A glass break defection system includes a unidirectional acoustic transducer directed toward an area of glass to be monitored. The transducer is narrowband and has a sharp frequency response peak in the 4 to 8 kHz range. An electronic audio discriminator connected to the transducer output is responsive...http://www.google.de/patents/US4837558?utm_source=gb-gplus-sharePatent US4837558 - Glass break detector
