Methods and apparatus for using Fourier techniques to detect and isolate the fundamental frequency sweep of echolocation calls produced by bats, or of other narrow-band whistles as produced by other kinds of animals such as birds and whales. According to one example, a Fourier transform is applied on blocks of input samples to produce output frames, and a narrowband frequency modulated signal is detected and traced through the output frames. A filter is applied in each frame to attenuate frequencies above or below the narrowband signal, and an inverse Fourier transform is applied to produce an output signal. The resulting signal may then be analyzed with zero crossing techniques to measure the precise instantaneous frequency sweep.