GB1059481A - - Google Patents

Abstract

1,059,481. Electrostatic transducers. MATSUSHITA ELECTRIC INDUSTRIAL CO. Ltd. Jan. 22, 1964 [Jan. 25, 1963], No. 2799/64. Heading H4J. In an electrostatic transducer having a fixed electrode and a movable electrode formed by a vibratable diaphragm, the electrostatic driving forces applied to different areas of the vibratable diaphragm are graduated in order to ensure that the mode of vibration of the diaphragm corresponds with a single one of its natural modes despite change of frequency. The graduation may be effected, for example, by graduating dielectric constant, interelectrode distance, electrode area, bias voltage or driving voltage. The transducer may be a loud-speaker or microphone and all parts of the vibratable diaphragm may vibrate with the same phase, or the diaphragm may vibrate in stable nodal circles. In one arrangement (Fig. 3, not shown) the fixed electrode (2) has the form of concentric annuli which diminish in area from the centre outwards. Alternatively the fixed electrode may have perforations or be in the form of electroconductive mesh. Alternatively (Fig. 4, not shown) the fixed electrode (2) has a spherical surface whilst the movable electrode is planar. In a further arrangement (Fig. 5, not shown) the fixed electrode (2) has the form of concentric annuli, each of which is at a different potential. In another example (Fig. 6, not shown) the dielectric is in the form of concentric rings (51 to 56) of differing dielectric constant. The bias voltage may be supplied by a D.C. source (Fig. 2, not shown). Alternatively the fixed electrode may be of plastics such as a polycarbonate which is prepolarized at an elevated temperature. Since, with such material the charge leaks away, such fixed electrode may be charged periodically. In alternative arrangements the bias voltage may be obtained by rectification of a high-frequency signal (Fig. 7, not shown) or by one or both of pulse length and pulse height modulation (Fig. 8, not shown). For push-pull transducers full-wave rectification may be used. To obtain a substantially flat frequency characteristic the overall electrical impedance of the transducer may be matched to its mass. To assist in such matching the inductance of an output transformer may be utilized. Alternatively a feedback circuit may be used, the vibrational amplitudes of the movable electrode being detected by an auxiliary electrode (10) (Figs. 12, 13, not shown). In another arrangement (Fig. 14, not shown) high frequency power is amplitude- or phase-modulated by the variation of capacity between the fixed electrode (2) and movable electrode (1). Alternatively the oscillation frequency of a high-frequency oscillator is varied (Fig. 15, not shown). In another arrangement (Fig. 16, not shown) the movable electrode (1) has attached to it a piezoelectric element (20) which provides a feedback signal. Two flat electrostatic transducers may be placed adjacent and parallel to each other (Fig. 22, not shown) and connected to operate in opposite phase. Alternatively, a plurality of transducers may form a closed vibrating surface which vibrates as a whole.