CA2039745C - Single cavity automobile muffler - Google Patents

Abstract

A noise suppression system for use on internal combustion engines and small enough for use in automotive applications is disclosed. A cancellation noise generator and actuator speakers produce a noise to combine with and cancel the engine noise carried in exhaust gases in a mixing chamber.
The resultant noise leaving the mixing chamber is measured by a circulr tubular microphone array to control the noise generator.

Description

. , ,.

2 i ~

g " ~ ~ '~ ' ' B~IdGLE CAVITY AUT~MOHTLE I~IiCIF'FL~~
11 . , , 17 TI;CIINICAL FILLD
1& Ths present invsnti.on relates to sound muEt?~.i~tg ds~rice~q 19 particularly those of the type used 'in contlec~:aon with tulaes or ducts which emit sounds which ode wish~~ to ,, ~, - 2 '" ,.
1 silence such as, for examplef the exhaust pi~e~ o~ internal 2 combustion engines.

3

4 ~5 nA~K~ItOU~(D
6 Very early in the evolutiota of the int~x~~~. combus~tio~1 7 engine, it was discovered that the relati~r~~~r ~~c~h ~.evels 8 of noise emitted during operation of the ~~c~~pe could b~
controlled, to a large extent, by resonant ~punc~ muffl~.r~c~
devices. At least as early as about a cent~r~r ac~p, i~ w~
11 discovered that a major portion of the sound ~r~i~~sd day 1~ - internal combustion engine. _ exited through t~~ _ t~:ll ~ip~ t _ 13 which serves the primary purpose of exh~l~~t$pg, ~pep~
14 combustion gases.
16 The approach toward the attenuation of the~~ upc~estrably 17 high levels of noise was to pass air exi~~.~~ ~p e~lgirde 18 through an acoustic filter. In principle, ~it~er high past 19 acoustic filters or low pass acoustic fi7.tprg may be employed to muffle sounds in a duct. For e~~~pl~, a lob 21. pass filter is useful in order to~prevent the tr~psmi~siop 22 of relatively high frequency sounds. on the other hand 23 the low frequencies of acoustic ,energ~r which arp 24 predominant :ln explosive discharges, such as hose created by the explosion of a gun or found in an autoa~obilp e~t~au~~.
26 system may be filtered out using a high pass filter.

~G~ ~r8~ ~~7 2 Likewise, a combination of both ~bigh pass bpd 1oW pass 3 acoustic filters may be used to achieve the q~.imination of 4 noise. The elimination of noise may be vieW~c~ ~s generally involving the cancellation of the alter~at~pg flora of 6 gases, representing sound transmission, whid.~ poi impeding 7 the steady flow of gas out from the exhaust a~r~te~ Which is -8 necessary in order to discharge spent combustion ~~od~cts.

As a general rule, mufflers hays volume~ i~ ~~as range o~
11 six to eight times the piston displacement p:~ ~~e engine - 12 and may contain baffles .with or .without -hole. ~ primary 13 aspect of their operation involves the aa~l,cellation of 1~ sound waves by interference, usually involving breaking the, waves into two parts which follow different ~~thq and meet 16 again out of phase before leaving the mu~:~~.~r. Another 17 important aspect is that exhaust back prs~~ure must be 18 minimized in any muffler design, insofar as a#~ increase of 19 only one psi in back pressure decreases the ~naxi~um power output of an engine by about 2.5~. About l~ of this loss 21 is due to additional work being expended by ~t~e ~ngiye to 22 exk~aust the gases. The balance of the los~x :~.p dL~e to tt~e 23 effects of increased gas pressure on volumetric ef:eiciencyt Turning to t'he case of ventilating ducts, a dagre~ of noise 26 suppression is usually obtained by lining th$ ducts on ~t , . ~ ~ C11 8l~ 1~~ ~~ hJ , -- 4 - .
1 least two non-opposite wa118 YJith an ~ff~.cie~at aound°
2 absorbing material for a distance of three ~o six ~nete~~
3 from both the inlet and the outlet, wher~t dq~ to tl~~
4 length of available duct, this is insuffici~~t, ~dditiona~.
noise suppression may be provided by introc~uci~c~ baffles 6 into the duct and covering the baffles with s~~tnd-absorbi~tg 7 materials.

9 Tn the .case of duct associated noise co~~rol systeme~
1~ increased speed of air flow introduces ad~~.ti4~al noi~~e 11 through the generation of turbulence. ~~is mutt by 12 addressed by. _ additional . baffles _ and/or soxlnd ~bs4~:bi~c~
13 materials. ;
l~
Some understanding of baffle filter systems may b~ obtained 16 if we consider a quarter wavelength resonant cav~.~y. Such 17 a cavity, known as a Helmholz cavity is a claafi~er closed ~t 18 one end and open at the other. Secause .fit is ~ quarter 19 .wavelength in length, sounds entering the op~:n egad of the 2a chamber pass through~the chamber and are rs~lect~c'i back to 21 the open end of the chamber with a phase defy o~ one--half 22 a wavelength. The half wavelength delay is caused becauap 23 the time of transit of the acoustic dis;~urbanoe tY~rough tip 29 chamber includes a forward transmission path of o~~e-qt~a~-ter wave7.ength and a reflected transmission bac~C to the open 26 end of an additional quarter wavelength. a .~ ~ ,"~ ,~ ~ .
. ' ~~ P3 ~.1~

