CA2116695A1

CA2116695A1 - Muffler

Info

Publication number: CA2116695A1
Application number: CA 2116695
Authority: CA
Inventors: Owen Jones; Michael C. J. Trinder
Original assignee: Individual
Current assignee: Noise Cancellation Technologies Inc
Priority date: 1991-08-29
Filing date: 1992-08-28
Publication date: 1993-03-18
Also published as: JPH07500646A; WO1993005282A1; KR100272313B1; AU2489192A; EP0602091A1; GB9118779D0; AU668100B2

Abstract

A muffler for muffling noise in a fluid flow, such as exhaust gases from an internal combustion engine comprises a chamber (10), loudspeakers (7) for injecting anti-noise into the chamber under the control of processing circuitry (12) in dependence upon residual noise detected by a microphone (11). The gas flows from an inflow pipe (5) to an outflow pipe (3) past a radial aperture (13) that produces an acoustic coupling between the chamber (10) and the gas flow, to allow the anti-noise to mix with noise in the gas flow and produce noise cancellation. The coupling is so arranged that the chamber (10) becomes pressurised by the gas flow. in use. so as to restrict hot gas flow through the chamber and thereby provide an essentially benign environment for the loudspeaker.

Description

~ 093/0~2X2 PCT/GB92/0155~
~ii6b95 HUE~

FI~ OP q~ lNY~.-iclN
The present im erltion relates to a muffler for ~uffling noise in a fll-~ia flow, for eYA~rle the ~YhAllct flow ~.-nm of an int~prnAl combustion engin~.

Most noise from an intern~l combu_tion engine propagates from thE Pngins-s eYhA~ct ports and has coG~only been su~essed ~T ~eans of mufflers, also known as silP~cP- , located in an PYhAllCt pipe co---.o~-Led;to the ports. Such ~lfflPrs are entirely passive devices.

It has been ~ ~o~ed to ~e active noise col.~.ol techniques to conL~ol the noise prop~gating along the PYhAll~t pipe. In one such system, the PYh~llqt pipe is CAllCe~ to~ pass th~ougl- a chamber, ~n which a U~ eAl~Pr is l~~t~ ~eA . ~he 1 r~..rlc~ r iS used to ~,u~_a anti-noise in the ch~mh~r, for cAn~Dlling the ~st ~ ise. It has been found that in order for W093/05282 PCT/~.B92/0159~

2~,66g5 there to be sllffiriont acoustic coupling between the 1OUdSPeA~er and the inside of the ~YhA~ct pipe, the portion of the oYh-tllCt pipe passing throush the chamber sho~ be perforated. However, a problem with this S ~rr~n~ement is that ~yhA~~ct gases pass out through the perforations located tow~rds the chAmber inlet. This ~8cAt~ of ~YhA~t g ces results in a pressure differential along the length of the peLfoL~ted portion of the pipe. As the pressure in the perforated portion of the pipe is lower towards the chamber outlet th n towards the chAmh~r inlet, ~yhAtlet gaçes in the ch~Pr ~re ~r_wn back into the ~YhA--~t pipe thLou9ll the tions loc~ted towards the chamber outlet. Thus, there is a constant flow of hot eYhA~et gases through the chamber ~llri~g ope~aLion. These hot ~Yh~st gases are deleterious ~o the loudsp~A~or m~unted in the rh-~r .

In an alternative arrangement, an inflow pipe pAsses completely th~o~gh the cha~ber and te~minates w~thin an o~f'ow pipe. ~ v~r, it has been found that when such an A~ Jement is u~ed in a virtual earth noise cAnr~llAtion system, ~he positio~ing of the system I

~ W0 93/05282 PCTIGB92/0159~
21,1~6~5 microphone is e~L ~ ly critical. Even with the mielo~o..e optimally plAre~ it is still not possible to achieve effective cancellation over a wide range of frequencies.
s It is an ob~ect of the present in~ention to overcome the aforementionQ~ disadvA~tAges of the prior art and provide a muffler which enables satisfactory acoustic co~pli~g to the int~riQr of an eYhA~ct pipe while providing a h~nign environment for a lGI~A.peA~er.

