The chorus effect revisited-experiments in frequency-domain analysis and simulation of ensemble sounds

The so-called 'chorus' or 'ensemble' effect is interesting both musically and perceptually. It is usually imitated in effect devices using slowly varying time shifts, giving the impression of rotating speakers rather than of an ensemble. M. Dolson (1983) found that the quasi-random amplitude modulation of beating partials alone can cue the perception of ensemble. The small changes in frequency, he found, are less salient perceptually. This suggests an alternative simulation of the chorus effect. Attempts were made to corroborate Dolson's finding, and to simulate ensembles in the frequency domain by modulating only partial tone amplitudes, using three approaches: filter banks, real FFTs and complex FFTs. The exact partial envelopes of a choral sound were found to be elusive, partly because the sidebands of one partial will overlap its neighbours at higher frequencies. The outcome of these trials is discussed and illustrated with sound examples.