The precedence effect, the observation that sound-source localization is determined largely by the interaural cues associated with the earlier-arriving direct sound to the neglect of later-arriving reflections, was investigated in several psychophysical experiments. The first experiment employed a stimulus composed of a continuous noise and its delayed repetition to simulate a direct sound and a single reflection. Comparison of the interaural amplitude and phase differences in this stimulus with its judged lateral position showed that the interaural amplitude and phase differences in this stimulus with its judged lateral position showed that the interaural differences do not predict lateralization judgments as simply as does a knowledge of the temporal pattern of stimulation, given the precedence effect. Most of the experiments were attempts to outline the time course of the precedence effect by measuring just-noticeable differences in interaural time and intensity of brief (less than 5 ms) wide-band noise bursts. The principal finding of these experiments was that interaural sensitivity to changes in both time and intensity follows a nonmonotonic course after the abrupt onset of as ound. Sensitivity is degraded for a period from approximately 0.5 to 10 ms after onset, with the largest jnd's at delays of 2-3 ms. The precedence effect can be understood as resulting from this temporary lapse of interaural sensitivity. It was speculated that this temporal variation in interaural acuity serves to inhibit: (1) the representation of multiple and ambiguous interaural time delays between corresponding points on the waves of periodic stimuli; and (2) the extraction of interaural phase at frequencies greater than approximately 1400 Hz.

In this paper evaluations of a two-microphone adaptive beamforming system for hearing aids are presented. The system, based on the constrained adaptive beamformer described by Griffiths and Jim [IEEE Trans. Antennas Propag. AP-30, 27-34 (1982)], adapts to preserve target signals from straight ahead and to minimize jammer signals arriving from other directions. Modifications of the basic Griffiths-Jim algorithm are proposed to alleviate problems of target cancellation and misadjustment that arise in the presence of strong target signals. The evaluations employ both computer simulations and a real-time hardware implementation and are restricted to the case of a single jammer. Performance is measured by the spectrally weighted gain in the target-to-jammer ratio in the steady state. Results show that in environments with relatively little reverberation: (1) the modifications allow good performance even with misaligned arrays and high input target-to-jammer ratios; and (2) performance is better with a broadside array with 7-cm spacing between microphones than with a 26-cm broadside or a 7-cm endfire configuration. Performance degrades in reverberant environments; at the critical distance of a room, improvement with a practical system is limited to a few dB.

A simple model to summarize the precedence effect is proposed that uses a single metric to quantify the relative dominance of the initial interaural delay over the trailing interaural delay in lateralization. This model is described and then used to relate new measurements of the precedence effect made with adjustment and discrimination paradigms. In the adjustment task, subjects matched the lateral position of an acoustic pointer to the position of a composite test stimulus made up of initial and trailing binaural noise bursts. In the discrimination procedure, subjects discriminated interaural time differences in a target noise burst in the presence of another burst either trailing or preceding the target. Experimental parameters were the delay between initial and trailing stimuli and the overall level of the stimulus. The model parameters (the metric c and the variability of lateral position judgments) were estimated from the results of the matching experiment and used to predict results of the discrimination task with good success. Finally, the observed values of the metric were compared to values derived from previous studies.

Saberi and Perrott [Acustica 81, 272-275 (1995)] found that the in-head lateralization of a relatively long-duration pulse train could be controlled by the interaural delay of the single pulse pair that occurs at onset. The present study examined this further, using an acoustic pointer measure of lateralization, with stimulus manipulations designed to determine conditions under which lateralization was consistent with the interaural onset delay. The present stimuli were wideband pulse trains, noise-burst trains, and inharmonic complexes, 250 ms in duration, chosen for the ease with which interaural delays and correlations of select temporal segments of the stimulus could be manipulated. The stimulus factors studied were the periodicity of the ongoing part of the signal as well as the multiplicity and ambiguity of interaural delays. The results, in general, showed that the interaural onset delay controlled lateralization when the steady state binaural cues were relatively weak, either because the spectral components were only sparsely distributed across frequency or because the interaural time delays were ambiguous. Onset dominance can be disrupted by sudden stimulus changes within the train, and several examples of such changes are described. Individual subjects showed strong left-right asymmetries in onset effectiveness. The results have implications for understanding how onset and ongoing interaural delay cues contribute to the location estimates formed by the binaural auditory system.

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