2 The result is a half wavelengtta or 1800 phass sh$ft 11~ t~~
3 output of the cavity with respect to the sound pag~inc~ aver 4 the top of the cavity. Hecause the sic~r~~ls ire ~h~~e shifted with respect to each other by 1~0 dec~rea~~ aid 6 because, for a first approximation, we ~a~ aq~ume th~~t 7 during the ~emissi.on of a particular sounds the amplitude a and frequency of one wavelength of ~~e found is 9 substantially identical to the amplitude ~~~ fr~quer~cy q~
the next wavelength produced by the sources ~'hu~~ ~ giv~H.
11 undulation corresponding to one wavelength i~ e~act~.~r 12 cancelled by the prior undulation of the found ~hic~~olle 13 wishes to cancel. Naturally,~this is only true for souHd 14 having the particular frequency which results in a quarter .. .
wavelength relationship between the Helmholz cavi~x ap.d t~~
16 sound. However, if the frequency is not far re~oved from 17 the resonant frequency of cancellation, ttl~ ca~cellatiop 18 effect will still occur to a substantial extent.

20In early automobile mufflers, the approach takeg~was t4 21pass the exhaust gases over a matrix of paffles which 22together defined a plurality of tuned cavities. This 23structure acted as a filter and to a 1:mitPd extent 28cancelled a range of sound Frequencies produced by tl~e 25intei:raalcombustion engine, propagated through the manifold 2Gto the ~ngi~e tailpipe, and which would otherwise edit the 1 in the form of acoustic disturbances.

3 Todayr the quieting of such muffler systems i~ on the order 9 of eight decibels.
fi Notwithstanding the numerous disadvantages o~ th$~ cart of 7 noise muffling system, modern mufflers rem~~~ sub~~tan~ial~.x 8 identical in their essentials. Generally ~Nch ~rio~° ~~t . 9 mufflers are constructed of sheet ~~~al~ Moxp particularly, such mufflers comprise an o~i~er ~h~~l c~~
~11 casing made of sheet metal and a sheet metal baffle 12.: structure' .secured _ within the casing.. .~ ~at~ ..fob ~ t~~
13 oanduction of combustion gases and attendant acoustic 19 disturbances is provided in the muffler adj~aent the Z5 various noise absorbing cavities.

17 Because the exhaust gases are both teat and copros~.ve (beil~g 18 the praduct, of the combustion of gaeoline)~ they cause 19 relatively quick corrosion and otherwise dateriorat~ tkae sheet metal components of the muffler. The ,r~su~.t is that 21 the muffler must be periodically replaced.

23 Still another problem with conventional mufflers is the 2~1 vi.scous resistance which they provide to spews: combustion products. trot is the viscous resistance of tye muffler of 26 no significant effect. Rather, the resistance encountered , ~, Vin' ~-~ ~3 m ~

1 by escaping combustion products is significant enough to 2 adversely affect fuel effiaie~cy and the collcentratipn of 3 pollutants in the exhaust gases. This is c~~~ed~ in part 4 by the failure of the engine to exhaust spelt combustion products from the cylinders with tlae ea~a c~sgree of 6 efficiency that an internal combust:Lon engine Without a 7 muffler achieves.

9 d~lhile, to some extent, the problems, involves in 'the rap~.d daterioration of awtomobile mufflers nark ~e addressed 11 through the use of relatively expensive alloyst sash as 12 pertain types of stainless steel, and tha use. of.~Plativel~r 13 thick material;, the additional coat of such high quality 14 materials renders this uneconomical. l~oreoyer~, the additional labor r>osts involved in manuf.act~~inc~ muffler 16 with relatively thick sheet metal componerkts adds cost 17 which clearly makes such mufflers impractical, 19 Likewise, while it is conceivable that a muffler design including relatively wide passages for the e~chaugt o~
21 combustion products and numerous cavities to cancel pounds 22 passing over them could improve the incomplete scavengirkg 23 of spent gases from the cylinders, the, increa~xe ika size of 24 a device made using such an approach would make ~.t impractical in the environment of today~s automobile. Hlere4 26 space is at a premium and even 'the present clay ~:~latively , 1 small muffler represents a significant po~tio~ a~ tl'a~
2 volume of the automobile. 3n any event, t~p~ ~~ffler $s 3 also often the lowest point op the automo~aile and thus 9~ represents the limitation on clearance over ~:he xoad. In connection with this, it is noted that even $pl t)~p case of 6 diesel-engine trucks, where the problem of ~ac~C pressure 7 has required the use of relatively large m~a~~le~~ aid tie 8 aesthetics and size of the truck have a11o4~~d tie qse of 9 large mufflers, adequate snuffling of combu~~$an poise h~~
l0 not been satisfactorily achieved by ex~.~~in~ mqffl~r 11 systems.
' . 12 - . . - . _ : "
13 ; SUMMA~tSt' Q1L' TI3E 2~d'6~'ENTIODJ
14 The invention is intended to provide a remedy. Tt solves the problem of how to muffle noises in Ia.:.duc~, such as a~
16 engine exhaust or air.-conditioning duct y~ith ~ simply' 17 durable and effective device. This ponfi.gutratior~
18 integrates a mixing diameter with an integral microphone 19 for improved cancellation over a wider frequency range thal~
2a previous attempts. ~ At the same time, back pressure 21 problems are minimized thus resulting ip goad fuel 22 efficiency and minimal exhaust of pollutants into the aid.
23 The same iS achieved through the use of a single or 2~1 multiply chambered dynamic cavity driven by aa3 electro-mechanical actuator which, effectively, generates a~n 26 acoustic signal used to cancel noise in the dqct.

