduit a portion of said acoustic energy is attenuated.
5. 11. An acoustic energy attenuator comprising at least two sealed chambers within a housing of a vehicle muffler each said chamber having a predetermined volume, a gas flow inlet conduit arranged to enter into a first sealed chamber having a plurality of apertures to form a communication path from said gas flow inlet conduit to said first chamber, a portion of said gas flow inlet conduit -16- passing into a second sealed chamber and terminating within said second sealed chamber at an open end, a gas flow outlet conduit a portion of which lies within said second sealed chamber and terminating in an open end within said second sealed chamber, said outlet open end being located a predetermined length within said inlet conduit and said inlet conduit being spaced from said outlet conduit such that a predetermined radial cross-sectional area exists therebetween to form a communication path from said gas flow inlet conduit to said second sealed chamber and said gas flow outlet conduit terminating at an open end external of said second chamber, such that when gas and acoustic energy flow into said inlet conduit and exit said external end of said outlet conduit a portion of said acoustic energy is S attenuated. *0*
6. 12. An acoustic energy attenuator according to claim 11 further comprising a third sealed chamber located adjacent said second sealed chamber having a portion of said first gas flow outlet conduit exit from said second sealed chamber and ee terminating at an open end within said third sealed chamber, 0. a second gas flow outlet conduit a portion of which lies within said third sealed chamber and terminating at an open end within said third sealed chamber and a portion of which terminating at an open end external of said third chamber and the housing of said vehicle muffler housing, said second outlet open end in said third sealed chamber being located a predetermined length within said first outlet conduit and said first outlet conduit being spaced from said second outlet conduit such that a predetermined radial cross- sectional area exists therebetween to form a communication path from said gas flow first outlet conduit to said third sealed chamber, such that when gas and acoustic energy flow into said first outlet conduit and exit said second outlet conduit a portion of said acoustic energy is attenuated. -17-
7. 13. An acoustic energy attenuator according to claim 1 wherein there are more than one gas flow inlet conduits and more than one gas flow outlet conduits arranged in parallel to form more than one gas flow path into said and out of said sealed chamber.
8. 14. An acoustic energy attenuator comprising a sealed chamber having a predetermined volume, a first gas flow inlet conduit a portion of which lies within said chamber and terminating within said chamber at an open end, 0S e• a first gas flow outlet conduit a portion of which lies ee. within said chamber and terminating at an open end within 0*00 said chamber, said first gas flow outlet open end being 06.0 located a predetermined length within or overlapping said first gas flow inlet conduit and said first gas flow inlet Q. conduit being spaced from said first gas flow outlet conduit 0C60 such that a predetermined radial cross-sectional area exists therebetween to form a communication path from said first gas flow inlet conduit to said sealed chamber, such that when gas O06O and acoustic energy flow into said inlet conduit and exit *040 said outlet conduit a portion of said acoustic energy is attenuated, 0 a second gas flow inlet conduit a portion of which lies within said chamber and terminating within said chamber at an open end, a second gas flow outlet conduit a portion of which lies within said chamber and terminating at an open end within said chamber, said second gas flow outlet open end being 33 located a predetermined length within or overlapping said second gas flow inlet conduit and said second gas flow inlet conduit being spaced from said second gas flow outlet conduit such that a predetermined radial cross-sectional area exists therebetween to form a communication path from said second -18- gas flow inlet conduit to said sealed chamber, such that when gas and acoustic energy flow into said inlet conduit and exit said outlet conduit a portion of said acoustic energy is attenuated. An acoustic energy attenuator comprising a sealed chamber having a predetermined volume, at least one gas flow inlet conduit a portion of which lies within said chamber and terminating within said chamber at an open end, .ie at least one gas flow outlet conduit a portion of which lies within said chamber and terminating at an open end within said chamber and also terminating at its other end external of said chamber, said at least one internal outlet open end being located a predetermined length within or overlapping said at least one respective inlet conduit and said respective conduits being spaced from each ct;. r such that a predetermined radial cross-sectional ar:l ts therebetween to form a communication path from said at least one gas flow inlet conduit to said sealed chamber, such that when gas and acoustic energy flow into said at least one inlet conduit and exit said at least one outlet conduit a portion of said acoustic energy is attenuated. Dated this 7th day of June 1994 WALKER AUSTRALIA PTY LTD By their Patent Attorneys R K MADDERN ASSOCIATES C f-J. -19- ABSTRACT This invention describes an apparatus to attenuate acoustic energy and in particular to an arrangement of acoustic energy conduitQ (12,16) in a cavity (28) to define a Helmholtz resonator, wherein the specific arrangement of conduits determines the quantity and frequency band of acoustic energy attenuated from a relatively broad acoustic energy spectrum passing into the arrangement. In the embodiment described input pipe (12) overlaps outlet pipe (16) for a predetermined length (26) and the arrangement of the overlap provides a predetermined cross-sectional area through which gases and acoustic energy can communicate from oo° the inlet pi'pe into a fixed and sealed volume. The pipes are fixed relative to each other and provide a simple and reliable means for attenuating exhaust acoustic energy when °o *oe used in a motor exhaust system. However, the arrangement may be usefully adapted to other applications. eoeo