Listeners attempted to localize 1500-Hz sine tones presented in free field from a loudspeaker array, spanning azimuths from 0°͑straight ahead͒ to 90°͑extreme right͒. During this task, the tone levels and phases were measured in the listeners' ear canals. Because of the acoustical bright spot, measured interaural level differences ͑ILD͒ were non-monotonic functions of azimuth with a maximum near 55°. In a source-identification task, listeners' localization decisions closely tracked the non-monotonic ILD, and thus became inaccurate at large azimuths. When listeners received training and feedback, their accuracy improved only slightly. In an azimuth-discrimination task, listeners decided whether a first sound was to the left or to the right of a second. The discrimination results also reflected the confusion caused by the non-monotonic ILD, and they could be predicted approximately by a listener's identification results. When the sine tones were amplitude modulated or replaced by narrow bands of noise, interaural time difference ͑ITD͒ cues greatly reduced the confusion for most listeners, but not for all. Recognizing the important role of the bright spot requires a reevaluation of the transition between the low-frequency region for localization ͑mainly ITD͒ and the high-frequency region ͑mainly ILD͒.

Human listeners, and other animals too, use interaural time differences (ITD) to localize sounds. If the sounds are pure tones, a simple frequency factor relates the ITD to the interaural phase difference (IPD), for which there are known iso-IPD boundaries, 90°, 180°… defining regions of spatial perception. In this article, iso-IPD boundaries for humans are translated into azimuths using a spherical head model (SHM), and the calculations are checked by free-field measurements. The translated boundaries provide quantitative tests of an ecological interpretation for the dramatic onset of ITD insensitivity at high frequencies. According to this interpretation, the insensitivity serves as a defense against misinformation and can be attributed to limits on binaural processing in the brainstem. Calculations show that the ecological explanation passes the tests only if the binaural brainstem properties evolved or developed consistent with heads that are 50% smaller than current adult heads. Measurements on more realistic head shapes relax that requirement only slightly. The problem posed by the discrepancy between the current head size and a smaller, ideal head size was apparently solved by the evolution or development of central processes that discount large IPDs in favor of interaural level differences. The latter become more important with increasing head size.

Previous headphone experiments have shown that listeners can lateralize high-frequency sine-wave amplitude-modulated (SAM) tones based on interaural time differences in the envelope. However, when SAM tones are presented to listeners in free field or in a room, diffraction by the head or reflections from room surfaces alter the modulation percentages and change the shapes of the envelopes, potentially degrading the envelope cue. Amplitude modulation is transformed into mixed modulation. This article presents a mathematical transformation between the six spectral parameters for a modulated tone and six mixed-modulation parameters for each ear. The transformation was used to characterize the stimuli in the ear canals of listeners in free-field localization experiments. The mixed modulation parameters were compared with the perceived changes in localization attributable to the modulation for five different listeners, who benefited from the modulation to different extents. It is concluded that individual differences in the response to added modulation were not systematically related to the physical modulation parameters themselves. Instead, they were likely caused by individual differences in processing of envelope interaural time differences.

It is difficult to localize the source of a tone in a room because standing waves lead to complicated interaural differences that become uninterpretable localization cues. This paper tests the conjecture that localization improves if the listener can move to explore the complicated sound field over space and time. Listener head and torso movements were free and uninstructed. Experiments at low and high frequencies with eight human listeners in a relatively dry room indicated some modest improvement when listeners were allowed to move, especially at high frequencies. The experiments sought to understand listener dynamic localization strategies in detail. Head position and orientation were tracked electronically, and ear-canal signals were recorded throughout the 9 s of each moving localization trial. The availability of complete physical information enabled the testing of two model strategies: (1) relative null strategy, using instantaneous zeros of the listener-related source angle; and (2) inferred source strategy, using a continuum of apparent source locations implied by the listener's instantaneous forward direction and listener-related source angle. The predicted sources were given weights determined by the listener motion. Both models were statistically successful in coping with a great variety of listener motions and temporally evolving cues.

Reflections and standing waves in a room cause distorted binaural information, making it difficult for listeners to localize ongoing sound sources. Possibly head motion allows a listener to sample a distorted sound field, gaining dynamic information that enables more accurate localization. The present research tries to discover listener strategies for localizing pure tones using a variable acoustics environment. Over the course of 9-s trials, a head tracker recorded head position and orientation while probe microphones in the ear canals recorded the signals and interaural differences. Therefore, the experiment provided complete information. Two model strategies are considered. In the “nulling” strategy, the listener discovers temporally and spatially discrete head pointing directions where the dominant interaural difference is zero. Candidate directions are weighted by head-pointing perseverance in the vicinity of the interaural zero. In the “inferred source” strategy the listener registers temporally and spatially continuous inferred-source candidates based on instantaneous interaural differences and head pointing directions. Each candidate is weighted by the instantaneous ratio of head-angle change to inferred-source change evaluated at the candidate location. Model strategies are evaluated by comparing weighted candidate source locations with listener response locations.