Electrical audio signal processing systems and devices – Acoustical noise or sound cancellation – Adjacent ear
Patent
1983-04-11
1985-07-02
Rubinson, Gene Z.
Electrical audio signal processing systems and devices
Acoustical noise or sound cancellation
Adjacent ear
381 94, G10K 1116, F16L 5502
Patent
active
045272824
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to an improved method of, and improved apparatus for, the active attenuation of low frequency gas-borne vibrations (e.g. noise). The method finds particularly useful applications in the attenuation of very low frequency noise in exhaust gases from engines.
DISCUSSION OF PRIOR ART
The invention can be considered as an extension of the simple virtual earth active attenuation system which is diagrammatically illustrated in FIG. 1 of the accompanying drawings. In FIG. 1, a pressure-sensing microphone 1 is located a distance d from a loudspeaker (or driver) 2 in a position where the microphone can receive primary vibrations, travelling in the direction of the arrow X, from a source, such as an engine exhaust, (not shown), and secondary vibrations from the loudspeaker 2. The loudspeaker 2 is of the closed-back type and is disposed adjacent to the path of the primary vibrations. The microphone 1 is coupled to the loudspeaker 2 via an electronic circuit which includes a phase-inverting amplifier 3.
LIMITATIONS OF PRIOR ART
The arrangement shown in FIG. 1 is theoretically capable of producing a constant or null pressure in the composite vibration field generated by the source and the driver 2 in the immediate vicinity of the microphone 1. The simple virtual earth system shown in FIG. 1 will, however, oscillate at a critical frequency fr which is, inter alia, a function of the distance d. The smaller is the distance d, the higher will be the frequency fr, and the greater is the band width over which the system can be used to obtain useful active attenuation. Unfortunately, however, with a conventional driver operating into free space, the field pattern of the secondary vibrations emitted thereby becomes increasingly more non-uniform the closer one comes to the driver 2. Thus there is a practical limit as to how close one can locate the microphone 1 to the driver 2, and yet obtain useful attenuation remote from the microphone 1. Thus the non-uniformity of the near-field of the driver 2 sets an upper limit to the frequencies which can be cancelled with a simple virtual earth system shown in FIG. 1.
In order to minimise the delay round the loop 1, 3, 2 and hence reduce the instability, the microphone 1 must be placed as close as possible to the loudspeaker 2. The microphone 1 is thus located in the near field of the driver, where the sound pressure changes much more rapidly with distance, compared to some position more remote from the driver. It follows, therefore, that the pressure waveform produced precisely at the microphone 1 by the driver 2 matches the primary vibration field only over a very localised region of space, thus severely limiting the region of resultant null pressure.
Furthermore, the pressure waveform received by the microphone 1 is also affected by nearby objects creating reflections.
To prevent a system such as that shown in FIG. 1 oscillating at the frequency fr, it is known to arrange for the gain of the amplifier 3 to be decreased to unity before the critical frequency fr is reached. This frequency dependent characteristic of the gain is known as the "roll-off" and although a rapid (or high order) roll-off might appear to be desirable to maximise the useful band width of the system, in practice generating a rapid roll-off, by means of a gain-control network in the electronic circuit (such as the network N shown in FIG. 1) reduces the stability of the system at lower frequencies. Thus the "roll-off" generally adopted with a system shown in FIG. 1 is of the form shown by the dashed line in FIG. 2.
Thus from the standpoint of the poor near-field uniformity of known drivers working into free space and the natural limitations set by high order electronically-induced roll-offs, the simple virtual earth system shown in FIG. 1 finds little practical use.
BRIEF DISCLOSURE OF INVENTION
We have now found that the useful working range of a virtual earth system can be greatly enhanced by the simple expedient of feeding the vibrational output of the driver, into
Chaplin George B. B.
Jones Andrew
Jones Owen
Dwyer James L.
Rubinson Gene Z.
Sound Attenuators Limited
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