Note: Descriptions are shown in the official language in which they were submitted.
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1
2
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B~IdGLE CAVITY AUT~MOHTLE I~IiCIF'FL~~
11 . ,
,
16
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
1
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.
9
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
24
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.
8
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~
1
1
2 The result is a half wavelengtta or 1800 phass sh$ft 11~ t~~
3 output of the cavity with respect to the sound pag~inc~ aver
4 the top of the cavity. Hecause the sic~r~~ls ire ~h~~e
shifted with respect to each other by 1~0 dec~rea~~ aid
6 because, for a first approximation, we ~a~ aq~ume th~~t
7 during the ~emissi.on of a particular sounds the amplitude
a and frequency of one wavelength of ~~e found is
9 substantially identical to the amplitude ~~~ fr~quer~cy q~
the next wavelength produced by the sources ~'hu~~ ~ giv~H.
11 undulation corresponding to one wavelength i~ e~act~.~r
12 cancelled by the prior undulation of the found ~hic~~olle
13 wishes to cancel. Naturally,~this is only true for souHd
14 having the particular frequency which results in a quarter
.. .
wavelength relationship between the Helmholz cavi~x ap.d t~~
16 sound. However, if the frequency is not far re~oved from
17 the resonant frequency of cancellation, ttl~ ca~cellatiop
18 effect will still occur to a substantial extent.
19
20In early automobile mufflers, the approach takeg~was t4
21pass the exhaust gases over a matrix of paffles which
22together defined a plurality of tuned cavities. This
23structure acted as a filter and to a 1:mitPd extent
28cancelled a range of sound Frequencies produced by tl~e
25intei:raalcombustion engine, propagated through
the manifold
2Gto the ~ngi~e
tailpipe,
and which
would
otherwise
edit the
-6-
1 in the form of acoustic disturbances.
2
3 Todayr the quieting of such muffler systems i~ on the order
9 of eight decibels.
fi Notwithstanding the numerous disadvantages o~ th$~ 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.
16
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.
22
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
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.
8
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,
18
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 ~.$~Cinc~ ct~amb~~
surrounded by a circular sensing micropho~~y ~it~ tie
proper components and cavity volume and port ~~lep~.ior~ hic~~
11 efficiency cancellation can be achieved over ~ 50 to 300 I~z
12 frequency r.ange~: ~ ~ - ~ w
13
14 . As compared to previous designs a single cirpL~la~' port is
s
used with multiple speakers as opposedwto an gray of
16 individual ports from multiple speakers arranc~~d mound tt~e
17 exhaust outlet. A preferred embodiment avoidq loa~tillg the
18 microphone and anti-noise port a distance a~ay from eack~
19 other for acoustic mixing in air with limited high
,frequency results. The inventive system brings all of the
21 components together at the exhaust port producing a higher
22 degree of cancellation with higher frequency response than
23 previous designs.
2~
:Ln accordance with the invewtion, engine or pthe~ exhaust
26 noise is .introduced into a mixing region wit? an acoustic r
_10-
1 cancellation signal where they era caused to c~~cel each
2 other: A ring-shaped microphone array is di~pogpd grouNd
3 the noise source and the acoustic cancali.~tio~ signalq
4 which is produced by the actuator, to gene~ata an error
signal proportional to the degree to which ca~cel~.~tion hao
6 not occurred. 'Phis error signal is then ~t~pd ~o control
7 the signal produced by the actuator. Sansirl~ of ~ha souNd
a pressure within the tubular member is done w$~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.
22
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 '
-11-
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 $.llu~strate
9 only one specific embodiment of the in5~s~lti~p aid
l0 whi.ch:-
11
1z ' Figure 1~ is 'a tOp plan View in CrOB~1 ~~..'Ct~..On C)f ail
13 engine muffler constructed in accordapce Wit~1 tlla
14 present invention; ,
Figure 2 is a view along lines 2-2 0 ~ Figure 1:
16 Figure 3 is a view along lines 3-3 of Fic~urs 1 showing
1~ the construction of the muffler in crass sec~.ion;
lg Figure 4 is a transverse cross-sectional vi~eW of the
lg muffler illustrated in Figure 1 along the likes 4-~ of
2Q Figure l;
21 Figure 5 is a detail along lines 5-5 of Figyre 2
22 illust rating the construction of a microphone assembly
23 useful in conjunction with the muffler of Figure l;
2~ Figure 6 is a cross-sectional view along lies (-6 of
Figure 2 illustrating the placement of a microphone
26 within the m:Lcrophone assembly%
~,~~~'~-
-12-°
1 Figure 7 is a block diagram of the inventive ryst~m;.
2 Figure 8 is a diagrammatic v$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~ .
13
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.
24
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.
2
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.
9
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-
1
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.
16
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
--15-
1 forward wall 54 are both made of synthetic m~teri~l such as
2 that of outer casing 12.
3
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
~ ~~,
-16-
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.
8
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.
16
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.
26
~~~"~~'~
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.
19
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.
4
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,
22
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,
".
-20-
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
7
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.
14
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.
19
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 ,
-21-
1 circular microphone assembly 72 Will have ~ ~.ess serious
2 impact on system operation.
3
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.
5
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.
22
23
~Aa~ r~ ~~
-22-
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
