Image Number 2 for United States Patent #4229811.
An improved method and apparatus for carrying the method are disclosed which provide for selection by the user of a surface used as a reference in the detection of an intrusion. This is achieved by periodically projecting pulses of acoustical energy from a source towards a reference surface and detecting the reflection of such pulses of acoustical energy at a receiver and providing a first indication or signal representative of the reflected pulses. A second indication or signal is generated which is adjustable and representative of the time elapsed between the projection of a pulse of acoustical energy from the source and the reception at the receiver of the pulse of acoustical energy when reflected from the selected reference surface. The variations between the first and second indications are detected and compared and an alarm is generated when a variation is detected in the indications. In the embodiment disclosed, the absence of variations between the first and second indications is measured by the coincidence of the first and second indications.