>':~s r _g_ 1 The inventive muffler cavity is based upon the use of a so-t called ported enclosure or symm~trically lqade~ s~stefi.
3 This type of enclosure is characterized b~ the use of a 4 closed rear volume, together with a front volu~re Goup~.ed to a radiating tuned port. '.this novel tuned ds~~gn p~tilizea a 6 single circular port driven by multiple ~~~ak~rs y~hich 7 surround the exhaust pipe to provide improved par~c~ll~tion.
8 Integrated into the port design is a ~.$~Cinc~ ct~amb~~
surrounded by a circular sensing micropho~~y ~it~ tie proper components and cavity volume and port ~~lep~.ior~ hic~~
11 efficiency cancellation can be achieved over ~ 50 to 300 I~z 12 frequency r.ange~: ~ ~ - ~ w 14 . As compared to previous designs a single cirpL~la~' port is s used with multiple speakers as opposedwto an gray of 16 individual ports from multiple speakers arranc~~d mound tt~e 17 exhaust outlet. A preferred embodiment avoidq loa~tillg the 18 microphone and anti-noise port a distance a~ay from eack~
19 other for acoustic mixing in air with limited high ,frequency results. The inventive system brings all of the 21 components together at the exhaust port producing a higher 22 degree of cancellation with higher frequency response than 23 previous designs.
2~
:Ln accordance with the invewtion, engine or pthe~ exhaust 26 noise is .introduced into a mixing region wit? an acoustic r _10-1 cancellation signal where they era caused to c~~cel each 2 other: A ring-shaped microphone array is di~pogpd grouNd 3 the noise source and the acoustic cancali.~tio~ signalq 4 which is produced by the actuator, to gene~ata an error signal proportional to the degree to which ca~cel~.~tion hao 6 not occurred. 'Phis error signal is then ~t~pd ~o control 7 the signal produced by the actuator. Sansirl~ of ~ha souNd a pressure within the tubular member is done w$~y oN~ or more 9 microphones where the output of the multi~~~ 7tl~cr~phona 1c~ are. combined by averaging of their indiv~~ua~. outputs.
11 Noise due to turbulence and other assal~tial~.y xandom 12 factors .is~ cancelled' through the use of a pl~xality of 13 sound-sensing points.
14 .
In accordance with the preferred .embodimeii~, ~ plurality of 7~6 such sound-sensing points is achieved through the use of a 17 tubular member with a plurality of sound-~en~ing hobs 18 disposed along its length. This tubular member i~ disposed 19 concentrically with .and downstream from the emis~i.on point of sound exiting the mixing region and down~traam of arid 21 concentric with the acoustic output of the actuator.

23 This is achieved through the use of~a first pipe which 24 corresponds to the duct with naise in it being contained within a second larger pipe which is provided with the 26 acoustic energy generated by the actuator. The use of ' 1 concentric sources and a plurality of sound ~ens~.pg parts 2 in the inventive configuration rea~ults i~p ~nors uniform 3 noise cancellation, minimal mixing region siz$ anc~ immunity A to random noise.

5 nRIIGF aFSCRIPTION OF TFi~ pR~WINON
7 one way of carrying out the invention .is descpibed i~
8 detail below with reFerence to drawings whips $.llu~strate 9 only one specific embodiment of the in5~s~lti~p aid l0 whi.ch:-1z ' Figure 1~ is 'a tOp plan View in CrOB~1 ~~..'Ct~..On C)f ail 13 engine muffler constructed in accordapce Wit~1 tlla 14 present invention; , Figure 2 is a view along lines 2-2 0 ~ Figure 1:
16 Figure 3 is a view along lines 3-3 of Fic~urs 1 showing 1~ the construction of the muffler in crass sec~.ion;
lg Figure 4 is a transverse cross-sectional vi~eW of the lg muffler illustrated in Figure 1 along the likes 4-~ of 2Q Figure l;
21 Figure 5 is a detail along lines 5-5 of Figyre 2 22 illust rating the construction of a microphone assembly 23 useful in conjunction with the muffler of Figure l;
2~ Figure 6 is a cross-sectional view along lies (-6 of Figure 2 illustrating the placement of a microphone 26 within the m:Lcrophone assembly%

~,~~~'~--12-°
1 Figure 7 is a block diagram of the inventive ryst~m;.
2 Figure 8 is a diagrammatic v$ew of an ~pxod~~tamio 3 microphone design;
4 Figure 9 is a diagrammatic represantatio~ of an alternative embodiment of tile inventive ~luff~.er

6 system; and

7 Figure to is a view along l~.neg 10-l0 0~ Figure 9 a illustrating the outside appearance of tie muffler 9 system of Figure 9. .
to 11 .
12 . kiEST. PiODE FOit Cd~RRYIhIG oUT THE II~VE3~'~"ldP~ .

