PURPOSE: To detect the power spectrum of a wide-frequency band noise source generation signal in a underwater noise by finding the time difference of maximum correlation from the mutual correlation between the stored waveforms of a waveform memory part and an integral memory part.

CONSTITUTION: A time lag estimation part 1 estimates time lag corresponding to the repetitive period of a signal waveform generated by a sound source from the autocorrelation of a received signal obtained from one passive buoy. The waveform memory part 2 stores received waveforms corresponding to a time which is M+1 times (M: integer larger than 1) as long as the time lag and the integral memory part 4 stores the cumulative result of the waveforms. A correlation detection part 3 finds the time difference of maximum correlation between both memory parts 2 and 4, a waveform extraction part 5 extracts the stored part of the waveform memory part 2 where the correlation is maximum for a time which is M times the time lag, and a cumulation part 6 adds the output to the storage contents of the integral memory part 4 and analyzes a frequency to detect the power spectrum.