Electrical audio signal processing systems and devices – Acoustical noise or sound cancellation – Adjacent ear
Patent
1983-07-25
1986-01-21
Dwyer, James L.
Electrical audio signal processing systems and devices
Acoustical noise or sound cancellation
Adjacent ear
381 94, F10N 106
Patent
active
045661189
DESCRIPTION:
BRIEF SUMMARY
It is known from U.S. Pat. No. 4,153,815 that repetitive vibrations (e.g. noise) emanating from a source of such vibrations can be at least partly nulled at some selected location (which may or may not be close to the source) by feeding to that location a specially generated secondary vibration which is synchronised to the source. If the source is a piece of machinery (e.g. an engine), the generation of the required waveform for the secondary vibration can be synchronised by a triggering signal extracted from the machinery (e.g. by using a magnetic or optical sensor placed close to a toothed wheel forming part of the machinery). With the secondary vibration locked to the primary vibration by the triggering signal, generation of the necessary secondary vibration to optimise the cancellation at the selected location, requires an adjustment of the waveform of the secondary vibration and this can be effected by a variety of different algorithms, the simplest of which would be a trial and error approach based on a monitoring of some parameter of the residual vibration sensed at the said location.
This invention relates to a method of and apparatus for cancelling vibrations from a source of repetitive vibrations which does not require a triggering signal to be extracted directly from the source of the primary vibrations.
According to one aspect of the invention a method of generating a synchronising signal for an active vibration cancelling system in which a primary vibration, from a source of repetitive vibrations, entering a location is at least partially nulled by a specially generated secondary vibration fed to the location, the synchronising signal being used to synchronise the secondary vibrations to said source, is characterised in that the synchronising signal is obtained from the output of a vibration sensor located at said location and influenced there by both the primary and secondary vibrations.
In one arrangement, the output from the residual vibration sensor can be monitored to extract therefrom a component (e.g. a low-frequency component) which has a repetition rate locked to the repetition rate of the source of the primary vibration, the monitored component being used to generate the synchronising signal.
Where, as could often be the case, it is desired to null all the frequency components of the primary vibration at the desired location, the arrangement described above will be in danger of losing synchronisation as the cancellation becomes increasingly successful, and it may then be desirable to reconstruct the primary vibration that is being nulled by adding to the residual signal a component derived from the secondary vibration source.
According to a further aspect of the invention, apparatus for cancelling a primary vibration entering a location from a source of repetitive vibrations, using a waveform generator synchronised to said source to generate a secondary vibration which is fed to said location and a vibration sensor in the location to sense the residual vibration remaining after the primary and secondary vibrations have interacted in the said location, is characterised in that the apparatus includes circuit means for deriving a synchronising signal for the waveform generator, which circuit means receives an input from said residual sensor.
The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:
FIG. 1 is a schematic representation of a prior art apparatus for cancelling repetitive noise, and
FIG. 2 to 5 are schematic representations of four different embodiments of apparatus according to the invention.
Referring to FIG. 1, in a known arrangement, a machine 1, which is a source of a primary repetitive vibration P, feeds that vibration into a location (shown dotted at 2) which includes a residual vibration sensor 3. A waveform generator 4 synthesises an electrical signal fed to a line 5 which causes an actuator 6 to generate a secondary vibration S, also fed to the location 2. Synchronising pulses are derived from the machin
Chaplin George B. B.
Smith Roderick A.
Dwyer James L.
Sound Attenuators Limited
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