Read/Search this Article
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 projectors: i.e., one is an inner disc projector and the other is an outer ring projector. The active areas of these projectors are the same. The outer diameter of the source is 20cm. Both the projectors are driven individually at the same frequencies of 38 and 40kHz but different phase angles. Especially, the study focuses on two extreme cases of the usual in-phase driving and out-of-phase driving. Numerical computation using the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation demonstrates that when the driving signal is in phase the difference frequency wave of a 2-kHz wave provides quite a narrow beam a parametric acoustic array usually creates. The beam has a similar directivity when the signal is 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 10dB or 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.