Active sound and/or vibration control

Details Active sound and/or vibration control Active sound and/or vibration control

1992-11-10

1994-11-15

Isen, Forester W.

Electrical audio signal processing systems and devices

Acoustical noise or sound cancellation

Adjacent ear

G10K 1116

Patent

active

053655940

DESCRIPTION:

BRIEF SUMMARY
FIELD OF INVENTION

This invention relates generally to systems for controlling sound or vibration, and more especially to active control systems which use a plurality of actuators to produce a controlling sound or vibration field and a plurality of sensors to measure the residual field.
In contrast to previous systems aimed at controlling periodic sound or vibration, the system of the invention can be used even when the fundamental period of vibration is changing rapidly. For example, it can be used to control the engine noise in the interior of a vehicle.
The improved method in accordance with the invention uses orthogonal transformations to,reduce a multichannel control system to a series of single channel systems and provides a method by which the output of each such system can be adapted to maintain good performance of the control system even when the fundamental frequency of the vibration or sound source is changing.


BACKGROUND OF THE INVENTION

The principles of active sound and vibration control have been known for many years and there is a wealth of published literature. Most patent specifications in this field relate to methods applicable to particular situations. The method and system described herein relate to the control of periodic or almost periodic sound and vibration. The two main approaches to this problem are:
(i) Wave shaping or filtering, eg U.S. Pat. No. 4,506,380 and published UK Patent Application No. 2,201,858, where a reference signal containing one or more frequencies of the unwanted sound and vibration is filtered to produce the signals to send to actuators which in turn produce the desired sound or vibration.
(ii) Waveform synthesis, where a waveform-generator istriggered by a signal derived from the source, eg UK Patent Specification No. 1,577,322.
The two methods are equivalent only if the vibration source is exactly periodic. If the source characteristics are changing in time it is usual to use an adaptive control system in which sensors in the region to be controlled sense the residual sound or vibration and pass the information to a processor which alters the filter coefficients or the synthesized waveform so as to provide better control. Published UK Patent Application No. 2,201,858 describes methods for adapting filter coefficients. UK Patent Specification No. 1,577,322 recognises the need for adaption and a later patent specification, UK Patent No. 2,107,960, describes a simple technique for such a system using a single actuator and sensor. This latter patent specification does not explain how to control vibration where the period is changing, except to suggest that in this case the transform technique should produce frequency components from the lowest expected frequency to the highest, rather than just at frequencies corresponding to the harmonics of the source.
A further Patent Specification No. 2,122,052, uses a waveform synthesis technique for vibration control. In this method a sensor and actuator are placed at each of a number of locations. This results in a system with equal numbers of sensors and actuators and a method for adapting the waveform is presented for this special case. In most applications, however, the sources and sensors are not colocated and usually more sensors than sources are used in an effort to obtain a better measure of the resulting sound or vibration.


BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 depict equations described in the specification.
FIG. 4 depicts an embodiment of the invention.


THE INVENTION

The theoretical background to the present invention will now be described. The numbered mathematical equations referred to are set out in accompanying drawings.
The signal from each of a plurality of sensors is sampled using an analogue to digital converter (ADC) triggered by a signal related to the position of the source in its cycle. The data may be averaged over several cycles to improve accuracy. This gives an almost periodic sequence to which an orthogonal transform, such as the discrete Fourier transform, can be applied. This p

REFERENCES:
Nolet, "Solving or Resolving Inadequate and Noisy Tomographic Systems," J. Computational Physics, 1985, 61, No. 3, pp. 463-482.
Natterer, "Numerical Treatment of Ill-Posed Problems. Inverse Problems." Springer, Berlin-N.Y.; 142-167. Seines: Lecture Notes in Math, 1225, 1986.
Hansen, "The Truncated SVD as a Method for Regularization." Bit, Computer Science Numerical Mathematics, 1987, 27, No. 4, pp. 534-553.

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