~rcQr~;nq to the ~as~L invention there is provided a muffler fcr ~lffling nmise in a fluid flow, comprising a ch~ r, means for in~ecting anti-~ ise into the ch~ber, a co~lit for co..veying a fluid stream, means for ~eeiving fluid flowing from the cnn~it, and co~rtin~ means in~ ng a rA~iAl ~e,Lule ~eL~e.~ the c~ t and the receiving means, for acoustic~ly 20 cO~rl in~ the ch~h~ to the fluid stream, wherein the c~rlin~ meAnC is A ~ such that the chamber become~ ~essurised in use, such as to restrict flow of fluid from the fluid stream t~ou~h the chamber.

-W093/0s282 PCT/GB92/0159~
o p~66~5 Preferably, the muffler .s arranged to act as an acoustical filter.

Co..v-ni~ntly, the cQn~-lit passes through the ch~mh~r, S t~rrunating ~ust short of an op~nin~- n a wall of the c Amh~r~ in which case the con~nit terminstion and the a~e~Lule can be coAYiAlly Ali~nP~ and the acoustic coupling means comprises the gap between the con~ t tP~minAtion and the opnin~. Alternatively, the cQn~nit may run along the si.de of a chamhPr, the acoustic CO~-rlin~ comprising a lot co~rling the int~rinr of the ch~ber with the gas flow in the c~n~it. The c~rling means may al~o include ~ri~in~ members for phy~ir~l ly ~o~rl i~g the cQn~"it to the receiving AdvantAgeoncly, the internAl diameter of the cQn~nit is le~s than t~he internal i~m~ter of the receiving -- ~ears. Eo~ewer, under certain circumstances, thia may not be nece~Ary to achieve satisfz_ Lu~ ~ pe~Lo mAnce by the muffler.

o 211669~

Preferably, the muffler will include a microphone.

Conveniently, the means for ~n~ecting anti-noise into the ch~Pr comprises a loudsp~A~Pr. A plurality of loudspeA~Pns may be employed.

Ad~AntAge~cly, the p_ssive frequency response of the muffler may be tAilQ~ed by resistively or capacitively 1OAA;nq the lo~d~eA~en. Also, the drive circuit for the lo~ yeA~er ma~- inrl~ an amplifipr circuit with a g~inJ~N~ur~c~ ,e~ ~e ~Qnfig~ed t~ drive the loud~oA~Pr so t~at its f~ue~y espQ~ce is ro~ifiP~
to have a desired rhA~Arteristic over a given frequency range.

~he muffler may be employed in an active noise cAn~ellAtinn syste~, for example in the ~YhAllct system of an int~rn~l combustiQn ~n~;n~.

ERI~Y ~ C~I OF TE~ DiuJnnYas Em}odime~ts of the p-r~-"t ~nvrnti~ Will nQW be sc- ihe~ by way of esample, with reference to the ~- accomç~nying drawings, in which:

--~093/0~282 PCT/GB92/~159~
.

Figure 1 is a side vieY of a muffler acco ling to the present invention; and Figure 2 ~s a diagra~atic representation of a muffler according to the pre~ent invention, employ~ed in an S active noise cAnrpll~tion s~stem.

PGf~rring to Figure 1 and 2, a muffler 1 comDrises a rect~ngular box 2 formed from sheet metal. An outflow pipe 3 P~t9n~c outvardly from a circular o~ening 4 located in a w~ll 2a of the b~x 2. An inflow pipe 5 e~tenAC t~ough a ~all 2b of the box 2, opposite the openinq 4, to a point ~ust short of and in axial alignment with the oF~nin~ 4. In a boY. 2 having a dimension A of 400 mm, the inflow pipe 5 terminates 2S
mm short of the a~LuLe 4. The space between the end of the inflo~ pipe and the op~ning 4 ~efinPc a radial ~6~Lu e 13 which in u~e ~.o~l ~es an acoustical r~nTling bet~een a h~mhnr 10 and gas flowing from-the inflow pipe 5 to the outflo~ pipe 3, as will be e-rlAin~d in ,more deta}l hereinafter.

WO 93/0~282 PC~/GB92/0159~
2I1 669~

A pair of partitions 8 and 9 run parallel to the inflow pipe 5 and divide the box into thr~e chambers.
c~tLol chA~er 10 is ~efin~ between the partitions 8 and 9. Two D ving coil loudspeakers 6 and 7 are S mo~nted in ~he partitions 8 and 9 respectively, such that they are ~irected towards the inflow pipe 5.