14 i2eferring to Figures 1-3, the structure of the inventive muffler l0 is Been to comprise an outer casing 12. Outer 16 casing 12 comprises a cylindrical. member lit ~ forwa~~ end 17 cap 16 and a rear end cap 10. Cylindrical member 1~, 18 forward end cap 16 and rear end cap 18 are Made of a 19 relatively inexpensive material such as ple~tic which is selected for mechanical strain and durabilit~r under ~ wide 21 :range of temperatures and other environmer~t~al factors as 22 would be experienced by a muffler positioned at vhe bottom 23 of an automobile.

The exhaust pipe 2o is mounted within casing 12, being 26 supported in forward end cap .~.6 by an insulative annular ~ 3 I9 y, _13_ 1 member 22.

3 the exhaust pips 20 is made of steal, stai~lesq steel ox 4 any other suitable material having a thic~C~~~s ~uff~.cier~t to result in mechanical integrity. 7Cn ~ddi~ion~ the 6 exhaust pipe 20 is made thick enough to ~ritllptand the 7 expected degree of corrosion during the lift of the a automobile without losing the required str~r~c~~h.

When installed, it is contemplated that muff~.~r ~.0 w~.ll ~e 11 secured to the underside of the automobile ar~c~ th~~ ~xhau~t 12 2.0 is also secured to the automobile. ln~ofar ~as it~ ia.
13 connected to the exhaust of the engine, tie bpd 24 of 14 exhaust pipe 20 is held irt position ;by a plqxality of ...
radial support members 26, 28, 30 and 32~. ~adi2~l support 16 members 26-32 are secured between exhaust pipe 20 and 17 mixing chamber pipe 36, by being welded or otherwise 18 suitably attached 'to both of these members. Tn accordance 19 with the preferred embodiment, mixing chamber pike 3~ and support members 26--32 are all made of steel, or stainless 21, steel or ether suitable materials. Likewise extlyust pipe 22 20, xadial support members 26-32, and mixing chamber pipe 23 36 may be made of stainless steel in view of the resistance 24 of this material to long exposures of high temperr~tur~s arid the various combustion products created during the 26 operation of the internal combustion engine.

a,'a~~ J
~~~~~I~'~!~r:
~14-2 As illustrated in Figure l, mixing chamber pipe 34 (~hich 3 may also be made of plastic) ig securely mounted within 4 rear end cap 18 by being securely attached or dam-fitted ip a circular bola 36 within end cap 18. Final~,y, additional 6 support is provided by a pair of transverse ~adi~l support , 7 members 38 and 40, as illustrated in F'ic~ure 4. The

8 transverse radial support members are made of material

9 similar to that of support members 26-32. ~~p ad~:Ltion, it l0 is noted that transverse radial support members ~8 and ~0 11 and radial support members 30 and 32 are made of ~~iangular - --12 shaped -pieces of relatively thick sheet metal iii order to 13 provide support when forces are applied to the muffler . ~ I4 structure in the direction parallel to the axis of syanmetry of exhaust pipe 20.

17 The isolation of heat present within, exhaust pipe 20 from 18 the remainder of the system is provided by a cyli~drically°
19 shaped layer of heat insulative material 42 which is disposed around exhau:.t pipe 20. Typic~lly~ this is 21 insulating fiberglass wrap, header wrap, or an isolating 22 air cavity. An acoustical chamber 44 is defined by a pair 23 of inner planar walls 46 arid 48, actuators 50 and 52 and a 24 forward wall 54 with a circular concentric hole 56 ~.~ its center. On the edge of the chamber opposlt~ flee f°rWarct 26 wall 54 is a rear wall member 58. Rear Mall member 58 and 1 forward wall 54 are both made of synthetic m~teri~l such as 2 that of outer casing 12.

4 Referring to Figure 4, wall 54 defines a chamber 60 which is filled with sound deadening material suo~ as ~:~berglass 6 62 in order to change the equivalent oavi~y volume arid 7 improve and simplify the acoustical propaptie~ of the 8 acoustical chamber 44. T~ikewise, rear W~~.l Member 58 9 defines a pair of chambers 64 and 66 which ~~~ f$l.led wi~~l acoustic deadening material such as fibergl~~s ~8 at~d 70 11 which again change the equivalent volume and si;~~li~~ the 12 operation of acoustical chamber 44 by prey~~ti~g ~°andom 13 oscillations and resanances from inter feri~ag ~ritt~ tk~e ' 14 operation of the muffler. , ' 16 Finally, a tubular microphone assembly 72 i~ provided at 17 the end of mixing chamber pips 4. A mixipg camber 7$
18 (Figure 1) is defined at the end of mixing ct~gmbe~ pipe 3~.
19 Referring to Figure 2, a plurality of holes 7,6 ark defined by a circular tubular member 78. Generall~~ holes 76 axe 21 equispaced along the circumference of member 7$ and one 22 such hole 76 is ll.lustrated in detail ~.1z Figure 5.
23 Typically four mi.craphones are also ec(u.ispacsd Within the 24 circumference of member 78. Actual detectioa~ of aou~d and conversion into an electrical signal is done by these 26 ~nicraphones 80 and 82 which have their acoustical inputs ~ ~~, 1 positioned within the annular cavity 84 defir~~d by tubular 2 member 78. Microphone assembly 72 is secured to tie end of 3 mixing chamber pipe 34 using any suitable mpan~ such as 4 rivets, adhesive, or the like. The electrical output of these multiple microphones are combined (ave~°~ge~~ wing a 6 mixing circuit to provide a composite r~~id~al error '7 signal.

9 The placement of microphones .80 and 82 is ~.~.lu~trat~d by the enlarged detailed diagram of Figure 6. #~~~e ~:~.croplao~e 11 80 is shown embedded in the sidewall 86 of ci~pul~~ tupular 12 member ~ 78. Microphones 80 and 82 . may be po~:l~l.pnec~ at a 13 variety of angular positions depending upon wh~tlzer ogle 14 wishes to route the microphone cable 88 on the inside ar ;
outside of the device.

17 Referring to Figure 7, during operation of the inventive 18 system, the noise generated by exhaust ~ipp 20 and 19 actuators 50 and 52 is detected by tubular microphone assembly 72 which generates an error signal which i~ sent 21 to a cancellation signal. generator 90. The cancellation 22 signal generator, in tuna, generates a cancellation signal 23 which is coupled to actuators 50 and ,52. ~. aar~cellation 24 signal generator such as that marketed by several, companies today may be used.

