Acoustic interferometer

An instrument that's sensitive to the interference of two or more acoustic waves. It provides information on acoustic wavelengths that's useful in determining the velocity and absorption of sound in samples of gases, liquids, and materials, and it yields information on the nonlinear properties of solids.

In its simplest form, an acoustic interferometer for use in liquids has a fixed piezoelectric crystal (acting as a transmitter) tuned to the frequency of interest and a parallel reflector at a variable distance from it. Driven by an oscillating electrical volt age, the piezoelectric crystal generates a sound wave, which in turn is reflected by the reflector. The acoustic pressure amplitude on the front face of the crystal depends on the velocity amplitude at the face and the distance to the reflecting surface. The amplitude ratio (radiation impedance) of the acoustic pressure to the velocity and the relative phase shift between the two oscillating quantities depend solely on the distance to the reflecting surface. If the reflector acts as a rigid surface, this amplitude ratio is ideally zero whenever the net round-trip distance between the crystal and the reflector is an odd number of half-wavelengths because the reflected wave is then exactly out of phase with the incident wave at the crystal’s location. The crystal then draws the maximum current since the oscillations are unimpeded.

During operation, the current drawn by the crystal is monitored as the reflector is gradually moved away from the crystal. Whenever the reflector position is such that the crystal is at a pressure anti-node (place of maximum pressure in a standing wave), there is a strong dip in the current drawn due to the relatively high radiation impedance presented by the standing wave to the crystal face. Consecutive antinodes are a half-wavelength apart. For a given frequency f, a measured distance L between the location of anyone anti-node and that of its nth successor yields the wavelength 2L/n and the speed of sound c = 2Lf/n. An acoustic interferometer based on this principle can achieve a precision of 0.01%. Since the current drawn by the crystal is relatively in sensitive to the frequency for a given radiation impedance, the sound speed can also be determined by keeping the distance between the crystal and the reflector fixed and gradually sweeping the frequency.

The pressure nodes and antinodes correspond to the local maxima and minima, respectively, in the current drawn. The peak of the current amplitude decreases with the distance traversed by the reflector. If the separation distance is sufficiently large that the exponential decrease associated with absorption dominates any spreading losses, the absorption coefficient for the medium can be derived by measurement of the ratios of current amplitudes at two successive points where the current drawn is a local maximum.