The USRD type J15-3 projector is an underwater electro-acoustic transducer for use as a CW source in the frequency range 10–600 Hz. The transducer contains three diaphragm assemblies, each driven by a large moving coil in the field of a permanent magnet. When the diaphragms are connected in series, 750 W can be applied to yield a sound pressure level of 178 dB re 1 µPa. The diaphragms also can be driven in parallel or individually. At frequencies below 100 Hz, the input power is limited by the allowable displacement of each diaphragm. The J15-3 has been used successfully at a depth of 200 m at Lake Pend Oreille, Idaho, and at 300 m in ocean measurements. Pressure compensation is provided by a closed gas system that requires no high-pressure gas bottles; when water enters the compensation chamber, it compresses a butyl rubber bag until the internal air pressure equals the external water pressure.