~~~"~~'~
17._ 1 7Cn particular, in the embodiment illustrated in ~~.gures 1-2 6, an acoustical chamber 44 which is eubetantially 3 completely closed except for an annular outpqt dgct region 4 92 defined between exhaust pips 20 and mixi~c~ chamber pipe 34, is provided. Thus, the acoustic energy gerl~rated by 6 actuators 50 and 52 is transformed into a conoentxic source 7 which is concentric with the noise output a~ exhaust pipe 8 20. These two concentric sources are m~x~c~ t~get~ler in 9 mixing region 94 where, in the ideal posed because successive undulations are substantially com~let~~.y out of 11 phase with each other and of equal magnitude, they add 12 ' together and cancel etch other resulting in hero noise a't 13 the output of the exhaust system ad~aael~t ~licrophap~
14 assembly 72. It has been found that a mixing region 94 on the order of ten centimeters in diameter end three 16 centimeters in length is sufficient to aahieye an 17 acceptable degree of cancellation in an automobile muffler 18 system.

In order to permit the flow of spy liquid ~ha~t may have 21 accumulated .in the muffler out of 'the muffler, a drip hole 22 urith a short, small diameter tube 96 is provided at the 23 bottom of 'the muffler, as illustrated in ~'~Lgure 4.
24 Depending upon the actual configuration of the tube microphone, it may alsp be desirable to put an additional 26 drip hole 9a adjacent to the tube microphone assembly 72.

~' 1 ~ ~ c.~ :J ~:,~
_lg_ , 1 From a practical standpoint, such drip ho es Will not ' 2 affect the performance of the system in at~~ s~bstanti~l 3 matter from an acoustic standpoint.

