Toward orthogonal non-individualised head-related transfer functions for forward and backward directional sound: cluster analysis and an experimental study.

Individualised head-related transfer functions (HRTFs) have been shown to accurately simulate forward and backward directional sounds. This study explores directional simulation for non-individualised HRTFs by determining orthogonal HRTFs for listeners to choose between. Using spectral features previously shown to aid forward-backward differentiation, 196 non-individualised HRTFs were clustered into six orthogonal groups and the centre HRTF of each group was selected as representative. An experiment with 15 listeners was conducted to evaluate the benefits of choosing between six centre-front and six centre-back directional sounds rather than the single front/back sounds produced by MIT-KEMAR HRTFs. Sound localisation error was significantly reduced by 22% and 65% of listeners reduced their front-back confusion rates. The significant reduction was maintained when the number of HRTFs was reduced from six to five. This represents a preliminary success in bridging the gap between individual and non-individual HRTFs for applications such as spatial surround sound systems. STATEMENT OF RELEVANCE: Due to different pinna shapes, directional sound stimuli generated by non-individualised HRTFs suffer from serious front-back confusion. The reported work demonstrates a way to reduce front-back confusion for centre-back sounds generated from non-individualised HRTFs.

The precedence effect: no evidence for an "active" release process found.

Hafter et al. [Auditory Function: The Neurobiological Bases for Hearing (Wiley, New York, 1988)] have reported that "post-onset adaptation," as measured with trains of high-frequency clicks in a lateralization paradigm, can instantly be released by presentation of an additional, short trigger signal, which is spectrally different from the click trains. As post-onset saturation may be one of the psychoacoustic components of the precedence effect, it has been investigated whether suitable trigger signals might also induce a release from echo inhibition. However, no evidence for such an "active" release process could be observed in a number of exploratory precedence-effect settings.

Auditory spaciousness: some further psychoacoustic analyses.

Psychoacoustic investigations have confirmed the known high correlation of auditory spaciousness and preference in concert-hall types of sound fields. Further, they have provided exploratory results which gave rise to the following interpretations: Auditory spaciousness is a multidimensional perceptual attribute, predominantly caused by early lateral reflections. Low-frequency reverberation may also contribute slightly, but is sensitive with respect to individual taste. All spectral components of early lateral reflections contribute to spaciousness. Spectral restriction of the reflections leads to less favorable judgments. Early lateral reflections which do not contain spectral components above about 3 kHz mainly create image expansion in the front-back direction, thus adding to the sense of envelopment of the listener. If components above about 3 kHz are present, image broadening is prominent. By means of a model of binaural signal processing which takes interaural arrival-time differences as well as interaural level differences into account, a quasiobjective measure of auditory spaciousness can be derived.

Spatial mapping of intracranial auditory events for various degrees of interaural coherence.

The broadening and splitting of auditory events in dichotic listening conditions with various degrees of interaural coherence are discussed. By using a psychoacoustical mapping method, it has been possible to observe broadening and splitting for a wide range of stimuli, including broadband pink noise as well as bandpass noises with different relative bandwidths and center frequencies. The spatial extents of the auditory events decrease with increasing center frequencies for bandpass stimuli of constant relative bandwidth. The number of partial events for bandpass stimuli decreases with increasing degrees of interaural coherence. These results are, for example, of interest with respect to auditory spaciousness in architectural acoustics.

Binaural localization.

In this paper we review some issues which are relevant to the formation of special positions and extents of auditory events in the case of two normally functioning ears. Physical aspects under consideration are measurement and analysis of the transfer functions of the external ears. These transfer functions are responsible for encoding information about the spatial characteristics of sound fields into the ear input signals. The hearing system can decode this information by evaluating interaural and monaural cues of these signals. A rough outline of psychoacoustical effects and mechanisms involved in this evaluation is given and a conceptual model of binaural interaction is described.

Lateralization of jittered tones.

Lateralization thresholds based on differences in interaural arrival time have been measured for jittered 4-kHz tones. The results are in good agreement with those of other authors obtained with bandpass noise as well as with AM and FM signals. However, thresholds are about two orders of magnitude higher than those of Nordmark [J. Acoust. Soc. Am. 60, 870-880 (1976)] obtained with jittered tones. These results suggest an explanation of the lateralization of jittered tones and other FM signals on the basis of an hypothesized FM-to-AM conversion which may take place peripherally.