The present invention is directed towards an acoustic concealed item detector and related methods for detection using acoustics. In an illustrative embodiment, a multi-frequency ultrasonic wave generator generates in a gaseous medium at least first and second ultrasonic waves. The multi-frequency ultrasonic wave generator is arranged such that in operation, the first ultrasonic wave and the second ultrasonic wave mix in a prescribed mixing zone to produce a difference-frequency acoustic wave. A receiver sensor detects the difference-frequency acoustic wave and produces corresponding electromagnetic signals. The electromagnetic signals are processed by a system processor and signals indicative of concealed items are identified. Preferably the ultrasonic waves are focused to a small prescribed mixing zone. Parametric and multi-transducer embodiments are disclosed.