A mic~o~h~ 11 is mounted on the outflow pipe 3, ad~acent to where it joins the box 2. An electronic ~,~essing ~nit 12, such as ~icclose~ in our copc~ n~
U~ Patent ~FlicAtion No. gll6433.5 has an input which is elec~ricAlly c~lp1e~ to the mic~o~ho.~e 11 and o~L~u~s which are coupled to .e~e~tive sp4Ah~rs 6 and 7.
In o~e~aLion, a portion of a gas stre~m flowing along the infl~r pipe 5 will initially enter the o .)L.~l chamho~ 10. r7~ ._~, th~ c~-.l ~l chAmh~ 10 will ~ k~y become prPC~lri~e~ thus L~sL~icting the further iDsress of ~YhAllct gases- ~nC~~ Lly~ an ~Yh~ct gas flow path does not b~rome es~Ahlich~ through the chamh~r. Hence, the environment in the c~ l chi~nber U'O 93/05282 PCI/GB92/Cl~i9~
~66~5 10 1~ ~inc essen~ially benign for the spe~e~s 6 and 7 as they are not exposed directly to a hot gas flow.

Accompanying the gas stream are sound waves, comprisino r~gine noise, which propagate along the inflow pipe 5 into the c~h~mber 1 and out through the op~ni~g 4. The geometry of the muf~ler is designed so that higher fr~x~enri~c, in the order of a few hund~ed ~.ertz, are attenuated in the muffler.
Sound emerging f~ the ~lffl~r and propagating along the outflow pipe 3 is ~cence~ by the mic,o~-h~ 11 which sends an electrical signal e~e~nt~tsve of this sound to the input of the p,~cessing unit 12. The processing ~it 12 O~L~u~s electrical cAnrelling 5i~Als which dri~e the spe~rrs 6 and 7 to ~l~uce anti-noise. The anti-noise mises with the engine noise in the c~tY~l ch~h~r 10, which results in the sound propagating along the outflow pipe 3 having a much sm2ller amplitude than that propagating along the inflow pipe 5. Once the ool.L,~l loop inr~ in~ cessing unit 12 has come into op~,aLion, the mic,o~k~- ~ 11 s~nces the W093/05282 PC~GB9!/01~
~ 2~166g5 result of t~e ~iYi~5 of the ~ngine noise and the . anti-noise ~lt~ by the speA4Prs 6 and 7.

With the Arrangement described above, it has been found that a certlin de~ee of ~hAn~ement can occur at noise f~et~ es in the 10 to 50 hertz region. The passive ~lfo mAn~e of the muffler in this re~io~ can be mc~i~ie~ by re~ing the elec~ri~Al damping of the o--~sl~A~r. This may be a~hieved by either conu~Ling a lo~ value resistor, for example 5 ohms, or a rAr~it~nce, for ~mrle 1000 microfarrads, in pArallel ~ith ea-h l~creA~pr 6 and 7. The e~act value~ for these additiGnsl co~o.~.Ls will ~e~ l on the gesmetry of the muffler employed and the degree of mo~ifirAtion of the muffler response which is ~d.

The lo~ 7~e~or may be driven by circuits (within element 12) ;~rl~l~in~ _mpl ~fi~rs with fre~Pn~y/sai rh~r~teristics selected ~o modify the natural chAr~teristies of the lol~cp~A~Prc. For example, the ~o-~7~P~ers 6 and 7 may be made to AppeAr acoustirAlly 8s simple masses over a given fre~ n~y range. This is ~'093/0~282 PCT/GB92~01~9~
.

~66g~

achieved by arrA~qing for the circuits driving the loudspeakers 6 and 7, to have o~ L irrP~A~ces which are the cQmplex conjugates of the loudspeA~er i~o~ces over ~he given frequency range.

Whilst in the embc~ir~~t ~scrihe~ hereinhefore, ~ ving coil loudspeAkPrc have been employed, other forms of trAQC~l~c~ may usefully be employed such as piezo ' electric de~ices.