Conventional radio speakers ruggedized for ~~~ in an active 5 muffler may be used as actuators. Ruggediz~~ioi~ consists 7 of using waterproof materials such a~ Keylarr or a impregnated materials, with a neoprene muf~I~r purround.
9 In particular, an acceptable degree of perfo~~ncg haq been achieved using circular thirteen centimets~ tk~~.rty watt 11 speakers of the type manufactured by AUDAX ~nde~ Catalog 12 No. HIF13JVX.: In addition, it~has been-found adyant~geous 13 to ruggedize the speakers through the ap~l~.cation of a 14 protective Coating of Kevlar.
16 It has been found that good canaellatio~ results for 17 typical automotive sound pressure levels mad be achieved 18 using 2o to 50 taatts of eleotrical power into the speaker 19 actuators 50, 52. Tn principle, while a single microphone speaker will also work, the provision of two or more 21 speakers provides some redundancy and allows a smaller 22 enclosure to b~ used and would appear to improve the 23 symmetry of the system. Generally,, cancella~Lion is 24 achieved in the range below 800 hertz. If it is desired to achieve complementary cancellation l.n the rage above 8o0 26 hertz, a thin steel wool liner l0U (other liner materials ~~~~~~~°~~~
y19~
1 are also acceptable) may be positioned within exhaust papa 2 20 as illustrated in Figure 4. ether trad~.~ional p~ssivs 3 muffler attenuation methods can also be int~c~rat~d in the 4 dynamic muffler for higta frequency attenuatio~e 6 'Phe tubular ring microphone system disclosed ~bova is both 7 durable and has excellent performance charac~~ri~~ic~. As B can be seen, with reference to the figures, ~$~p m~.crophones 9 BO and ~2 are protected from the ~nviro~~~~t ~y being l0 positioned within circular tubular member '~~. ~'huq the 11 microphone is protected from weather and heat ef~pat~. In 12 addition, the use of numerous holes 76 in ciraul~~ tubular 13 member 78 results in numerous individual inputs to the 1~ microphones and has the result of acoustically averaging random noise, thus drastically reducing wind and exhaust 16 turbulence effects. The tubular configuration with the 17 multiple microphones produces a residual error signal which la is the integrated-averaged error as measua~ed along the 19 perimeter of the ring. For a dynamic muffler measuring this error at 'the Zone Of caricellatio~ produces a 21 symmetrical cancellation zone treat is optimum, 23 The tubu7.ar microphone assembly 72 is ,constructed from an 24 insulating tubular material such as plastic tube. This creates a thermally insulating medium to protect the 26 microphone. The tubular material is perforated at regular ,~ ~ r, ".

1 intervals, corresponding to 30-°50 holes per Wavelength at 2 the highest frequency of interest (i.e.~ O.Q4 meters 3 separation between holes corresponding to X00 Hz). For 4 best operation, the k~ole size needs to be smell, typically around 0.062 meters. The hole size and numbed of holes can 6 be varied to adjust the amount of sound picku~~t a The plastic tube of the microphone aasambl~ protects the 9 .microphones by surrounding them with a oap~~.ve thermally insulating air medium. Ths use of open doles at the 11 exhaust outlet provides an accurate mes~lg of sound 12 transmission without directly exposing .the micro~hone._ 13 elements to the corrosive and~hot exhaust ga8ss.

An alternative configuration .is to cover the perforations 16 in the tubular member with thin ( . 001°°} Kapton ('fM} tape.
17 This provides all the above noted advantages of the tubular 18 microphone while providing a sealed configuration.

In connection with this, it is noted that s variety of 21 profi:Les may be used in order to minimize ~tu~plzlence about 22 holes 76. For example, turbuloncs reduciHg aerodynamic 23 surfaces 102 and 109 may be used to reduce turbulence, as 24 illustrated in Figure 8. ' 26 In addition, blodkage of one or more of the hol.es in the , 1 circular microphone assembly 72 Will have ~ ~.ess serious 2 impact on system operation.

4 An alternative embodiment is illustrayed i~ Figures 9-10.
Generally, similar parts or party ~erfoacmi~ac~ analogous or 6 corresponding or identical fu~ctio~~ ~~~ ni~~l~ered herein 7 with numbers which differ from tho~p of the earlier 8 embodiment by multiples of one hundred.

l0 As can be seen from the alternative embodimar~~ of Figures 9 'll and l0, it is not necessary that the muffler Af 'the present' 12. inv~entiori .tale a -conventional form. . For example, ~ it _is 13 possible that a bumped 104 may accommodate the inventive' 14 muffler 110. In particular, an exhaust pipe 12o may feed its output to a mixing chamber 17~ which, .~.n turn, receives 16 the acoustic output of a pair of actuators 150 and 3.52.
17 Additional advantage may be obtained by providing an 18 annular membrane 106 to receive the output of actuators 150 19 and 152 and couple that output generally in the directions indicated by arrows 108 while isolating the actuators from 21 the environment.

~Aa~ r~ ~~

1 ~Jhile an illustrative embodiment of the invention has been 2 described above, it is, of coezrseo ttpderstooe~ that vaxious a modifications will be apparea~t to those of ordinary ~~Cill 4 in the art. Such modifications ire Within t~,p spirit and scope of the invention, which is limited anc~ defined only 6 by the appended claims.
i

Claims (36)

CLAIMS:

1. A system for cancelling undesirable noise in a duct, wherein said duct has an output end, comprising:

(a) a chamber member defining a mixing volume and an output port, said mixing volume being acoustically coupled to the output end of said duct, said mixing volume being defined adjacent said output end;
(b) a cancellation noise generator having an output acoustically coupled to said mixing volume and generating a cancellation noise;
said mixing volume being configured for combining the undesirable noise and the cancellation noise; and (c) a plurality of audio inputs disposed around said output port of said chamber member for sensing noise at said output port and generating a control signal to control said cancellation noise generator.

2. a system for cancelling noise in an open ended duct, comprising:
(a) a pipe coupled to the end of said duct;
(b) an actuator speaker;

(c) a ported actuator speaker enclosure containing said actuator speaker and having an audio output;

(d) a cancellation noise generator;

(e) a mixing chamber for receiving the output of said ported speaker enclosure and said pipe and (f) a plurality of audio inputs disposed around the end of said mixing chamber opposite the end of said mixing chamber coupled to said pipe and said ported speaker output, said audio inputs being coupled to said cancellation noise generator.

