EXPERIMENTS ON ACOUSTIC MODULATION OF LARGE-AMPLITUDE WAVES

A series of experiments were preformed to study the parametric relationship between two high frequency carrier waves and the difference frequency wave created by their nonlinear interaction. The measurements were made in a freshwater lake in the farfield of all radiations. A split face piston projector, 2 in. Ã 2 in., was driven by two pulsed power amplifiers, and the resulting acoustic fields were measured with three separate calibrated hydrophones. Before the experiments were conducted, the usefulness of a frequency selective acoustic filter (SOAB) in identifying pseudosound was established. In the first experiments, a 143 kHz difference frequency wave having an extrapolated source level of 81 dB re 1 μbar at 1 yd was created from the nonlinear interaction of a 981 kHz and a 1124 kHz radiation whose source levels were 121 and 113 dB, respectively. The difference frequency sound propagated according to the spherical spreading law, independently of the existence of the carriers, with a 1.7° half power beamwidth. The difference frequency SPL was found to be proportional to the product of carrier wave amplitudes, except at high carrier wave source levels, and proportional to the difference frequency to a power of 1.7. With few exceptions, the results showed good agreement with a mathematical model of the problem devised by Westervelt. © 1969, Acoustical Society of America. All rights reserved.