PURPOSE: To attenuate the sound pressure level of a primary wave received by a listener without destroying the directional characteristic of a secondary wave by providing an ultrasonic generator, an acoustic filter and a sealing side wall for interrupting the primary wave and transmitting the secondary wave.
CONSTITUTION: There are provided the ultrasonic generator 1 and a sealing container made of a material in which the transmission loss of the primary wave emitted from the ultrasonic generator in large and the transmission loss of the secondary wave is small. For instance, the ultrasonic generator 11 is formed by arranging 190 piezoelectric ceramic ultrasonic oscillators in approximate circular shape and has a diameter of about 16cm and a driving frequency of 40kHz. A pipe 2 made of punching metal of the diameter of about 18cm and the length of 1m is attached thereto as the side wall and further the sealing side wall 3 formed by various sealing materials is externally wound. At the end of the pipe, the acoustic filter 4 composed of 5 layers alternately superposed with urethane foam of the thickness of 15mm and polyethylenefilm of the thickness of 20μm is disposed.
KIMURA YOICHI
NAKAMURA AKIRA
| JPS60201799A | 1985-10-12 | |||
| JPS60167597A | 1985-08-30 |