3. A system as in claim 2 wherein said pipe is substantially concentric with and surrounded by said mixing chamber, and is substantially concentric with said plurality of audio inputs.

4. A system as in Claim 1, wherein laid chamber member together with said cancellation noise generator and said duct defines a single chamber.

5. A system as in Claim 1, wherein said output end is contained within said chamber member, said mixing volume being downstream of said output end and said cancellation noise emanates from points upstream of said output end.

s. A system as in claim 5, wherein said chamber member defines at least one audio input port, said audio input port being closed by a cancellation noise generator.

7. A system as in Claim 6, wherein said cancellation noise generator is a first audio loudspeaker having a front surface which is acoustically coupled to said mixing volume and a rear surface.

8. A system as in Claim 7, wherein a volume defined between said duct and said chamber member forms a front volume of a ported audio enclosure system and further comprising a closed rear volume acoustically coupled to the rear surface of said first loudspeaker.

9. A system as in Claim 8, wherein said plurality of audio inputs comprises a closed tubular loop having at least one microphone acoustically coupled to the interior of said tubular loop and a plurality of holes disposed in said tubular loop, said tubular loop being disposed substantially along the periphery of said output port.

10. A system as in Claim 8, further comprising a second loudspeaker positioned, configured and dimensioned to generate a signal substantially symmetrical with respect to the cancellation noise generated by said first loudspeaker.

11. A system as in Claim 1, wherein a volume defined between said duct and said chamber member forms a front volume of a ported audio enclosure system and further comprising a closed rear volume defining member acoustically coupled to the rear surface of said first loudspeaker.

12. A system as in Claim 11, wherein said plurality of audio inputs comprises a tubular loop having at least one microphone acoustically coupled to the interior of said tubular loop and a plurality of holes disposed in said tubular loop, said tubular loop being disposed substantially along the periphery of said output port.

13. A system as in Claim 12, further comprising a second loudspeaker positioned configured and dimensioned to generate a signal at a point substantially displaced with respect to the cancellation noise generated by said first loudspeaker.

14. A system as in Claim 11, further comprising a second loudspeaker positioned, configured and dimensioned to generate a signal substantially symmetrical with respect to the cancellation noise generated by said first loudspeaker.

15. A system for preventing undesirable noise from emanating from an output end of a ducting arrangement, said system comprising a plurality of audio inputs disposed around said output end of said ducting arrangement for sensing noise at said output end and generating a control signal, a cancellation noise generator responsive to said control signal and having an output acoustically coupled to a mixing volume defined within said ducting arrangement, said generator generating a cancellation noise, and said audio inputs positioned outside said mixing volume, said mixing volume being configured for combining the undesirable noise and the cancellation noise to cancel a substantial portion of the undesirable noise.

16. A system as in Claim 15, wherein said plurality of audio inputs comprises a tubular member having at least one microphone acoustically coupled to the interior of said tubular member and a plurality of holes disposed in said tubular member, said tubular member being disposed substantially along the periphery of said output end.

17. A system as in Claim 16, wherein said tubular member is closed and said cancellation noise generator is a loudspeaker, having front and rear surfaces and a volume defined by said ducting system forms a front volume of a ported audio enclosure system and further comprising a closed rear volume defining member acoustically coupled to the rear surface of said first loudspeaker.

18. A system as in Claim 16, further comprising a second loudspeaker positioned, configured and dimensioned to generate a to reinforce the cancellation noise generated by said first loudspeaker.

19. A system as in Claim 16, wherein said ducting system comprises a duct within a second volume defining member, said second volume defining member defining said mixing volume in a space between said duct and said volume defining member.

20. A system as in Claim 15, wherein said ducting system comprises a duct within a second volume defining member, said second volume defining member defining said mixing volume in a space between said duct and said second volume defining member.

21. A system for preventing undesirable noise from emanating from an output end of a device, said system comprising a plurality of audio inputs for sensing noise at said output end and generating a control signal, a cancellation noise generator for generating a cancellation noise in response to said control signal and having an output acoustically coupled to a mixing volume acoustically coupled to receive said undesirable noise, said mixing volume being configured for combining the undesirable noise and the cancellation noise to cancel a substantial portion of the undesirable noise, said plurality of audio inputs comprising a tubular member having at least one microphone acoustically coupled to the interior of said tubular member and a plurality of holes disposed in said tubular member.

22. A system as in Claim 2l, wherein said tubular member is disposed substantially along the periphery of said output end.

