Abstract: For use in an intrusion alarm system, transmitting and receiving transducer structures adapted for mounting at the ceiling or other surface of a protected enclosure and providing modified energy patterns. Each transducer structure has reduced sensitivity along the normal or boresight axis thereof, and increased sensitivity along axes angularly displaced from the boresight axis. Each transducer structure includes a resonant cavity terminating in an aperture, the geometric characteristics and dimensions of which provide partial cancellation of energy reflections returned along the boresight axis while enhancing or cancelling to a lesser degree those reflections received along axes angularly displaced from the boresight axis.