This paper presents the design and development of a truly omnidirectional, low-frequency, high-power, underwater sound projector standard. The transducer is operational within the frequency range from 1 Hz to 10 kHz, with a transmitting response which is stable with hydrostatic pressures up to 1524 kPa (152-m depth) and temperatures from 5 to 35(degrees)C, and can be used as a hydrophone or a projector. The sensitive element is a 22.9-cm (9.0 in.) o.d. piezoelectric lead zirconate--lead titanate (MIL-STD 1376, Type 1) hollow sphere encapsulated in polyurethane. This transducer, designated the USRD type F80, produces an undistorted omnidirectional source level that is linear with driving voltages up to 1000 V rms, and can also be electrically tuned for a higher response at predetermined, discrete frequencies. This paper includes an electroacoustic equivalent circuit model of the transducer, compares the predicted to the measured transmitting response, and discusses the mechanical and electrical design.