23. An automotive noise suppresser for reducing noise emitted from an internal combustion engine, comprising: (a) an exhaust duct configured and dimensioned to be coupled at its input end to the output of said internal combustion engine, said exhaust duct defining an inside passage for the exhaust of hot gases and having an output end;
(b) an outer casing having an output port, said outer casing surrounding said exhaust duct and extending an length beyond said output end of said exhaust duct to define a protected space between g'aid exhaust duct and a portion of the inside surface of said outer casing and a mixing space acoustically coupled to said protected space and adjacent said output end of said exhaust duct;
(e) heat insulative material secured to the portion of said exhaust duct adjacent said protected space;
(d) an audio transducer positioned adjacent and acoustically coupled to said protected space and remote from said mixing spacer;
(e) a cancellation signal generator having an output coupled to said audio transducer; and (f) a microphone positioned at a point downstream of said mixing space for generating an audio feedback signal, said audio feedback signal being coupled to said cancellation signal generator to cause said cancellation signal generator to drive said audio transducer to generate an audio signal which will cancel a substantial portion of said noise emitted from said internal combustion engine.

24. A noise suppressor as in Claim 23, wherein said audio transducer has a front surface and a rear surface, said protected volume forming a front volume of a ported audio enclosure system, said front surface being acoustically coupled to said front volume and further comprising a closed rear volume acoustically coupled to the rear surface of said audio transducer.

25. A noise suppressor as in Claim 24, wherein said microphone comprises a tubular loop having at least one microphone acoustically coupled to the interior of said tubular loop and a plurality of holes disposed in said tubular loop.

26. A noise suppressor as in Claim 23, wherein said microphone comprises a tubular loop having at least one microphone acoustically coupled to the interior of said tubular loop and a plurality of holes disposed in said tubular loop.

27. A noise suppressor as in Claim 26, wherein said holes are disposed around said output port of said outer casing.

28. A noise suppressor as in Claim 26, wherein said tubular loop is disposed substantially along the periphery of said output port.

29. A noise suppressor as in Claim 26, further comprising a second audio transducer positioned, configured and dimensioned to generate a signal substantially symmetrical with respect to the audio signal generated by said first audio transducer.

30. A noise suppressor as in Claim 28, wherein audio transducer is a loudspeaker comprising a cone, a resilient member for supporting said cone and an electromechanical transducer-driver.

31. A system for preventing undesirable noise from emanating from an output end of a device, said system comprising an audio input for sensing noise at said output end and generating a control signal, a cancellation noise generator for generating a cancellation noise in response to said control signal and having an output acoustically coupled to a mixing volume acoustically coupled to receive said undesirable noise, said mixing volume being configured for combining the undesirable noise and the cancellation noise to cancel a substantial portion of the undesirable noise, said audio input being positioned external to said mixing volume.

32. A system as in Claim 28, wherein said audio input is a tubular loop containing at least one microphone and a plurality of holes along its length disposed substantially along the periphery of said output end.

33. A noise suppressor for reducing noise emitted from a device, comprising:
(a) a duct configured and dimensioned to be coupled at its input end to the output of said device, said duct defining an inside passage and having an output end;

(b) an outer casing having an output port surrounding said duct and extending in length beyond said output end of said duct to define a first space between said duct and a portion of the inside surface of said outer casing and a mixing space acoustically coupled to said first space and adjacent said output end of said duct;
(c) an audio transducer positioned adjacent and acoustically coupled to said first space and remote from said mixing space:
(d) a cancellation signal generator having an output coupled to said audio transducers and (e) a microphone positioned at a point downstream of said mixing space for generating an audio feedback signal, said audio feedback signal being coupled to said cancellation signal generator to cause said cancellation signal generator to drive said audio transducer to generate an audio signal which will cancel a substantial portion of said noise emitted from said device, said first and mixing spaces being substantially open to the environment of said device.

34. An automotive noise suppresser for reducing noise emitted from an internal combustion engine, comprising: (a) an exhaust duct configured and dimensioned to be coupled at its input end to the output of said internal combustion engine, said exhaust duct defining an inside passage for the exhaust of hot gases and having an output end;
(b) an outer casing having an output port, said outer casing surrounding said exhaust duct and extending beyond said output end of said exhaust duct to define a protected space between said exhaust duct and a portion of the inside surface of said outer casing and a mixing space acoustically coupled to said protected space and adjacent said output end of said exhaust duct;
(c) wall structure diseased between said protected space and said exhaust duct, said wall structure being in facing spaced relationship to said exhaust duct and positioned, configured and dimensioned to define an acoustic passage between said protected space and said mixing space. (d) an audio transducer positioned adjacent and acoustically coupled to said protected space;
(e) a cancellation signal generator having an output coupled to said audio transducer; and (f) a microphone positioned at a point downstream of said mixing space for generating an audio feedback signal, said audio feedback signal being coupled to said cancellation signal generator to cause said cancellation signal generator to drive said audio transducer to generate an audio signal which will cancel a substantial portion of said noise emitted from said internal combustion engine.

35. A noise suppressor as in Claim 34, wherein said audio transducer comprises a pair of loudspeakers each having a front surface and a rear surface, said protected volume forming the front volumes of a pair of respective ported audio enclosures, said front surfaces being acoustically coupled to respective ones of said front volumes and further comprising a pair of closed rear volumes acous- tically coupled to a respective one of said rear surfaces of said loudspeakers.

36. A noise suppressor as in Claim 35, wherein said microphone comprises a tubular member having at least one microphone acoustically coupled to the interior of said tubular member and a plurality of holes disposed in said tubular member.