Transaural experiments and a revised duplex theory for the localization of low-frequency tones.

The roles of interaural time difference (ITD) and interaural level difference (ILD) were studied in free-field source localization experiments for sine tones of low frequency (250-750 Hz). Experiments combined real-source trials with virtual trials created through transaural synthesis based on real-time ear canal measurements. Experiments showed the following: (1) The naturally occurring ILD is physically large enough to exert an influence on sound localization well below 1000 Hz. (2) An ILD having the same sign as the ITD modestly enhances the perceived azimuth of tones for all values of the ITD, and it eliminates left-right confusions that otherwise occur when the interaural phase difference (IPD) passes 180°. (3) Increasing the ILD to large, implausible values can decrease the perceived laterality while also increasing front-back confusions. (4) Tone localization is more directly related to the ITD than to the IPD. (5) An ILD having a sign opposite to the ITD promotes a slipped-cycle ITD, sometimes with dramatic effects on localization. Because the role of the ITD itself is altered by the ILD, the duplex processing of ITD and ILD reflects more than mere trading; the effect of the ITD can be reversed in sign.

Predicting the timing of dynamic events through sound: Bouncing balls.

Dynamic information in acoustical signals produced by bouncing objects is often used by listeners to predict the objects' future behavior (e.g., hitting a ball). This study examined factors that affect the accuracy of motor responses to sounds of real-world dynamic events. In experiment 1, listeners heard 2-5 bounces from a tennis ball, ping-pong, basketball, or wiffle ball, and would tap to indicate the time of the next bounce in a series. Across ball types and number of bounces, listeners were extremely accurate in predicting the correct bounce time (CT) with a mean prediction error of only 2.58% of the CT. Prediction based on a physical model of bouncing events indicated that listeners relied primarily on temporal cues when estimating the timing of the next bounce, and to a lesser extent on the loudness and spectral cues. In experiment 2, the timing of each bounce pattern was altered to correspond to the bounce timing pattern of another ball, producing stimuli with contradictory acoustic cues. Nevertheless, listeners remained highly accurate in their estimates of bounce timing. This suggests that listeners can adopt their estimates of bouncing-object timing based on acoustic cues that provide most veridical information about dynamic aspects of object behavior.

On the ability of human listeners to distinguish between front and back.

In order to determine whether a sound source is in front or in back, listeners can use location-dependent spectral cues caused by diffraction from their anatomy. This capability was studied using a precise virtual reality technique (VRX) based on a transaural technology. Presented with a virtual baseline simulation accurate up to 16 kHz, listeners could not distinguish between the simulation and a real source. Experiments requiring listeners to discriminate between front and back locations were performed using controlled modifications of the baseline simulation to test hypotheses about the important spectral cues. The experiments concluded: (1) Front/back cues were not confined to any particular 1/3rd or 2/3rd octave frequency region. Often adequate cues were available in any of several disjoint frequency regions. (2) Spectral dips were more important than spectral peaks. (3) Neither monaural cues nor interaural spectral level difference cues were adequate. (4) Replacing baseline spectra by sharpened spectra had minimal effect on discrimination performance. (5) When presented with an interaural time difference less than 200 micros, which pulled the image to the side, listeners still successfully discriminated between front and back, suggesting that front/back discrimination is independent of azimuthal localization within certain limits.

Lateralization of Huggins pitch.

The Huggins pitch is a sensation of pitch generated from a broadband noise having a narrowband boundary region where the interaural phase difference varies rapidly as a function of frequency. Models of binaural hearing predict that the pitch image should be well lateralized. A direct psychophysical experimental method was used to estimate the lateral positions of Huggins pitch images with two different forms of phase boundaries, linear phase and stepped phase. A third experiment measured the lateral positions of sine tones with controlled interaural phase differences. The results showed that the lateralization of Huggins pitch stimuli was similar to that of the corresponding sine tones and that the lateralizations of the two forms of Huggins pitch phase boundaries were even more similar to one another. Both Huggins pitches and sine tones revealed strong laterality compression (exponent approximately 0.5). Ambiguous stimuli, with an interaural phase difference of 180 degrees , were consistently lateralized on one side or the other according to individual asymmetries-an effect called "earedness." An appendix to this article develops a new first-order lateralization model, the salient phase density model, which combines attributes of previous models of dichotic pitch lateralization.

Lateralization of sine tones-interaural time vs phase (L).

Listeners estimated the lateral positions of 50 sine tones with interaural phase differences ranging from -150 degrees to +150 degrees and with different frequencies, all in the range where signal fine structure supports lateralization. The estimates indicated that listeners lateralize sine tones on the basis of interaural time differences and not interaural phase differences.

Binaural models and the strength of dichotic pitches.

Modern physiologically based models of the binaural system incorporate internal delay lines in the pathways from left and right peripheries to central processing nuclei. Different binaural models for the formation of dichotic pitch employ these delay lines in different ways. Consequently, the different models make different predictions for the relative strengths of dichotic pitches made with particular phase conditions. The differences are magnified for dichotic pitches at low frequencies where especially long delay lines may be required. Data from four low-frequency pitch strength experiments on pure-tone-like dichotic pitches (two on Huggins pitch and two on binaural coherence edge pitch) are consistent with models of the equalization-cancellation type and not consistent with the central activity pattern model.