Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ACTIVE NOISE CANCELLING MUFFLER
Introduction
In implementing a muffler system which relies on active cancellation of
the offensive noise source, problems of packaging and durability are critical.
Other
authors have described arrangements which permit high acoustical outputs over
a
predetermined frequency range in a relatively small package, for example,
U.S. 5,097,923 and PCT/LTS91/02731, "Improvements In and Relating to
Transmission Line Loudspeakers" to Hoge et al. The descriptions below utilize
the
transducers described by the above mentioned documents. The current invention
seeks to add enhancements which improve the packagability and durability of
these
active muffler devices. The importance of durability and low cost in such
systems
cannot be overstated. Passive devices which represent the current state of the
art
are inexpensive and very durable, sometimes performing for decades without
attention of any kind. Therefore, it is essential to utilize the lowest cost,
most
durable system to enhance the operation of active systems.
Background and Background Art
Several authors have described devices which cancel noise propagating
through a pipe or duct. For example, Chaplin in U.S. 4,122,303 and Kato in
U.S. 4,805,733 propose the use of undefined noise sources placed within the
duct
to cause a reflection of the propagated sound. Other authors, for example,
Eriksson
in U.S. 4,665,549 and Angelini et al in U.S. 4,177,874 and Bremigan in
U.S. 5,044,464 define the device being inserted into the duct. A refinement in
these systems is represented by the devices described by Ziegler et al. in
U.S. 5,097,923 and Hoge et al in PCT/LJS91/02731. These patents and
application
describe piping systems in which the active control anti-source is placed
concentric
to the duct and in the plane of the duct outlet. The active source described
in both
cases is a tuned acoustic enclosure which emits high power sound throughout a
limited frequency band. The sound output per unit volume is maximized through
the use of this type of source. Using this type of outlet configuration, the
highest
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possible frequency can be cancelled with the anti-noise source and many of the
environmental problems associated with placing a transducer in a corrosive gas
flow are avoided almost entirely.
The use of noise sources which are placed in close proximity to the
outlet of a pipe has been cited extensively in the technical literature. For
example,
Chaplin in U.S. 4,489,441 and Nelson and Elliott in their book Active Control
of
Sound, 1992, pages 233-244 describe this arrangement. Kido et al. in "A New
Arrangement of Additional Sound Source in an Active Noise Control System" from
Proceedings of Internoise '89, December 1989, pages 483-488, and Hall et al.
in
"Active Control of Radiated Sound from Ducts" from ASME Transactions Journal
of Vibration and Acoustics, July 1992, pages 338-346 describe several
different
pipe outlet configurations. However, these authors propose the use of a very
simple acoustic source or make no mention of the type of active transducer to
be
used.
Attempts to use active sources on mufflers include the work of Cain,
U.S. Patent No. 5,272,286 which shows an active noise cancelling device
surrounding an exhaust pipe in a generally concentric configuration. The
problem
with such an arrangement is the tremendous expense involved in building
something in direct contact with a hot exhaust pipe, the lack of being able to
retrofit the system to existing tailpipes and its enormous bulk as well as
other
problems in its operation. A similar device is shown in Japanese Application,
60-22010, entitled "Exhaust Noise Reducing Device" by Toshiyuki Kaminaga,
published on February 4, 1985. Scherrer, in French Patent No. 1,190,317,
published October 12, 1959 shows a system very much like Cain, supra, where
concentric pipes empty into a mixing chamber. Finally, U.S. Patent No.
4,487,289,
December 11, 1984, entitled "Exhaust Muffler with Protective Shield", shows an
extension fitting over a tailpipe, again like Cain.
Summary of the Invention
The invention relates to the enhancement of active acoustical attenuation
by coupling an engine exhaust pipe with the acoustic exhaust of an active
enclosure.
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When active noise control is applied to an offending noise source, a
secondary source is placed in close proximity to the offensive noise source.
The
secondary source can be placed either around the offensive noise source,
concentrically, or beside the noise source, in a dipole configuration, as long
as the
separation between the two source centers is much smaller than the wavelength
of
the highest frequency of cancellation. The secondary source creates an
acoustic
wave form equal in amplitude and 180 degrees out of phase from the offensive
source. The secondary source is driven by an adaptive controller system that
requires a feedback microphone. The feedback microphone measures the
effectiveness of the destructive interference and is used to adjust the signal
of the
secondary source and optimize cancellation.
