The first modern device for air acoustic use was created in 1998,[REF] and is now known by the trademark name "Audio Spotlight", a term first coined in 1983 by the Japanese researchers[REF] who abandoned the technology as infeasible in the mid-1980s.
A transducer can be made to project a narrow beam of modulated ultrasound that is powerful enough, at 100 to 110 dBSPL, to substantially change the speed of sound in the air that it passes through.