Active ultrasonic room occupancy sensors are equipped with an ultrasonic transmitter having an output with a fixed amplitude. The size of the zone of coverage was adjusted by selection of the transmitter plate which was mechanically connected to the output crystal. This configuration often resulted in sensors with too large a coverage area being selected for a given area. With fixed energy transmitters, distant targets reflect significant energy leading to inappropriate activations. The sensitivity of the receiver is often ineffective in minimizing inappropriate activations.