Telephonic communications – Terminal – Housing or housing component
Patent
1993-04-20
1994-08-23
Dwyer, James L.
Telephonic communications
Terminal
Housing or housing component
379434, H04M 100
Patent
active
053414209
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
I. Field of the Invention
The invention relates to noise-cancelling methods for microphones, and in particular to a noise-cancelling telephone handset.
II. Related Art and Other Considerations
Telephones are often situated in areas with a high level of background noise, for example at railway and underground (subway) stations, airports and adjacent to busy roads. Noise-reducing housings are not always provided for such telephones, as space may be limited, or fire regulations may not allow their use.
In a telephone handset, sound arriving from a distant sound source will tend to result in a uniform pressure over parts of the surface of the handset. Hence, at such points, a microphone system which detects pressure differences will produce little or no output. Sound propagating from a close source will, however, produce large pressure differences over the surface of the handset, and a microphone system which detects pressure differences can be made to produce a large signal.
The conventional approach to noise-cancelling handset design relies on an open mouthpiece structure around a first order pressure gradient microphone. A first order pressure gradient microphone is a microphone which detects a single pressure difference between two pressure-sensing locations. In this approach, both sides of the microphone diaphragm are exposed to the sound vibrations from a pair of holes (ports) in the mouthpiece housing, and distant noise is cancelled out because it creates substantially equal pressures on both sides of the diaphragm. The ports constitute the two pressure-sensing locations for the microphone. This approach is unsuitable for rugged payphone applications because the open structure is fragile, and would be prone to the ingress of dirt and moisture. Traditional theory suggests that the separation of the two pressure sensing locations must be much smaller than half a wavelength for useful cancellation in order that there is sufficient correlation between the noise signals. In practice, this means that the distance between two pressure-sensing locations is of the order of 5 mm, with a maximum separation of about 10 mm.
In non-telephony areas, second order boom microphones are used, for example by pilots, but these are not appropriate for telephony because they rely on the microphones being placed in position close to the mouth, and do not work properly if poorly positioned.
SUMMARY
The object of the invention is to provide an improved form of noise-cancelling in a telephone handset.
The invention is based upon the unexpected realization that correlation between signals from widely-spaced pressure-sensing locations is sufficient to permit useful noise cancellation to occur.
The present invention provides a noise-cancelling telephone handset comprising a housing containing a first order pressure gradient microphone, the housing having two pressure-sensing locations which are connected to the microphone by means of ducts whereby sound travels from the pressure-sensing locations to the microphone, the pressure-sensing locations being spaced apart by a predetermined distance which is sufficiently large to permit locally-generated speech signals to produce a pressure gradient between the pressure-sensing locations whilst sound signals from distant sources produce substantially equal pressures at the pressure-sensing locations, whereby, in use, the output of the microphone is substantially entirely that resulting from the locally-generated speech signals.
Advantageously, said predetermined distance is at least 20 mm. Preferably, said predetermined distance is less than 100 mm. More preferably, said distance lies within the range of from 30 mm to 80 mm. In the most preferred embodiment, said predetermined distance is 65 mm.
Conveniently, the pressure-sensing locations are positioned substantially along the longitudinal axis of the handset, and are constituted by holes formed in the housing.
In a preferred embodiment, the ducts comprise tubes extending in the handset from the pressure-sensing locations to
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Patent Abstracts of Japan, vol. 11, No. 320 (3-550)(2767), Oct. 17, 1987 & JP A 62110349, May 21, 1987.
Hollier Michael P.
Welsby Kevin
British Telecommunications public limited company
Chiang Jack
Dwyer James L.
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