A Small Anechoic Tank for Underwater Research

A small inexpensive anechoic tank (2.5 ft high, 3 ft wide, and 4.5 ft long) was designed, built, and tested for use in ultrasonic research. The tank is shaped like a pair of “butterfly wings” in all three principal planes passing through its center. This reduces the effects of standing waves and provides for a trapped mode of operation that forces reflected energy into the corners of the tank. The walls are lined with flexible polyurethane sheets, specially designed for maximum absorptivity in the frequency range 0.5–2.0 MHz. A simple manually operated transducer-positioning system allows for variable-range separation along a track and enables rotation and tilting of the transducers through their acoustic centers. Data on the acoustic properties of the tank are presented to demonstrate such design parameters as pressure reflectivity and absorption of the liner, reverberation time, and critical angles for trapped-mode operation. Some examples of its use as a research tool are given. [This work sponsored by the U. S. Navy Office of Naval Research.]