パラメトリックスピーカ実用化への検討

抄録

Parametric loudspeaker uses the self-demodulation effect of a finite amplitude ultrasound being amplitude modulated by audio signal, and the consequent special feature of this loudspeaker is very sharp directivity. In the present paper, a new amplitude modulation for primary wave is proposed to reduce the harmonic distortions of demodulated audio sound. The input signal has the form of square rot of envelope function. If ultrasonic transducers that construct parametric loudspeaker have wide and uniform frequency response, the demodulated audio sound does not contain any harmonic distortions as far as quasilinear approach for nonlinear acoustic process is valid. Experimental verifications were done for reduction of harmonic distortion using this modified double-sideband(MDSB)excitation, and it has been made clear that MDSB excitation suppresses 2nd harmonic generateon by about 10dB less than the commonly used DSB excitation. In order to attenuati the large amplitude ultrasound which is dangerous to auditory sense of human being even in farfield region of parametric array, the authors discuss the acoustic filtering of primary and secondary waves