We describe a device designed to detect interference (vandalism or tampering) by acoustic means. The design employs both a piezoelectric vibration transducer and a common microphone in a novel mechanical arrangement. In contrast to conventional acoustic sensors that simply respond to vibrations above a threshold, this design analyses the outputs of the two to trigger an alarm. The method confers a near-zero susceptibility to triggering by external loud noises. No complex calculations are required so that only a low-cost, embedded microcontroller is required and the whole sensor can be very cheap. Extreme sensitivity can be achieved with little risk of false alarms.