SUBHARMONIC GENERATION IN AN ACOUSTIC FABRY-PEROT INTERFEROMETER.
MICHIGAN STATE UNIV EAST LANSING ULTRASONICS LAB
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Above a certain energy density threshold in an intense ultrasonic standing wave, subharmonics of the driving frequency are sometimes present. To study this effect, an acoustic Fabry-Perot interferometer was constructed, consisting of two air-backed quartz transducers submerged in water. One of the transducers is driven at frequencies around its 3 MHz resonance. Electronic analysis of the waveforms present in the cavity is accomplished with a spectrum analyser connected to the other quartz transducer. Optical methods are used as a supplementary means of analysis. It was observed that subharmonics exist only at frequencies for which the cavity is in resonance and always occur in pairs, such that the sum of their frequencies is equal to the driving frequency. These discrete frequencies appear only in regions near integral sub-multiplies of the driving frequency. The pressure in the cavity at the onset of subharmonic oscillation may be as low as 1.5 atmosphere. In this study, subharmonics were obtained only when the cavity reflector had a frequency dependent reflection coefficient. Author