Abstract: An apparatus for detecting defects in empty containers, such as dents in cans, is disclosed. The containers are magnetically struck, causing them to ring at their natural frequencies. A microphone senses this resonance; a bandpass filter filters out the known frequencies generated by an acceptable container. The energy associated with other frequencies is examined to detect defects in the containers. A circuit for reducing the effects of background acoustic noise and a circuit for sensing the amount of energy imparted to the container when it is struck are also disclosed.