Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Thermal response to – or generation of – sound vibration
Patent
1988-03-07
1989-12-12
Ng, Jin F.
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Thermal response to, or generation of, sound vibration
381174, 381191, H04R 1904, H04R 1900
Patent
active
048873004
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a pressure gradient microphone comprising a membrane and a back electrode the surface of either the membrane or the back electrode being a film of an electrostatically charged, electret material divided into preferably semicircular sections.
BACKGROUND ART
The U.S. Pat. No. 3,588,382 describes a pressure gradient microphone of the electret type with the electret being divided into semicircular sections, said sections being positively or negatively charged. It is difficult to manufacture good pressure gradient microphones according to this principle for measuring purposes. Such microphones are, apart from being sensitive to pressure gradients, also sensitive to pressure, i.e. to pressure equally distributed all over the membrane. The polarized sections divide the microphone into separate transducer sections, each section contributing to the signal of the microphone. Ideally, the contributions of the sections should neutralize each other with equal pressure all over the membrane whereby the microphone should transmit no signal. Due to different charges, unequal distances between the membrane and the charged sections, unequally distributed membrane voltages etc. between the components this can never be achieved in practice. That is why such pressure gradient microphones are not used for the acoustic measuring of particle speed and sound intensity, although it would be an advantage compared to the state of the art.
DESCRIPTION OF THE INVENTION
An electret microphone of the above type is according to the invention characterised by only some of the sections being permanently charged, the back electrode being electrically charged by means of an adjustable external voltage source.
In a pressure gradient microphone provided with a permanent charge as well as with a charge from an external voltage source, the pressure sensitivity can be adjusted to zero by adjusting the voltage source, while the membrane is supplied with equal pressure. The external voltage source is used for outbalancing the differences in all other important parameters. As a result the pressure sensitivity is reduced by a factor 10 or more compared to microphones with two permanent charges.
The electret microphone may be improved and become more easily adjustable, if the chamber of the microphone is so small that the deflection of the membrane is considerably reduced under equal pressure on the two halves.
In a preferred embodiment the electret microphone comprises four back electrode parts interconnected two by two for measuring the pressure gradient in one plane.
Measuring the pressure gradient is important because this parameter can be used for determining particle speed and sound intensity. Both values are of great interest in connection with acoustic measurements.
A pressure gradient microphone according to the present invention is on the outside formed like a typical microphone for measuring pressure. By placing such a pressure gradient microphone opposite a pressure microphone of the same outer size and form in such a way that their membranes are placed opposite to each other, an especially advantageous intensity measuring probe is obtained, since the pressure and particle speed can be determined within the same small area.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in greater details below with reference to the accompanying Figures, in which
FIG. 1 shows an electret microphone according to the invention for measuring pressure gradients,
FIG. 2 shows the electric circuit diagram to be used in connection with the pressure gradient microphone,
FIG. 3 shows a pressure gradient microphone in connection with a pressure microphone for measuring sound intensity, and
FIGS. 4a-4c show a pressure gradient microphone with its electric circuit diagram indicating the direction of propagation of the sound in one plane.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
The microphone shown in FIG. 1 comprises an outer microphone housing 1 formed substantially like
REFERENCES:
patent: 3588382 (1971-06-01), Reedyk
patent: 3944756 (1976-03-01), Lininger
patent: 4258235 (1981-03-01), Watson
patent: 4559418 (1985-12-01), Imai
Aktieselskabet Bruel & Kjaer
Byre Danita R.
Ng Jin F.
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