Patent ID: 7319639

Claim:
An acoustic concealed item detector, comprising: a multi-frequency ultrasonic wave generator for generating in air or a gaseous medium, at least a first ultrasonic wave having frequency f 1 , and a second ultrasonic wave having frequency f 2 , the frequencies f 1 and f 2 both being between 100 KHz and 200 KHz, the multi-frequency ultrasonic wave generator being arranged such that in operation, the first ultrasonic wave and the second ultrasonic wave are focused to a prescribed mixing zone having a characteristic size of less than about 0.2 cm, within which a difference-frequency acoustic wave is produced, the difference-frequency acoustic wave having a frequency less than about 20 KHz; a receiver sensor for detecting interactions of the difference-frequency acoustic wave and producing corresponding electromagnetic signals; and a system processor for processing the electromagnetic signals and identifying signals indicative of concealed items.