JPS5830391Y2 - ultrasonic microphone - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an ultrasonic microphone that uses a piezoelectric polymer film as a vibrating membrane, and provides an ultrasonic microphone that can suppress a decrease in sensitivity and a change in band due to a decrease in temperature.

An embodiment of the present invention will be described below with reference to FIG.

In FIG. 1, reference numeral 1 denotes a substrate made of an insulating material, and a semicircular plate-shaped protrusion 2 is formed at the center of the upper surface of the substrate 1. As shown in FIG.

Reference numeral 3 denotes a semicircular conductive protrusion fixed to one end of the substrate 1, and a similar protrusion is fixed to the other end of the substrate.

Reference numerals 4 and 4' denote vibrating membranes made of a piezoelectric polymer film adhered to the projections 2 and 3, and electrodes are formed on both surfaces of the vibrating membranes 4 and 4' by vapor deposition.

Reference numeral 5 designates a semi-cylindrical grid plate fixed to the substrate 1 so as to cover the piezoelectric polymer films 4, 4', and this grid plate 5 has holes 6 formed therein.

Reference numeral 7 denotes a cover having a mesh 8 attached to an opening on the upper surface, and the substrate 1 is attached to the opening on the lower surface of the cover 7.

When sound pressure is applied to the vibrating membranes 4, 4', the vibrating membranes 4, 4' vibrate, and a signal is generated between electrodes formed on both surfaces of the vibrating membranes 4, 4'.

The feature of the present invention is that the resonance frequency f1 of the vibrating membranes 4, 4' is
The point is that the resonance frequency f2 of the grid plate 5 is lower than the resonance frequency f2 (the resonance frequency of the grid plate determined by the diameter, thickness, aperture ratio of the holes in the grid plate, and the distance between the diaphragms), and in this way, L<f2. In this case, it is possible to reduce the decrease in sensitivity and change in band due to temperature decrease.

Figure 2 shows the frequency characteristics of the diaphragm. When the temperature is lower than room temperature, the resonant frequency of the diaphragm increases, as shown by A in Figure 2, and the reception sensitivity decreases; on the other hand, when the temperature is higher than room temperature, Then, as shown by B in FIG. 2, the resonant frequency of the diaphragm decreases and the reception sensitivity improves.

This is due to the reasons shown below. In other words, the resonant frequency f1 of the vibrating membrane is determined by the Young's modulus C of the piezoelectric polymer film.
It is expressed as follows using E, density ρ, and radius of curvature R of the diaphragm.

Here, the Young's modulus CE of the polymer piezoelectric film (polyvinylidene fluoride film) gradually increases as the temperature decreases, and becomes constant around -30 to -40°C.

Further, the tan δ of the piezoelectric polymer film, which determines the resonance Q of the vibrating membrane, reaches its maximum at around -30 to -40°C.

As described above, since the piezoelectric polymer film of the vibrating membrane itself has temperature characteristics, the resonance of the vibrating membrane changes with temperature changes.

Figure 3 shows the transmission sensitivity characteristics of a conventional ultrasonic microphone with fl > f2. When the temperature drops below room temperature, the resonant frequency f1 of the diaphragm increases, and the transmission sensitivity characteristics change from a in Figure 3. b.

On the other hand, in the present invention, f<f2 is used, so that even if the temperature drops below room temperature, the large sensitivity drop and large band change that occur in the conventional example do not occur.

[Brief explanation of the drawing]

Figure 1 is a partially cutaway perspective view of an ultrasonic microphone according to an embodiment of the present invention, Figure 2 is a diagram showing changes in sensitivity due to temperature changes in the diaphragm, and Figure 3 is a diagram showing a conventional ultrasonic microphone. FIG. 3 is a diagram showing changes in sensitivity of a sonic microphone. 1... Board, 2... Protruding piece, 3...
... Protruding piece, 4.4'... Vibration membrane, 5...
-Grid plate, 6...hole, 7...case, 8...net.

Claims (1)

[Scope of utility model registration request] While bending the piezoelectric polymer film into a semi-cylindrical shape,
In an ultrasonic microphone in which a semi-cylindrical grid plate with a plurality of holes is disposed in front of the piezoelectric polymer film, ultrasonic waves are generated in which the resonant frequency of the piezoelectric polymer film is lower than the resonant frequency of the grid plate. Microphone.