In general the invention provides improved coupling between a dipole
oriented engine exhaust and an active enclosure acoustic port. It increases
the
amount of attenuation achievable with a dipole oriented engine exhaust and an
active enclosure acoustic port and decreases the amount of power required to
achieve a certain amount of attenuation for a given active noise cancellation
system.
It also allows for the acoustic port of the active enclosure to be shorter
thereby increasing the acoustic output of the active enclosure and allows for
surface
mounting of an error sensor. The arrangement provides protection to the error
sensor from road debris, provides a way to integrate the error sensor cable
into the
active enclosure to minimize cables and protects the cable by encasing it in a
conduit. The arrangement has a flared port to reduce turbulence and an
internal
heat shield to protect the error sensor or sensing microphone. The duct
extension
can be styled in a variety of shapes.
Accordingly, it is an object of this invention to provide improved
coupling between a dipole oriented engine exhaust and an active enclosure
acoustic
port.
Another object of this invention is to increase the amount of attenuation
achievable with a dipole oriented engine exhaust and active enclosure acoustic
port.
A further object of this invention is to decrease the amount of power
required to achieve a certain amount of attenuation for a given active noise
cancellation system in a dipole orientation.
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A still further object of this invention is to allow the active enclosure to
be mounted further behind the automobile muffler and further from the road
surface.
Yet another object of this invention is to provide a channel which will
allow harmful engine exhaust gases to exit out from underneath the vehicle at
the
regulatory distance.
Additional objects of the invention include:
(i) allowing the acoustic port of the active enclosure to be shorter
thereby increasing the acoustic output of the active enclosure;
(ii) allowing a surface for mounting the error sensor;
(iii) providing protection to the error sensor from road debris and
foreign matter;
(iv) providing a way to integrate the error sensor cable into the active
enclosure so that the active enclosure and the error sensor may be powered
from
one input cable;
(v) providing protection to the error sensor cable by enclosing the
cable in a built in conduit which mates with the active enclosure; and
(vi) providing an internal heat shield to protect the error sensor from
extreme exhaust gas temperatures.
In accordance with one aspect of the present invention there is provided
an active muffler noise cancellation system for use on stationary or vehicle
applications which involve an exhaust pipe, said system comprising: an active
noise enclosure; an active noise attenuator in said active enclosure adapted
to
produce a counter noise wave to cause destructive interference with a noise
wave
emanating from said exhaust pipe; an adaptive controller connected to said
active
noise attenuator; a counter noise wave and outlet pipe, said outlet pipe
adapted to
terminate immediately adjacent said tailpipe at an acute angle thereto, so
that
exhaust gas pulses emitted from said outlet pipe form an acoustic dipole to
thereby
attenuate noise coming from said tailpipe; and a transducer listening means
associated with said outlet pipe and counter noise source and adapted to be
located
in a plane generally perpendicular to a plane equidistant from the acoustic
centers
of the exhaust outlet and outlet pipe.
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In accordance with another aspect of the present invention there is
provided an active noise cancelling muffler system for use on stationary or
vehicle
applications which involve an exhaust pipe, said system comprising: an active
noise enclosure; an active noise attenuator in said active enclosure adapted
to
5 produce a counter noise wave to cause destructive interference with a noise
wave
emanating from said exhaust pipe; an adaptive controller connected to said
active
noise attenuator; an extension duct connected to said active enclosure through
a
port and adapted to receive a terminus of said exhaust pipe so as to receive
both
said exhaust pipe gases, said noise and said counter noise at one end of said
extension duct, said noise and counter noise combining to form a single plane
wave
at an opposite end of said extension duct; and a transducer listening device
on said
extension duct and adapted to provide a residual signal to said adaptive
controller
to allow it to adjust said active noise attenuator to provide the necessary
counter
noise; wherein a dipole is created where the noise and counter noise enter
said duct
extension, the shape of said duct extension, the shape of said duct extension
forcing
said dipole pattern into a plane wave adjacent said transducer listening
device.These
and other aspects will become apparent when reference is had to the
accompanying
drawings and the detailed description below.
Brief Description of the Drawings
Figure 1 is a plan view of one embodiment of this invention.
Figure 2 is a perspective view of the embodiment of Figure 1.
Figure 3 is a diagrammatic view of the speaker enclosure.
Figure 4 is an end view of the speaker enclosure of Figure 3 showing its
relationship to a tailpipe.
Figure 5 shows an alternative embodiment of the speaker enclosure.
Figure 6 is a perspective view of a third embodiment of the invention.
Figure 7 is a cross-sectional view of the port connection of Figure 6.
