Electrical audio signal processing systems and devices – Acoustical noise or sound cancellation – Adjacent ear
Patent
1994-05-26
1997-06-10
Kuntz, Curtis
Electrical audio signal processing systems and devices
Acoustical noise or sound cancellation
Adjacent ear
381 94, A61F 1106
Patent
active
056384548
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to noise reduction systems.
BACKGROUND TO THE INVENTION
In the past unwanted noise and vibration has been controlled by muffling or isolation. However, the principle of superposition means that noise and vibration can also be controlled by means of so-called "anti-noise", that is the production of an acoustic signal having the same spectral characteristics as the unwanted noise or vibration but 180.degree. out of phase.
U.S. Pat. No. 4,527,282 discloses a system where a speaker generates a cancelling acoustic signal which is mixed with an unwanted acoustic signal. A microphone senses the residual acoustic signal which is then amplified and inverted to drive the speaker. Systems of this type are prone to instabilities and are restricted in the range of frequencies over which they are effective.
A system which avoids the instability problems of simple systems, such as that disclosed in U.S. Pat. No. 4,527,282, is described in U.S. Pat. No. 4,490,841. In the described system, the residual signal is analysed by means of a fourier transformer. The resultant fourier coefficients are then processed to produce a set of fourier coefficients which are then used to generate a cancelling signal.
Systems which process signals in the frequency domain, following fourier transformation, perform their function well under steady-state conditions. However, if the fundamental frequency of the noise signal changes, the system requires several cycles to re-establish effective cancellation. This is due to the time taken to perform the fourier transformation. If such apparatus is used in an internal combustion engine noise control system, bursts of noise will occur during acceleration and deceleration. These bursts may, in fact, have a higher peak value than the unsuppressed steady-state engine noise. Furthermore, the need to carry out high-speed digital signal processing means that these systems are expensive to implement.
SUMMARY OF THE INVENTION
It is an aim of the present invention to overcome the above disadvantages associated with prior art noise control systems, which process signals in the frequency domain, whilst avoiding the stability problems that have bedevilled simple feedback systems. Surprisingly, it is not recourse to ever more sophisticated, and expensive, digital signal processing which provides a key to overcoming the aforementioned disadvantages in accordance with the invention.
The present invention provides an apparatus for the cancellation of noise or vibrations, comprising: means for producing an electrical error signal representative of the sum of the instantaneous amplitudes of an unwanted periodic acoustic signal and a cancelling acoustic signal; filtering means for filtering the electrical error signal to produce an electrical cancelling signal comprising the filtered electrical error signal; means responsive to the electrical cancelling signal to produce the cancelling acoustic signal for cancelling the unwanted periodic acoustic signal; and control signal generating means for generating a control signal, harmonically related to the unwanted periodic acoustic signal; wherein the filtering means includes a tunable bandpass filter means for filtering the electrical error signal, the filter means being tuned, in response to the control signal, so as to maintain within its passband a frequency harmonically related to the unwanted periodic acoustic signal. Additionally, the gain at resonance of the filter means maybe reduced as a function of the fundamental frequency of the unwanted periodic acoustic signal.
Advantageously, a plurality of narrowband bandpass filters may by provided, tuned to harmonically related frequencies. Preferably, these filters due implemented using switched-capacitor filter techniques.
However, other conventional techniques such as LC filters, using inductors or gyrators, comb filters, transposing filters or digital filters may usefully be employed. If a very high Q switched-capacitor filter is used, a servo loop may be req
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Jones Owen
Trinder Michael C. J.
Chang Vivian W.
Kuntz Curtis
Noise Cancellation Technologies, Inc.
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