--Alt~.~ugh the Lnflow pipe 5 has been shown entering the chamk#~- 1 throu~h a wall opposite the ape~Lu~e 4, alternati~e a~.~.gements may be employed so long as the inflow pipe 5 terminates in alignment with an ad~acent to the opeoing' 4 so as to define a rA~iAl ape~LuLe such as the ap~LLuLe i~.

~, :
Since, ~i~ing of the en Lne noise and the anti-noise o~c~ within the c~ l chamber 10 the position of the mic~phs~e 11 is ~ t ~riticzl. CQ~C~Ue.~1Y, the mic~opbQ~ 11 may be rlp~e~ i~ any c~..v~ t position e.g. within the ce~L~l chambe.- 10 or at the ~ischArge end of the outflow pipe 3.

~ 93/05282 PCT/GB92/0159~
.

The passive attenuation of high~r frequency noi~e means that ~he p~ocessing unit 12 may be of a s~mple design since it need only function over a re~tricted fre~lenry range.

In a mo~ifiçA~ion, the muffler includes ~ compliant membrane (not shown) to isolate the reg$on of the ~h~ r 10 cont~ini~g the loudspeA~e~s 6 an~ 7 from the pipe 5, the membrane being substantially acoustically L~ arent for transmission of the antl-no$se, but mechAnicAlly isolating gas LLa~ the ~Y~tlRt from re~hing the lo~JspeA~ers. As anot~.er modlflcat~on, the inflow pipe 5 may be c~r.~ ed to the outflow pipe 3 on its lower side, thereby exten~ing Acrc~ ~he lower side of the aperture 13 in order to provid~ ~ flow path for liquid co~ nCAte in the pipe 5 to the outflo~ pipe 3. In this way an acct~m~lation of l t~-i~l co,.~r.l~ates in the cb~ber 10 is avoided.

20 It may al80 be A~sirF~hl~ to include at le~st one i ~ oLL in the baffles 8 and 9 to modify t~ ~coustic characteristics of the cha;~er lû.

:

66g~

In the aforegoing description the term anti-noise is used tG mean acoustic signals controlled in phase and a~plitude so as to t~nd to cancel an unwanted noise.

Claims

1. A muffler for muffling noise in a fluid flow, comprising a chamber, means for injecting anti-noise into the chamber, a conduit for conveying a fluid stream, means for receiving fluid flowing from the conduit, and coupling means including a radial aperture between the conduit and the receiving meals, for acoustically coupling the chamber to the fluid stream, wherein the coupling means is arranged such that the chamber becomes pressurised in use, such as to restrict flow of fluid from the fluid stream through the chamber.

2. A muffler according to claim 1, wherein the muffler acts as as acoustical filter.

3. A muffler according to claim 1 or 2, wherein the conduit extends through the chamber.

4. A muffler according to claim 1, 2 or 3, wherein the acoustic coupling means is within the chamber.

5. A muffler according to any preceding claim, wherein the acoustic coupling means comprises a gap between an end of the conduit and an opening to said receiving means.

6. A muffler according to any preceding claim, wherein the conduit extends into the chamber and the receiving means includes an opening in a wall of the chamber, the conduit terminating adjacent to and in axial alignment with the opening.

7. A muffler according to any preceding claim, wherein the internal diameter of the conduit is less than that of the opening of the receiving means.

8. A muffler according to any preceding claim including a microphone for detecting residual noise to produce an electrical error signal.

9. A muffler according to any preceding claim, wherein the means for injecting anti-noise comprises a loudspeaker.

10. A muffler according to claim 9 wherein the means for injecting anti-noise comprises a plurality of loudspeakers.

11. A muffler according to claim 9 or 10, wherein the or each said loudspeaker is resistively loaded.

12. A muffler according to claim 9, 10 or 11, wherein the or each loudspeaker is capacitively loaded.

13. A muffler according to any one of claims 9 to 12, wherein the loudspeaker is caused to appear acoustically as a simple mass over a given frequency range.

14. A muffler according to any preceding claim, wherein the conduit is an exhaust pipe connected to an internal combustion engine.

15. An active noise cancellation system including a muffler according to any preceding claim.

15. A muffler according to any preceding claim including a membrane for isolating said fluid flow from said anti-noise injecting means.