Electrical audio signal processing systems and devices – Acoustical noise or sound cancellation – Adjacent ear
Patent
1983-06-17
1984-12-25
Schreyer, Stafford D.
Electrical audio signal processing systems and devices
Acoustical noise or sound cancellation
Adjacent ear
381 94, G10K 1116
Patent
active
044908415
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an improved method 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. The invention also extends to vibration nulling apparatus based on the said improved method.
The invention can be applied equally to the cancellation of repetitive vibrations or random vibrations.
Earlier vibration cancellation methods used direct feedback from a residual vibration sensor, influenced by both the primary and secondary vibrations, which acted both to sense the uncancelled vibrations and to provide a null point for a negative feedback system. These known methods were quite limited in applicability because they provided only local cancellation in a region where the vibration field might be changing rapidly with position. The residual sensor could not be moved to a more uniform vibration (e.g. sound) field, because the additional delay thereby introduced into the feedback system would cause instability in the feedback loop, or an unnacceptably poor cancellation performance.
It has also been proposed (see UK Application No. 1,577,322), that where the primary vibrations are of a repetitive nature, the waveform of the required cancelling, or secondary vibrations can be synthesised from waveform elements synchronised with the repeat cycle of the primary vibrations. The most important feature of such a repetitive cancelling method is that by generating a synchronised waveform for cancellation, the feedback can be applied on the following repeat cycle, thus providing a capability to compensate for the inherent acoustic delay. Heretofor a time domain approach has been used for adjusting waveform elements of the secondary vibrations to minimise the power or the waveform of the residual vibrations at the null point.
In cases where the response from the source of the secondary vibrations (cancelling actuator) to the residual vibration sensor is particularly difficult (in terms of its phase response) considerable advantages can be gained by using a frequency domain approach. Earlier attempts to do this (see FIG. 1 of the accompanying drawings) involved separating the measured output of a residual sensor 1 into a number of frequency bands using frequency domain analogue fulters 2a, 2b, 2c (etc.), correcting the amplitude of each frequency band with adjusters 3a, 3b, 3c and re-combining in a summer 4 to produce a compensated output waveform fed to the source 5 of the secondary vibrations.
FIG. 1 shows a case where the primary vibrations (P) are noise travelling in a gas-filled duct D, but the principle applies equally to free-space noise transmissions, or vibrations transmitted through solid bodies.
According to one aspect of the invention a method for cancelling vibrations comprises sensing the residual vibrations resulting from interference between primary vibrations from a source of vibrations and secondary vibrations from a driven actuator, transforming the sensed residual vibrations into a plurality of independent pairs of components which together define the residual vibrations at a plurality of different locations in the frequency domain, separately modifying said independent components representative of each different frequency domain location, transforming the independent pairs of components back into a drive signal for the actuator and controlling the separate modification of the independent pairs of components to reduce the power or amplitude of the residual vibrations.
The independent pairs of components defining the residual vibrations at each of the different locations in the frequency domain can be the amplitudes of the real and imaginary sinusoidal components or the amplitude of the real sinusoidal component and a phase component.
Applying the method of the invention to the cancellation of repetitive primary vibrations it is possible to synchronise the defining pairs of components for each frequency location to the fundamental frequency of the repetitive primary vibration
REFERENCES:
patent: 3662108 (1972-05-01), Flanagan
patent: 4066842 (1978-01-01), Allen
Journal of the Acoustical Society of America, vol. 70, No. 3, Sep. 1981, (New York, US), Burgess: "Active Adaptive Sound Control in a Duct: A Computer Simulation.
Chaplin George B. B.
Smith Roderick A.
Dwyer James L.
Schreyer Stafford D.
Sound Attenuators Limited
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