Auditory vertical localization in the median plane with conflicting dynamic interaural time difference and other elevation cues.

Both dynamic variation of interaural time difference (ITD) and static spectral cues provide information for front-back discrimination and vertical localization. However, the contributions of the two cues are still unclear. The static spectral cue has conventionally been regarded as the dominant one. In the present work, psychoacoustic experiments were conducted to examine the contribution of dynamic ITD and static spectral cues to vertical localization in the median plane. By modifying the head-related transfer functions used in a dynamic virtual auditory display, binaural signals with conflicting dynamic ITD and spectral cues that were either static or dynamically modified according to instantaneous head position were created. The results indicated that the dynamic ITD and static spectral cues contribute to vertical localization at low and high frequencies, respectively. For full a bandwidth stimulus, conflicting dynamic ITD and static spectral cues usually result in two separated virtual sources at different elevations corresponding to the spatial information conveyed by the low- and high-frequency bands, respectively. In most cases, no fused localization occurs in the high-level cognition system. Therefore, dynamic ITD and static spectral cues contribute to vertical localization at different frequency ranges, and neither of them dominates vertical localization in the case of wideband stimuli.

Artificial enveloping reverberation for binaural auralization using reciprocal maximum-length sequences.

Binaural auralization through proper room-acoustic simulation can produce a realistic listening experience as if the listener were sitting in a room with spatial perception, including enveloping reverberance. Based on analysis of experimentally measured binaural room-acoustic data, this paper discusses an approach to creating artificial but natural-sounding reverberation for binaural rendering that can be employed in simulating such an environment in an efficient way. Approaches to adjusting the spaciousness of enveloping reverberance within the context of artificially generated reverberation are investigated via hearing tests. This paper exploits the excellent pseudorandom properties of maximum-length sequences to generate deterministic and controllable decorrelations between binaural channels for artificial reverberation for room-acoustic simulations with high computational efficiency. To achieve natural-sounding enveloping reverberance in an enclosed space, and thereby an immersive environment, the shapes of both the reverberation energy decays and the spatial characteristics are found to be decisive. This paper discusses systematic hearing test results that support the mentioned finding.

Near-field head-related transfer-function measurement and database of human subjects.

Near-field head-related transfer functions (HRTFs) of human subjects are essential to those researching spatial hearing. By using a carefully designed measurement system, near-field HRTFs of human subjects were measured and a database was constructed. The database includes 56 Chinese human subjects, seven source distances from 0.2 to 1.0 m, and 685 directions at each distance for each subject. In the present work, the technique of near-field HRTF measurement is outlined, the performance of the measurement system is assessed and validated, and the resultant database is reported. The database can provide fundamental data for future research.

Recovery of individual head-related transfer functions from a small set of measurements.

Head-related transfer functions (HRTFs) vary with individuals, and in practice, measuring HRTFs with high directional resolution for each individual is tiresome. Based on a basis functions representation of HRTFs, the present work proposes a method for recovering individual HRTFs from a small set of measurements. The HRTFs are represented by a combination of a small set of spatial basis functions (SBFs) with frequency- and individual-dependent weights. The SBFs are derived by applying spatial principal component analysis to a baseline HRTF dataset with high directional resolution. The individual weights for any subject outside the dataset are estimated from measurements at a few source directions, and then the HRTFs with high directional resolution are recovered by combining the SBFs and the individual weights. In an illustrative case, the SBFs derived from a baseline dataset that includes 20 subjects are used to recover the HRTF magnitudes for six subjects outside the baseline dataset. Results show that individual HRTF magnitudes can be recovered from measurements at 73 directions with a mean signal-to-distortion ratio of 19 dB. The proposed method is also applicable to recovering head-related impulse responses. The results of psychoacoustic experiments indicate that in most cases the recovered and measured HRTFs are indistinguishable.

Maximal azimuthal resolution needed in measurements of head-related transfer functions.

The head-related transfer function (HRTF) is a continuous function of sound source position. Measurement of the HRTF can only be undertaken at discrete positions in space, however. Determination of angular resolutions so as to reconstruct HRTFs at unmeasured positions has been an open problem. Azimuthal Fourier analysis was proposed to analyze the variation in the HRTF in each elevation plane. As a result, an azimuthal sampling theorem and a corresponding interpolation formula were derived. It was proved that the maximal azimuthal resolution of measurements is 360 degrees /(2Q+1), where Q represents the highest order in the truncation of the azimuthal Fourier expansion of the HRTF. The maximal azimuthal resolutions for the HRTF with and without arrival time correction were investigated. Results show that the arrival time correction can reduce the burden of measurements, since a larger azimuthal resolution is possible without introducing obvious interpolation error. A psychoacoustic experiment was also conducted to evaluate the proposed theorem.