A signal enhancement method and apparatus including a computer arranged to process underwater signals at discrete spectral intervals for facilitating spectral signature recognition of submarines. An average of the spectral intensities of broadband noise on either side of the discrete spectral interval is computed and then subtracted from the spectral intensities of the specific narrow-band frequencies generated by the submarine, thereby eliminating any false combinations of nonrandom sidebands with random background noise. The remainder is then integrated with time producing a continuously increasing cumulative energy level of the discrete spectral interval when the energy thereof is originating from the submarine.