Active acoustic attenuation mixing chamber

Active acoustic attenuation apparatus is provided for cancellation of noise from an exhaust pipe (12). A chamber (50) has an input (70) receiving exhaust from the exhaust pipe and directing the exhaust along a flowpath (76) to a chamber output (74). Speakers (88, 90) are transversely spaced and offset from the flowpath and introduce sound into the chamber at a space (56) having a transverse area at least as large as each speaker, to minimize acoustic loading of the speakers and to protect the speakers from the heat of hot exhaust for automotive applications, including electronic mufflers.

BACKGROUND AND SUMMARY 
The invention relates to active acoustic attenuation systems. 
Active acoustic attenuation is accomplished by sound wave interference. 
Undesirable noise is attenuated by the introduction of cancelling sound 
which ideally is a mirror image of the undesirable sound, to thus cancel 
same. 
The present invention provides a chamber for mixing the cancelling sound 
with the undesirable sound. The invention particularly arose during 
development efforts directed toward providing an electronic muffler for a 
motor vehicle exhaust by means of active acoustic attenuation, though the 
invention is not limited thereto.

DETAILED DESCRIPTION 
Prior Attempts 
FIG. 1 shows a top view of the rear portion of an automobile 10 having an 
internal combustion engine (not shown) with an exhaust pipe 12 and muffler 
14 shown in dashed line. Mufflers typically include acoustically 
absorptive or baffling structure for passively attenuating the engine 
exhaust noise. 
FIG. 2 shows an early attempt of applicant to provide an electronic muffler 
using active acoustic attenuation. A mixing chamber 16 has a first inlet 
18 from exhaust pipe 12, and a second inlet 20 for receiving cancelling 
sound, and has an outlet 22 for discharging engine exhaust which hopefully 
has been quieted. Inlet 20 is connected through a funnel-like structure 24 
to a cancelling speaker 26. Funnel-like structure 24 was required to 
couple the larger diametered speaker 26 to the smaller diametered inlet 
20. This arrangement was found to be unsatisfactory because of the 
acoustic loading placed on the speaker due to the reduction in cross 
sectional area of the flowpath for the cancelling sound before it reached 
the mixing chamber 16 to mix with the undesirable sound from exhaust pipe 
12. 
FIGS. 3 and 4 show a second attempt by applicant to provide an electronic 
muffler. A rectangular box-like chamber 30 was provided with openings 32 
and 34 in its sidewalls which substantially matched the diameter of 
respective cancelling acoustic sources, such as speakers 36 and 38. 
Cancelling sound is thus introduced into the box at a space having an area 
at least as large as each speaker, to minimize acoustic loading of the 
speakers. However, it was found that the heat of the hot exhaust along 
flow path 40 from pipe 12 to outlet 42 was detrimental to the speakers 
Preferred Embodiment 
FIGS. 5-9 show preferred embodiments of active acoustic attenuation 
apparatus for cancellation of noise from exhaust pipe 12. A mixing chamber 
50 has left and right sidewalls 52 and 54, FIGS. 5 and 7, facing each 
other across space 56. Each sidewall has an upper generally semi-circular 
portion as shown at 58 in FIG. 6, and a lower generally rectangular 
portion as shown at 60. The mixing chamber has a front wall 62, a rear 
wall 64, a top wall 66, and a bottom wall 68, extending transversely 
between sidewalls 52 and 54 and enclosing space 56. Front wall 62 has an 
opening 70 therein at the bottom thereof adjacent bottom wall 68 and 
providing a chamber input receiving exhaust pipe 12. Input opening 70 has 
a diameter 72 substantially the same as pipe 12. Rear wall 64 has an 
opening 74 therein at the bottom thereof adjacent bottom wall 68 and 
providing a chamber output. Sidewalls 52 and 54 have lower rear extension 
portions 52a and 54a extending rearwardly therefrom adjacent bottom wall 
68. Exhaust flows along a path 76 through the chamber along bottom wall 68 
from opening 70 in front wall 62 at the chamber input to opening 74 in 
rear wall 64 at the chamber output. Each of sidewalls 52 and 54 has a 
circular opening 78, 80, respectively, having an axial centerline 82. The 
upper portion of the circular opening is in the upper semi-circular 
portion 58 of the sidewall. The lower portion of the circular opening is 
in the lower generally rectangular portion 60 of the sidewall 
Opening 70 in front wall 62 and opening 74 in rearwall 64 define flowpath 
76 therebetween which is perpendicular to and offset below centerline 82, 
and preferably below the bottom of circular openings 78 and 80 in 
sidewalls 52 and 54. Lower rear extension portions 52a and 54a have a 
height 84 greater than or equal to the diameter 72 of exhaust pipe 12 and 
preferably less than or equal to the height 86 of the bottom 80a of each 
circular opening in each respective sidewall above bottom wall 68. The 
bottom 80a of opening 80 can be below the top of exhaust pipe 12, while 
still maintaining centerline 82 above the exhaust pipe, though it is 
preferred that bottom 80a of opening 80 be spaced above exhaust pipe 12. 