Figure 8 is a cross-sectional view of the speaker enclosure of Figure 6.
Figure 9 is a block diagram of the control system.
Figure 10 is a partial perspective view of a heat shield/air vent/cable
conduit.
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Description of the Invention
The proposed invention utilizes basic configurations similar to those
described by Ziegler et al. and Hoge et al. in the above mentioned patent and
patent application. The instant device, however, instead of being arranged
concentric with the pipe is non-integral with the pipe as shown in Figure 1.
Anti-source, 2, is placed such that the outlet, 3, is placed near the outlet
of pipe, l,
connected to passive muffler, 15, which contains a flow of gas containing
pressure
pulsations. Passive muffler, 15, is used to reduce noise at frequencies above
the
capability of the active anti-source. Active transducer means, 2, consists of
outlet
acoustic mass, 4, acoustic compliances 5 and 6, speaker driver, 7, and,
optionally,
an acoustic mass, 8. Figure 2 shows a variation. Figure 2 shows the two
outlets
from the end. If a microphone, 9, is placed on the plane, 10, between the
pipe, l,
outlet and the active source outlet, 3, the electronic controller will cause
the two
sources to form an acoustic dipole. A dipole has a directional radiation
pattern, but
if the acoustic centers of the two sources are within approximately one tenth
of a
wavelength the minimum cancellation will be approximately 10 decibels. This
minimum will occur along the line through the centers of the source and anti-
source. For this reason, it is sometimes advantageous to orient the two
sources
above and below each other, as shown in Figures 3 and 4, since microphones or
listeners are less likely to be located above or below the sources if the
device is
mounted on a vehicle. However, 10 decibels is generally sufficient to result
in
what is perceived to be a significant reduction in the noise and is sufficient
to
reduce the offensive tone to the level of the other system noise sources.
Since a
passive element, 15, is generally used with this type of active source to
eliminate
the high frequency sound, the one-tenth wavelength rule will rarely be
violated in
practice.
There are several advantages to this orientation of active sources and the
use of this type of source. First, the active source can be located remotely
from
the hot exhaust pipe. This increases the potential that packaging solutions
can be
found, particularly on automobiles, in which the space limitations are severe.
More
importantly, though, the remote location of the active source allows different
materials to be used in the construction of the active source to save weight,
reduce
cost and improve durability. For example, whereas the challenges of using
plastic
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to construct the anti-noise source were severe when the source was in direct
contact
with the exhaust pipe, the use of plastic is a simple matter with the new
outlet
arrangement.
The active source now can be disguised as a traditional "dual" exhaust
package, which reduces the possibility consumers will react negatively to its
appearance. The non-integral active muffler can now be placed within the
vehicle's trunk if necessary and its use in what were near-impossible
applications is
now easier. For example, marine mufflers, in which a flow of water is mixed
with
the hot gases are now possible without exposing the active transducer to
water.
The transducer can be mounted above the waterline.
One alternative arrangement is shown in Figure 3 in which the non-
integral active muffler outlet is pointed 90 degrees away from the pipe
outlet. In
this manner, the acoustic centers of the two noise sources can be moved closer
together to extend the upper frequency limit of the system. Other outlet
arrangements and shapes are similarly possible and will be obvious to those
skilled
in the art.
Figure 6 shows the perspective of a third preferred embodiment of the
invention. The apparatus, generally denoted as 40 has a speaker enclosure 41
with
a connecting port 42 to duct extension 43. An opening in extension 43 is
adapted
to receive the end of tail or exhaust pipe 44 and be secured thereto by an
annular
clamping means 45 which is similar to a pipe clamp. Port 42 and pipe 44 enter
duct extension 43 side by side so as to create dipole radiation of noise. Duct
43
alters and compresses this radiation into a plane wave which is sensed by
microphone 46 as it exits the open end 47 of duct extension 43. Tailpipe 44 is
connected via clamp 48 to straight through muffler 49. The diameter of port 42
is
at least as large as the diameter of tailpipe 44. The cross-sectional area of
duct
extension 43 is such that the cut-on frequency of non-plane wave behavior or
propagation is above the operating frequency of the controller and its length
1 is 2
to 3 times its reduced circular cross section. Dimension "a" should be as
short as
possible to reduce acoustic mass.
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An extension duct 43 is fitted over the end of both the engine exhaust
pipe 44 and the acoustic port 42. The extension duct 43 allows for better
coupling
between the exhaust noise and the anti-noise emitted from the active enclosure
acoustic port 42. The results of this improved coupling is increased
acoustical
attenuation using less electrical power. These improvements are very important
for
active noise cancellation systems on automobiles since the enclosures are
small and
less powerful for packaging purposes and the available electrical power for
the
system is limited.
