Method and device for active noise reduction in a local area

Details Method and device for active noise reduction in a local area Method and device for active noise reduction in a local area

1995-03-21

1996-09-24

Kuntz, Curtis

Electrical audio signal processing systems and devices

Acoustical noise or sound cancellation

Adjacent ear

381 94, 381 83, 381 93, 381 731, A61F 1106, H03B 2900, H04R 2700

Patent

active

055598935

DESCRIPTION:

BRIEF SUMMARY
The invention concerns a method for active noise reduction in a local area in accordance with the introduction of claim 1. The invention also concerns a device for active noise reduction in a local area in accordance with the introduction of claim 9.
There is a known method of using an active noise reduction based on sound waves destructive interference in order thereby to reduce the energy in a sound field. A so-called cancelling sound source is used for producing a sound field with the same spectrum as the sound field which is to be suppressed, but opposite in phase thereto. When the amplitude of the two sound fields is identical, the result will ideally be a total suppression of the sound energy by phasing it out. The problem is to find the cancelling sound field which provides optimum noise reduction or noise suppression. The more acoustic dimensions there are in which the sound waves are propagated, the more difficult this problem becomes. In the space domain there will always be three acoustic dimensions.
By the use of active noise reduction based on destructive interference, the sound field which is required to be suppressed is detected by a special microphone arrangement, and after signal processing, the detected microphone signals are transmitted with the correct amplitude and phase to a loudspeaker which acts as the noise-cancelling sound source. In order that the noise cancellation should be effective, the sound which is detected by the microphone arrangement and the sound from the loudspeaker must be coherent, i.e. the distances between microphones, loudspeaker and the area in which the noise reduction or cancellation are to take place must be small. The problem is that small distances between microphone and loudspeaker which are connected in an electrical network will normally result in acoustic feedback, so-called howl.
For instance, U.S. Pat. No. 5,133,017 (Cain et al.) discloses a noise cancellation system providing a localized zone of noise suppression in the vicinity of, e.g., an individual person. This system uses a pair of loudspeakers--one for each ear--and a number of microphones to obtain a cancellation signal which is delivered to the loudspeakers. No distinction is made between the near field and the far field and the system does not address the problem of acoustic feedback.
Attempts have also been made to generate a local noise-suppressed area or a so-called quiet zone by using a reference signal. If, e.g., it is a case of noise from a rotating machine, which is common in cars and aircraft, the reference signal can be generated on the basis of the RPM of the rotating machine and the cancelling signal then generated on this basis. Thus the problem of feedback is avoided, but a system of this kind will only be capable of reducing noise which comes from the source of the reference signal and these sources should preferably be of such a nature that they limit a pure tone. This means that in practice this concept for active noise reduction is limited to noise from rotating machinery.
A further problem with active noise reduction in a local area is that the sound, i.e. the noise, is amplified in other areas. This will be a problem particularly in a noise reduction system which, e.g., is installed in a passenger seat, since noise reduction in one spot, i.e. in a passenger seat, can result in the noise being amplified in the area of the neighbouring seat.
The object of the present invention is to provide a method and a device for active noise reduction in a local area, whereby the above-mentioned problems are essentially eliminated.
This object is achieved with a method which is characterized by the features disclosed by claim 1 and a device which is characterized by the features disclosed by claim 9. Further features and advantages of the method and device according to the invention are disclosed by the dependent claims 2-8 and the dependent claims 10-14 respectively.
The method and the device according to the present invention will now be explained in more detail in connection with an e

REFERENCES:
patent: 4489441 (1984-12-01), Chaplin
patent: 4977600 (1990-12-01), Ziegler
patent: 5003606 (1991-03-01), Bordewijk
patent: 5133017 (1992-07-01), Cain et al.
patent: 5193117 (1993-03-01), Ono et al.
patent: 5400409 (1995-03-01), Linhard

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