Parametric Sound Fields Formed by Phase-Inversion Excitation of Primary Waves

Abstract

Two planar ultrasound projectors having identical rectangular apertures were placed side by side. Both projectors radiated bifrequency primary waves in air. The frequencies were 26 and 28 kHz, and the initial phases were different. Two driving modes were considered, namely, conventional in-phase driving and phase-inversion driving. The spatial profiles of sound pressure fields were measured along and across the sound beam axis for the primary waves and for the difference frequency wave of 2 kHz. The second and third harmonic components of the difference frequency waves were also measured. The pressure levels of the primary waves were considerably suppressed near the beam axis owing to phase cancellation when the driving signals were phase-inversed, i.e., 180 degrees out of phase. The beam pattern of the difference frequency was, however, almost the same as that for the case in which the signals were in phase. Interestingly, the harmonic pressure amplitudes of the difference frequency were reduced by more than 10 dB. The validity of the experimental results were confirmed based on their good agreement with the theoretical predictions based on the Khokhlov-Zabolotskaya-Kuznetsov equation.