Ripple density resolution for various rippled-noise patterns.

Ripple-density resolution was measured in normal humans using rippled noise with a phase-reversal test. The principle of the test was to find the highest ripple density at which an interchange of spectral peak and trough positions (the phase reversal) is detectable. Different rippled noise patterns were used: (i) either frequency-proportional or constant ripple spacing; (ii) various bandwidth; and (iii) either steep or shallow slopes of the spectrum envelope. When tested with frequency-proportional rippled noise, ripple-density resolution as expressed in relative units (the center frequency to ripple spacing ratio) little depended on frequency within a range of 1 to 8 kHz: from 11.4 at 1 kHz to 14.9 at 8 kHz, mean 13.1. These values were virtually independent on noise bandwidth. When tested with constant ripple spacing, the resolution was of similar values taking the relative ripple density at the lower part of the passband. Being measured by noise with steep spectral edges, the resolution was five units higher than it was for shallow-enveloped spectra, thus suggesting some edge effects at the spectrum boundaries. The resolution values obtained were about twice higher than those predicted by peripheral auditory filter tuning.