A waveform to be analysed is spectrally resolved into a plurality of frequency channel outputs (1). Amplitudes of the channel outputs are then compared with threshold values (4), with the threshold values being varied in dependance on 1) previous amplitude detection in the same channel (13, 14, 15) and 2) amplitude detection in adjacent channels (16). In this way features introduced by the spectral resolution of the waveform may be filtered out along with unwanted noise. In addition, smearing due to compression of the output of the spectral resolution may be counteracted. Hence, the present invention is particularly useful in the analysis of sound waves and in speech recognition systems.