Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Patent
1997-03-07
1999-09-28
Dombroske, George
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
G01F 166
Patent
active
059592176
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to the use of a fluid flow measuring device as a microphone.
Conventional microphones make use of the fact that the pressure waves associated with an acoustic signal give rise to mechanical vibrations in a membrane or the like, which vibrations are converted with the aid of suitable conversion means into an electrical varying signal, the frequencies arising in the electrical varying signal corresponding to those of the acoustic signal.
The invention is based on the fact that a pressure wave and a mass flow wave are always associated with an acoustic wave, the flow wave, however, having a phase shift relative to the pressure wave. However, the flow waves which are associated with an acoustic signal comprise the same frequency pattern as the pressure waves and are therefore also able to serve as the basis for sound measurement.
The use of a fluid flow measuring device as a microphone is known from R. O. Fehr, "Infrasonic thermistor microphone", Journal. of the Audio Engineering Society, April 1970, Vol. 18, Nr. 2, pages 128-132. This publication discloses the use of hot-wire anemometers for the measurement of turbulence. In such devices, a very fine wire is electrically heated and is cooled depending on the turbulence of the air. The temperature changes of the wire are registered as resistance variations of the wire. The resistance variations can be detected by suitable electronic measuring circuits, thus providing an electrical signal which is proportional to the volume flow. However, hot-wire microphones are insensitive to the direction of the volume flow. Moreover, hot-wire microphones double the frequencies of the volume flow variations. In the publication of R. O. Fehr, the application of two thermistors is disclosed to solve these problems of the hot-wire microphones. When there is a volume flow over one of the two thermistors, the windward thermistor will be cooled, whereas the other thermistor will receive heat transferred by the wind from the first thermistor. Again, the temperature variations of the two thermistors are translated into varying electrical signals by connecting the two thermistors in opposite arms of an electrical bridge circuit. The microphone described in this publication is reported to operate only in a frequency range from 0.1 to 20 Hz. The diameter of the thermistors used is reported to be about 330 .mu.m (13 mill). Since the publication of this article by R. O. Fehr about 25 years ago, no research has been carried out for developing a microphone based on fluid flow measurements for detecting acoustic waves in the audible range.
U.S. Pat. No. 4,932,250 suggests the use of a fluid flow measuring device as a microphone for the detection of ultrasonic waves. No arrangement is suggested to detect acoustic waves within the audible range.
From several documents a micro fluid flow measuring device is known which comprises at least one heating element and at least two temperature sensors arranged in opposite positions to the heating element; see e.g. the German patent application 36 11 614, the European patent application 0,268,004, British patent application 2,226,139, and T. S. J. Lammerink, et al., "Micro-liquid flow sensor", Sensors and Actuators A, 37-38 (1993), pages 45-50. In none of these documents reference is made to a possible use of the fluid flow sensor disclosed as a microphone.
The object of the invention is to provide a microphone which is able to detect acoustic waves in the audible frequency range based on fluid flow measurements.
In order to achieve this objective, use is preferably made of techniques and means for the measurement of fluid flow which are known per se. A typical example of a fluid flow sensor which can advantageously be used in the microphone according to the invention is described in: T. S. J. Lammerink, et al., "Micro-liquid flow sensor", Sensors and Actuators A, 37-38 (1993), 45-50, referred to above.
The invention provides the use of a fluid flow measuring device as a microphone for detectin
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R. Fehr, "Infrasonic Thermistor Microphone", Journal of the Audio Engineering Society, Apr. 1970, vol. 18, No. 2, pp. 128-132.
T. Lammerink et al., "Micro-liquid flow sensor", Sensors and Actuators, 1993, vol. 37-38, pp. 45-50.
De Bree Hans Elias
Elwenspoek Michael Curt
Fluitman Johannes Hermanus Josephus
Lammerink Theodorus Simon Joseph
De Bree Hans Elias
Dombroske George
Thompson Jewel
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