Active noise reduction

Details Active noise reduction Active noise reduction

1993-12-06

1995-07-18

Isen, Forester W.

Electrical audio signal processing systems and devices

Acoustical noise or sound cancellation

Adjacent ear

G10K 1116

Patent

active

054349257

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

This invention relates to apparatus for cancelling vibrations, which may be gas, liquid or solid-borne, by nulling primary vibrations, at least in part, with specially generated cancelling or secondary vibrations.
It has been proposed to cancel repetitive noise produced by e.g. an internal combustion engine, by generating cancellation noise with a controller such that the cancellation noise at least in part cancels the repetitive noise produced by the engine. In order to maintain the amplitude and phase of the cancellation noise in opposition to the noise itself, a detector in the form of a microphone is used to determine the residual noise or error between the cancellation noise and the noise itself. The error signal is used to control the output of the controller in accordance with a predetermined algorithm for maintaining a desired cancellation condition. Also, in order to allow the system to track changes in fundamental frequency of the noise, a synchronisation sensor is utilised to provide an indication of the fundamental frequency of the noise e.g. from the speed of rotation of the engine. An example of such an arrangement is disclosed in U.S. Pat. No. 4,417,098 (Chaplin et al). The apparatus may operate in a number of different frequencies each comprising a different harmonic of the fundamental frequency so as to produce cancellation over a broad frequency band.
The algorithm for the controller operates so that samples of the error signal are compared with one another and the output of the controller is modified in accordance with the algorithm to produce a iterative reduction of the error signal. However, as will be described in more detail hereinafter, the iterative technique utilised has a tendancy in conditions close to cancellation, to become unstable as a result of the error signals becoming small, so that the differences between successive error samples can contain a large percentage error that can drive the controller to produce a uncancelled condition.
In accordance with a first aspect of the invention, there is provided improved apparatus for cancelling vibrations wherein the controller is operated in accordance with an improved algorithm that is less prone to drive the apparatus out of the cancelled condition.
In accordance with a second aspect of the invention, there is provided improved noise cancellation apparatus implemented by means of an add-on unit for a personal computer (PC). The apparatus is conveniently embodied as a first circuit board for insertion into the expansion slot of a conventional PC, and a second circuit board driven by the first board and conveniently external to the PC, the second board including an output for a cancellation noise signal, an input for a noise error signal and an input for an external synchronisation signal. Also, software is provided, typically on a floppy disc, to configure the PC processor to perform cancellation in accordance with a predetermined algorithm.
The apparatus may also include means for producing demonstration noise waveforms to be cancelled. Such waveforms may simulate the noise produced by e.g. a helicopter or a motor vehicle so that the apparatus can be used as a demonstration device for active noise cancellation.
In order that the invention may be more fully understood an embodiment thereof will now be described by way of example with reference to the accompanying drawings, in which:
FIG. 1 is a schematic block diagram of an apparatus according to the invention for the active cancellation of repetitive noise from a machine;
FIG. 2 is a schematic functional diagram for the signals discussed in relation to FIG. 1;
FIG. 3 is a schematic block diagram of an example of the invention for use with a personal computer; and
FIG. 4 is a more detailed schematic block diagram of first and second circuit boards used in the apparatus of FIG. 3.
Referring firstly to FIG. 1, an active noise reduction apparatus is shown for cancelling noise from a repetitive source such as an internal combustion engine 1. Noise from the engine 1 is c

REFERENCES:
patent: 4417098 (1983-11-01), Chaplin et al.
patent: 4582963 (1986-04-01), Danstrom
patent: 4782459 (1988-11-01), Johnston
patent: 4894820 (1990-01-01), Miyamoto et al.
patent: 4937813 (1990-06-01), Schenk
patent: 5359656 (1994-10-01), Kim

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