| 摘要 |
PURPOSE:To cut harmful high-power ultrasonic waves for human bodies and to obtain an electroacoustic transducer with a little of a loss of secondary waves by providing an acoustic filter for interrupting a primary ultrasonic wave in a prescribed position away from a front of a piezoelectric vibrator. CONSTITUTION:A speaker employs a parametric function. The limited amplitude ultrasonic wave whose amplitude is modulated by an audible sound is radiated from a piezoelectric vibrator array 5, and propagates in air. During the propagation in air, self-demodulation of the wave is performed by influence of a non-linear characteristic of air, and accordingly a virtual sound source of a secondary wave (modulated audible sound) is formed in beams of the primary wave, whereby a secondary sound field of a sharp directivity is formed. Here, sound pressure of the primary wave is substantially large, and harmful for bodies. Consequently, an acoustic filter 6 is provided L (L=R0/2alpha; R0 is rayleigh length; alpha is attenuation coefficient of ultrasonic wave in air) away from the piezoelectric vibrator array 5, and the harmful primary wave is eliminated. |