The extension duct 43 also channels the harmful exhaust fumes and
allows the fumes to exit out from underneath the car at the regulatory
distance.
This feature allows the active enclosure to be positioned further underneath
the car,
from the bumper and from the road surface.
Since the extension duct 43 channels the exhaust fumes from under the
car, both the engine exhaust pipe 1 and the acoustic port 42 can be much
shorter.
Shortening the acoustic port 42 reduces the acoustic mass of the port 42 which
increases the acoustic output of the active enclosure 41. An increase in
output
power is always desirable.
The instant invention further contemplates the use of a duct extension 43
into which both the anti-noise and exhaust pipe noise enter, are mixed and
conformed to a plane wave and a sensing microphone samples the resulting plane
wave noise to cause adjustments to be made, if needed, to the control means.
The
duct extension 43 is connected via a port 42, usually the same size or larger
than
the exhaust pipe 44, to an enclosure containing the active noise cancelling
speaker
53 with accompanying front 52 and back cavities 57. A port 42 connecting the
cavities may be provided. The dimensions and relationship between the cavities
and the speaker are generally dictated by the theory and desired response
curves as
discussed by A.N. Thiele, "Loudspeakers in Vented Boxes; Part 1 ", Journal of
the
Audio En ing eerin~ Society, March 1961, pages 181-191 and "Closed-Box
Loudspeaker Systems Part II: Synthesis", Journal of the Audio Engineering
Society, pages 282-289, by Richard H. Small.
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Generally, the size of the duct extension 43 must be such that its cross-
sectional area is large enough so that the cut-on frequency of non-plane wave
(or
higher order) is above the operating frequency of the active noise controller.
Once
this criteria is established and met the actual shape of the duct extension 43
is not
that important even though it determines what the cut-on frequency will be.
The
end of the duct extension 43 can be beveled or styled in any number of ways to
conform to, e.g., an automotive style.
The length of the duct extension 43 is preferably greater and from two to
three times its reduced circular cross-section diameter. The extension changes
the
dipole radiation into a plane wave which is then sensed by a residual
microphone
which inputs to the controller how well the device is working and causes it to
rapidly adapt to achieve full attenuation.
The diameter of the port 42 connecting the enclosure with the duct
extension 43 should be at a minimum as large as the exhaust pipe 44 diameter
and
should be relatively short to reduce acoustic mass.
The control system for the invention may use the sync control described
in U.S. Patent No. 4,490,841 to Chaplin, the control described in U.S. Patent
No.
5,105,377 to Ziegler or that described in co-pending PCT Application Serial
No.
PCT/LTS92/05228, entitled, "Control System Using Harmonic Filters". All these
control systems use a residual microphone and a sync to an engine or motor.
The
control system also may use the digital virtual earth/adaptive feedforward
system
described in U.S. Patent No. 5,475,761, entitled "Adaptive Feedforward and
Feedback Control System". In such a case, no sync is required but a second
microphone is used to sense the exhaust noise upstream.
Figure 7 shows the inside of port 42 to be flared as at 50 to reduce flow
turbulence.
Figure 8 shows a cross-sectional view of speaker enclosure 41 with rear
cavity 57, front cavity 52 and speaker 53. If required, a second speaker 54
may be
added. A port 55 may also be provided to make the arrangement behave as a 6th
order speaker as described in PCT/US91/02731.
The control system is shown generally in Figure 9 with controller 60 and
amplifier 61 driving speaker 53 in enclosure 41. Power supply 62 is connected
to
controller 60 as is residual microphone 46. If the system is using only a
residual
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microphone a sync connection 63 to an engine flywheel 64 or the like is
necessary.
If no sync is used a digital virtual earth or an adaptive feedforward system
with an
upstream sensing microphone 65 can be used.
Figure 10 shows a combination hollow heat shield and conduit unit 70
mounted atop duct extension 43 and containing a cable 71 to microphone 46,
enclosed by conduit unit 70 which also has vent holes 72, 73 to allow outside
air to
ingress and egress to cool microphone 46. The unit also protects microphone 46
from road debris and the like. The conduit unit 70 is generally mounted on
heat
shield 74 which is held in a spaced relationship to duct extension 43 by
spacers 75.
This allows for further heat relief of microphone 46. Conduit 70 has two
passageways, one for the cooling air and one for the cable 71.
P
