The role of envelope modulation in spectrally unresolved iterated rippled noise

Iterated rippled noise (IRN) produces a pitch corresponding to the IRN delay. The pitch persists even when the sound is high-pass filtered at 12 times the reciprocal of the IRN delay, i.e., in the absence of resolved spectral peaks. Typically, when a sound produces a pitch in the absence of spectral cues, the pitch is explained in terms of periodic envelope modulation, for example, the pitch of a high-pass filtered cosine-phase harmonic complex, or the pitch of sinusoidally amplitude-modulated noise (SAMN). This study presents experiments designed to search for periodic modulation in IRN. The occurrence and significance of modulation is investigated in the envelope of the stimulus waveform as well as in the IRN envelope as represented after narrow-band filtering similar to that occurring in peripheral auditory filters. The results indicate that the envelope of band-pass filtered IRN reveals modulation but that the order of modulation (corresponding to the number of envelope maxima recurring every period) increases with increasing filter bandwidth. The occurrence of first-order modulation, like that of SAMN, is indirectly demonstrated for spectrally unresolved IRN in the lower unresolved frequency range between the 10th and 20th spectral peaks. The significance of recurring transients sometimes visible in the IRN waveform with respect to their contribution to the IRN pitch was assessed by replacing portions of the IRN period with random noise. The results of this experiment indicate that this 'waveform modulation' is not essential for the IRN pitch perception. The presence of temporal pitch in the absence of first-order modulation is demonstrated in two experiments involving the detection of phase delays and f0 differences for spectrally separated, narrow bands of harmonic complexes and IRNs.