Abstract:

``HyperSonic Sound'' is American Technology Corporation's commercial application of the parametric array to produce broadband audio sound (Business Week, 12-2-96 issue, pp. 108--109). E. G. Norris demonstrated the device at the Penn State ASA Meeting [J. Acoust. Soc. Am. 101, 3072(A) (1997)], but gave few technical details. An analysis of the device demonstrated is provided here. The primary fields are produced by sixty 40-kHz elements mounted on a ring, 4-cm i.d., 7-cm o.d., which produce an axial sound-pressure level of 140 dB at 30 cm. The values given imply a Rayleigh distance of about 1.2 m and a (primary) beamwidth of about 3(degrees). Estimated by the absorption length for the primaries, the interaction region is about 9 m. Those who witnessed Norris's demonstration can attest to the narrowness of the secondary sound (a synthesized musical melody) and apparent absence of sidelobes. However, the melody heard also exhibited distortion. Distortion may be predicted by recalling that when the primary wave is a modulated carrier, the secondary sound is (asymptotically) the second derivative of the modulation envelope squared. Prospects for reducing the distortion are discussed [see Yoneyama et al., J. Acoust. Soc. Am. 73, 1532--1536 (1983)].