Envelope regularity discrimination.

The ability to discriminate irregular from regular amplitude modulation was assessed using the "envelope regularity discrimination" test. The amount of irregularity was parametrically varied and quantified by an "irregularity index." Normative data were gathered for young subjects with normal audiometric thresholds. Parameters varied were the carrier and modulation frequencies, f and f, and the baseline modulation index, m. All tests were performed using a background threshold-equalizing noise. The main findings were (1) using f = 4000 Hz, f = 8 Hz, and m = 0.3, performance improved over the first two threshold runs and then remained roughly stable, and there was a high correlation between thresholds obtained at 80 dB sound pressure level (SPL) and at 20 dB sensation level; (2) using f = 8 Hz and m = 0.3 with a level of 80 dB SPL, thresholds did not vary significantly across f = 1000, 2000, and 4000 Hz; (3) using f = 8 Hz and f = 4000 Hz with a level of 80 dB SPL, thresholds did not vary significantly for m from 0.2 to 0.5; and (4) using m = 0.3 and f = 4000 Hz with a level of 80 dB SPL, thresholds improved with increasing f from 2 to 16 Hz. For all conditions, there was substantial individual variability, probably resulting from differences in "processing efficiency."