Sampling of a captured signal is synchronized to a tonal which may be unstable in frequency detected in the signal to cause it and all of its harmonics, sub-harmonics and fundamental to appear to have a substantially constant frequency relative to the sampling rate even if some related signals are otherwise undetectable amid noise. By adjusting an integration period of a fast Fourier transform to extract an intrinsic bandwidth, substantial signal processing gain can be obtained for the tonal and harmonically related signals, even if otherwise undetectable. Signal processing may be performed in either the time domain or the frequency domain. Recursive processing is performed to observe unrelated tones by grouping of tones which are harmonically related and supports detection of relationships between acoustic signal sources.