Scattering of Sound by Sound

Following recent theoretical work, another attempt has been made to seek evidence of scattering resulting from the nonlinear interaction of two finite-amplitude sources operating in water. Experiments were performed using two perpendicularly intersecting pulsed sound beams having frequencies of 7 and 5 MHz. These beams produced pressure amplitudes of 4.5 and 1.8 atm, respectively, at the center of the interaction region. A receiver tuned to the summation frequency (12 MHz) was swept about the interaction region to investigate the scattered field. Various theoretical descriptions of this experiment have predicted for the scattered wave maximum pressure amplitudes ranging between 2.40×104 and 5.07 dyn/cm2. Although pressure levels of 0.07 dyn/cm2 could be measured with a 10-dB signal-to-noise ratio, no scattered wave was found. The lack of scattering outside the interaction region is in agreement with the theoretical considerations of Westervelt, but contrary to the predictions of Ingard and Pridmore-Brown, Dean, and Berktay and Al-Temimi.