ASA 127th Meeting M.I.T. 1994 June 6-10

5pPP2. The effects of two types of level variation on the size of the binaural masking level difference.

Virginia M. Richards

G. Bruce Henning

Department of Psychol., Univ. of Pennsylvania, Philadelphia, PA 19104

Observers were required to detect 500-, 110-, or 20-ms bursts of 250-, 500-, or 5000-Hz tones presented in simultaneously gated narrow (30-Hz) noise bands using standard two-interval forced-choice tracking procedures. The noise was in phase at the observers' ears; the signal was in phase in one set of experiments and (pi) radians out of phase in another. Three different ``roving-level'' conditions were used: (1) fixed level the noise-power density, N[sub 0], was fixed (50 dB SPL), (2) inter-interval rove a random gain factor was chosen for every observation interval from a set uniformly distributed in decibels (40-dB range; 0.1-dB steps) to produce N[sub 0] centered on 50 dB SPL. The gain factor affected signal and noise in both ears equally and thus left the signal-to-noise ratio, interaural phase differences and amplitude ratios unaffected, (3) interaural rove the random gain varied from ear-to-ear in every observation interval. Reducing duration reduced BMLD's in all conditions; roving levels reduced BMLD's by small amounts except at 20 ms where interaural rove removes them. [This research was supported by The National Institutes of Health.] [sup a)]On leave from the Dept. of Exp. Psychol., Oxford.