Parametric audible sounds by phase-cancellation excitation of primary waves

Abstract

An ultrasound source with a simple configuration is considered as a theoretical model. The source with a circular aperture consists of two coaxially arranged planar emitters: i.e., one is an inner disc emitter and the other is an outer ring emitter. The active areas of these emitters are the same. The outer diameter of the source is 20 cm. Both the emitters are driven individually at the same frequencies of 40 and 42 kHz but different phase angles. Especially, we focus on two extreme cases of the usual in-phase driving and out-of-phase driving. Numerical computation using the KZK equation demonstrates that when the driving signals are in phase the difference frequency beam of a 2-kHz wave has a candle-flame-like directivity. The beam has a similar directivity when the signals are out-of-phase by 180 degrees, although the peak of the sound pressure level decreases by few decibels. Interestingly, the second harmonic pressure level of the difference frequency reduces by ten decibels and more. Needless to say, the pressure amplitudes of the primary waves are suppressed considerably near the beam axis. Experimental verification is done using an airborne ultrasound source with a 19.2-cm circular aperture.