The direct application of Thevenin's theorem to a complex electromechano-acoustical network results in a technique for obtaining within a bounded region the absolute calibration of pressure-sensitive hydrophones in the lower audio-frequency range. Two electrodynamic transducers are used in addition to the hydrophone being calibrated. It must be possible to monitor the motion of the diaphragm of one transducer, and also to block (either mechanically or electrically) the diaphragm. Four electrical measurements are required. Neither impedance nor the blocking force need be measured. Calibrations have been obtained up to 1.5 kc.