A calibrator for use with long line hydrophones (seismic streamers) has been designed, constructed, and tested. Comparison calibrations may be obtained for individual elements or small groups of interconnected elements of a line hydrophone array in the frequency range of 20 to 3000 Hz. Absolute calibrations may be obtained in the frequency range of 20 to 100 Hz. The calibrator may be operated in either of two configurations. The first configuration has the calibrator submerged in a water trough. In the second configuration the calibrator, with the hydrophone inserted, forms a closed chamber through which water is continuously circulated during operation. A comparison between results obtained by using the calibrator, and independent measurements is presented. The theory of operation of the device, and of the sonic resistors used to reduce standing waves within the calibrator, is presented in the appendixes.