Active/passive technique for the creation of a low-frequency traveling wave inside a water-filled tube

Abstract

Studies of sound reception in aquatic animals are often complicated by the problem of generating a suitable acoustic stimulus at low frequencies underwater. In this study, active and passive control techniques were used to generate a traveling acoustic wave inside a water-filled acrylic tube, at pure-tone frequencies from 10 to 800 Hz. The compliant walls of the tube reduced the wavelength and increased attenuation, both of which helped to reduce the amplitude of any reflected waves. Above 400 Hz, a traveling wave was present without any active control. Below 400 Hz, a secondary source was used to actively absorb sound at the tube end. The active control system consisted of two hydrophones, two sound projectors, and a personal computer with digital signal processing hardware. A simple pattern search algorithm was used to adjust the secondary source amplitude and phase in order to attain the desired acoustic impedance at the test location. This technique seems well suited for underwater sound tests on fish, since the relationship between pressure and particle velocity may be easily manipulated. [Work supported by ONR Grant No. N00014-94-1-0337.]