Mark B. Moffett
Naval Undersea Warfare Ctr., Fort Trumbull, New London, CT 06320
William J. Marshall, Jr.
Bolt Beranek and Newman, Inc., New London, CT 06320
Underwater sound projectors are usually operated near a resonance frequency in order to radiate high power at a reasonable efficiency. The width of the resonance peak is inversely proportional to the mechanical quality factor, Q[sub m], and therefore the bandwidth of a projector is often said to be determined by Q[sub m]. However, an equally important parameter affecting the bandwidth is the effective coupling factor, k[sub eff]. The connection between bandwidth and coupling factor, although expressed by Warren P. Mason in 1948, has not been generally appreciated by transducer designers and users. Recently, however, Dennis Stansfield [Underwater Electroacoustic Transducers (Bath U. P. and Inst. of Acoust., Bath, UK, 1990)] has elucidated this connection, showing explicitly how power amplifier and projector properties combine to determine practical bandwidth limitations. If the acoustic output of the projector is field-limited (i.e., by a maximum allowable drive field), the effective coupling factor furnishes a useful design starting point. Knowledge of typical values for k[sub eff] for various transducer classes permits initial sizing of the transducer active material. [Work sponsored by the Office of Naval Technology.]