Abstract: An adaptive filter for sonar signals which operates on the complex spectral components of the received signal, which signal has been transformed into the frequency domain by means such as the Fast Fourier Transform. The adaptive filter consists of a plurality of component filters, each of which operates on a single spectral component of the received signals. The transfer coefficient of each component filter is described by a complex number and is adaptively adjusted by means of a computational feedback loop. The feedback loop compares the product of the transfer coefficient and the complex spectral component of the received signal from a prior frequency transformation cycle, with the present spectral component to obtain an error signal. The error signal, in turn, adaptively alters the magnitude and phase of the transfer coefficient.A plurality of such component filters operate together to adaptively filter, in the frequency domain, the entire spectrum of the received signal.