ASA 126th Meeting Denver 1993 October 4-8

4pED6. The acoustical waveguide.

Alan B. Coppens Anthony A. Atchley Eric W. Moore

Phys. Dept., Naval Postgraduate School, Monterey, CA 93943

Fundamental concepts of normal mode propagation in bounded media can be studied simply and practically in a water-filled rectangular waveguide with pressure-release boundaries. Use of broadband source and receiver allows generation and study of well-formed gated sine-wave pulses. The source is designed to excite the lowest (1,1) normal mode of the system and suppress excitation of the next few higher modes. This provides a useful frequency range for studying the behavior of the lowest mode both above and below its cutoff frequency. The cutoff frequency can be estimated by a number of techniques; this relates cutoff to several different phenomena associated with the propagation. Tone burst operation allows study of the rise time of the propagating tone burst and thereby the group speed of the carrier frequency. With an appropriate absorber at the far end of the waveguide, cw transmissions are almost purely traveling waves so that the phase speed of the input frequency can be determined. The apparatus is simple to construct and the electronics are not exotic. This experiment is a direct descendant of one that originated in the graduate lab of the physics department at Brown University approximately 35 years ago.