Amplitude-modulated noise: the detection of modulation versus the detection of modulation rate

Modulation threshold, that is, the modulation depth required to discriminate a sample of amplitude-modulated (AM) noise from a sample of unmodulated noise, was measured as a function of modulation rate (16--320 Hz), modulator waveform (sine or square), and the bandwidth of the AM noise (0.5--8.0 kHz). Modulation threshold increases monotonically with modulation rate, sine-wave thresholds are greater than square-wave thresholds, and threshold rises as the bandwith of the AM stimulus decreases. These effects all support the use of some form of energy detection model to explain modulation threshold. The modulation thresholds were compared with pitch thresholds gathered under precisely the same conditions. Pitch threshold or, alternatively, rate threshold was taken to be the modulation depth required to decide which of two samples had the higher modulation; the rate difference was 20%--just over three semitones. In the region above about 70 Hz, rate threshold is essentially a constant multiple of modulation threshold, indicating that the primary constraint on rate threshold is the audibility of the modulation. Below 70 Hz, rate and modulation threshold diverge; it is argued that the limit on rate threshold in this region is probably the length of the correlation required to extract the periodicity.