Speakers 88 and 90 are mounted to the chamber at respective circular 
openings 78 and 80 in respective sidewalls 52 and 54. Speakers 88 and 90 
are directed along axial centerline 82, and introduce sound into chamber 
50 in space 56 to cancel undesirable noise in the exhaust from pipe 12. An 
input microphone 92 senses the input noise in pipe 12, and an output error 
microphone 94 senses the combined output noise. These signals are fed to a 
controller 96 which then outputs correction signals to speakers 88 and 90 
to control the cancelling sound such that the output sound at microphone 
94 is null, or otherwise reduced as desired. It is preferred that 
controller 96 be provided by the active attenuation systems shown and 
described in U.S. Pat. Nos. 4,677,676, 4,677,677, 4,736,431, 4,815,139, 
and 4,837,834, all assigned to the assignee of the present invention, and 
incorporated herein by reference. It is also preferred that the hybrid 
active silencing techniques shown and described in U.S. Pat. No. 
4,665,549, assigned to the assignee of the present invention, and 
incorporated herein by reference, be used as appropriate. 
Axial loading of the speakers is minimized because they introduce sound 
into chamber 50 at space 56 having a transverse area at least as large as 
each speaker. The speaker-mounting sections of the chamber provided at 
sidewalls 52 and 54 at openings 78 and 80 are transversely offset and 
spaced from the exhaust flowpath 76, to protect speakers from the heat of 
the hot exhaust. Speakers 88 and 90 coaxially face each other across space 
56. The axial center line 82 of the speakers is laterally offset and 
spaced from flowpath 76 of the exhaust. Chamber input 70, flowpath 76 and 
chamber output 74 are all rectilinearly aligned. Mixing chamber 50 thus 
has an acoustic source mounting section 78 and/or 80 mounting an acoustic 
source 88 and/or 90 directed along an axial centerline 82 perpendicular to 
and offset from the flowpath 76. Each acoustic source has a facing surface 
88a, 90a interfacing with chamber 50 at an interface lying in a plane 88b, 
90b parallel to flowpath 76. Axial centerline 82 extends perpendicularly 
through the respective planes 88b and 90b and never intersects flowpath 
76. The interfaces at 88a and 90a lie in spaced parallel planes 88b and 
90b parallel to flowpath 76 and define a parallel plane 91 therebetween 
containing flowpath 76. 
Chamber 50 has a height 98 transverse to flowpath 76 and greater than or 
equal to the height 100 of circular opening 80 in the sidewall. In the 
preferred embodiment, height 98 is greater than or equal to the sum of the 
height 72 of exhaust pipe 12 plus the height 100 of circular opening 80 in 
the sidewall, such that the entire opening 80 is transversely offset and 
spaced from flowpath 76. Chamber 50 has a length 102 parallel to flowpath 
76 and greater than or equal to the length of circular opening 80 in the 
sidewall. Tube 12a, which may be the rear section of exhaust pipe 12 
itself or may be a separate connected pipe, is within chamber 50 and 
extends from chamber input 70 and below centerline 82, and preferably 
below the bottom 80a of circular opening 80 in the sidewall, and has a 
rear end 12b stopping short of chamber output 74. 
Chamber 50 has a width 104 transverse to flowpath 76 and substantially 
comparable to the diameter of exhaust pipe 12 such that chamber 50 is not 
significantly wider than the exhaust pipe. Speakers 88 and 90 are mounted 
to the chamber at respective openings 78 and 80 in respective sidewalls 52 
and 54 and extend externally from the chamber. Openings 78 and 80 in the 
sidewalls of the chamber each have a diameter substantially larger than 
the diameter of exhaust pipe 12. As noted above, the height 98 of chamber 
50 is preferably greater than or equal to the sum of the diameter 72 of 
exhaust pipe 12 plus the diameter 100 of the circular opening in the 
chamber sidewall. The space 56 into which sound is introduced from the 
cancelling speakers has a transverse area larger than each speaker. 
In FIG. 8, heat insulating material 106 surrounds tube 12a within chamber 
50. The heat insulating material extends from chamber input 70 along tube 
12a and has a lower portion 106a between tube 12a and bottom wall 68, and 
has an upper portion 106b between tube 12a and axial centerline 82. 
In FIG. 9, an outer tube 108 is concentric to inner tube 12a and extends 
from chamber input 70 and has an air inlet 110 at chamber input 70 for 
receiving air into annular space 112 between inner tube 12a and outer tube 
108 for convective cooling of inner tube 12a. Outer tube 108 extends from 
chamber input 70 along inner tube 12a and directs cooling air along inner 
tube 12a. Outer tube 108 stops short of chamber output 74 and directs 
cooling air as shown at arrows 114 to chamber output 74 substantially 
parallel to flowpath 76. Inner tube 12a and outer tube 108 define the 
noted annular space 112 therebetween having a lower portion 112a between 
flowpath 76 and bottom wall 68, and an upper portion 112b between flowpath 
76 and centerline 82. 
It is recognized that various equivalents, alternatives and modifications 
are possible within the scope of the appended claims.