Acoustic measurements of capillary-gravity water surface waves

Abstract

For this work an optical method for measuring surface acoustic waves was adapted to acoustics using airborne ultrasound for measurements on water surface waves. With ultrasound incident on periodic traveling water surface waves, the reflected signal can be treated as the diffraction pattern from a moving corrugated reflection grating as long as the amplitude of the water surface waves is much less than the incident acoustic wavelength. The acoustic signal received at the first-order diffraction maxima is amplitude modulated at the frequency of the water surface wave. The intensity of this modulation is directly proportional to the amplitude of the water surface wave. Using spectral decomposition of the signal, the water surface wave amplitudes are precisely determined at sum and difference frequencies around the source peak. The transmission of water surface waves incident on a solid piercing boundary was measured using this method to understand capillary-gravity wave interactions with boundaries. [Work supported by NASA.]