Detecting damage to bulk material

Apparatus for detecting damage to sheet glass comprising:

• a transducer element of an amorphous piezoelectric material in a plate-like shape;
• a rigid housing in which said transducer element is supported, said housing comprising means rigidly supporting said transducer element about the periphery of said plate-like shape, and said housing having a surface portion attachable to a sheet of glass to allow transmission of virbrations from the glass to said transducer element;
• and an electric circuit connected to said transducer element to receive signals therefrom corresponding to such vibrations, said circuit including filtering means preventing response to any audio frequency signals generated by said transducer element while allowing response to signals in an ultrasonic frequency range about 100 kHz, and output means responsive to signals in said ultrasonic frequency range and constructed to provide a predetermined requirement of signal amplitude and/or duration in said ultrasonic frequency range to provide an output signal in response to ultrasonic signals meeting said requirement.