WO1994022403A1

WO1994022403A1 - Non-integral active muffler

Info

Publication number: WO1994022403A1
Application number: PCT/US1993/002596
Authority: WO
Inventors: Scott Miller; J. Clay Shipps
Original assignee: Noise Cancellation Technologies, Inc.
Priority date: 1993-03-24
Filing date: 1993-03-29
Publication date: 1994-10-13

Abstract

An active noise cancellation system for mufflers having an anti-noise source (2) connected to an outlet pipe (3) that terminates adjacent the end of a tailpipe (1) in the same plane or at an acute angle thereto.

Description

NON-INTEGRAL ACTIVE MUFFLER

Introduction

In implementing a muffler system which relies on active cancellation of the oflfensive 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 can not 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 Prior 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 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/US91/02731, both of which are herein incorporated by reference. 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 minimized through the use of this type of source. Using this type of outlet configuration, the highest possible frequency can be canceled 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. Dec. 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.

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, 1, 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 2a shows the two outlets from the end. If a microphone, 9, is placed on the plane, 10, between the pipe, 1, 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 Figure 2, 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 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.

Having described the invention and the preferred embodiments attention is directed to the claims. It will be apparent that changes and modifications to the invention can be made without departing from the scope of the appended claims.

Claims

1. An active noise cancellation muffler system which includes a standard tailpipe, an active noise canceling means including a controller means, an anti-noise source having acoustic compliance spaces, acoustic masses, a counter noise source and an outlet pipe, the system being characterized in that the tailpipe and outlet pipe terminate approximately adjacent to one another so that exhaust gas noise pulses emitted from said tailpipe and anti-noise pulses emitted from said outlet pipe form an acoustic dipole to thereby reduce the overall noise being emitted from said tailpipe.

2. A system as in claim 1 and including a listening means connected to said controller means and located proximate to said adjacent tailpipe and outlet pipe and on a plane between said pipe outlets.

3. A system as in claim 2 wherein said tailpipe and outlet pipe are located adjacent one another in a horizontal plane.

4. A system as in claim 2 wherein said tailpipe and outlet pipe are located adjacent one another in a vertical plane.

5. A system as in claim 2 wherein said anti-noise source is atop said outlet pipe.

6. A system as in claim 5 wherein said outlet pipe and tailpipe are located adjacent one another in a vertical plane.

7. A system as in claim 2 wherein said outlet pipe and said tailpipe terminate adjacent one another at an acute angle.

8. A system as in claim 7 wherein said listening means is located on a plane bisecting the acute angle between the outlet pipe and tailpipe.

9. A system as in claim 7 wherein said angle is 90°.

10. A system as in claim 7 wherein said system is silencing watercraft or other marine equipment.