The effect of impedance on interaural azimuth cues derived from a spherical head model

Abstract

Recent implementations of binaural synthesis have combined high-frequency pinna diffraction data with low-frequency acoustic models of the head and torso. This combination ensures that the salient cues required for directional localization in the horizontal plane are consistent with psychophysical expectations, regardless of the accuracy or match of the high-frequency cues, or the fidelity of experimental low-frequency information. This paper investigates the effect of a nonrigid boundary condition on the surface pressure and the resulting interaural cues used for horizontal localization. These are derived from an analytical single sphere diffraction model assuming a locally reacting and uniformly distributed impedance boundary condition. Decreasing the magnitude of a purely resistive surface impedance results in an overall decrease in the sphere surface pressure level, particularly in the posterior region. This produces nontrivial increases in both the interaural level and time difference, especially for sound source directions near the interaural axis. When the surface impedance contains a reactive component the interaural cues exhibit further changes. The basic impedance characteristics of human hair and their incorporation into the sphere diffraction model are also discussed.

The effect of hair on auditory localization cues

“…This is consistent with the general agreement that skin is sufficiently rigid throughout the audible frequency range ͑Katz, 2000͒. However, results from an earlier analytical study by Treeby et al ͑2007d͒ ͑using a spherical head model with uniformly distributed acoustic properties͒ suggest that impedance values representative of human hair can introduce noticeable modifications to the interaural azimuth cues. Empirical studies using mannequins ͑Riederer, 2005͒, spheres ͑Treeby et al, 2007c͒, and the boundary element method ͑Katz, 2001͒ have also shown that the addition of hair introduces asymmetrical perturbations to the HRTF in the order of several decibels.…”

Computing interaural differences through finite element modeling of idealized human heads

“…Replacing our box torso by a torso with sloping, or rounded, shoulders might bring the predicted ripples into better agreement with measured values. All of the surfaces in our model were perfectly reflecting; more realistic calculations would use surfaces with appropriate finite impedances, and those would have an effect on the outcome (Treeby, 2007).…”

An experimental study of the acoustic impedance characteristics of human hair

“…[3][4][5][6] These features are also noticeable in human HRTF, although little is currently known about their perceptual significance. The traditional use of sphere scattering models to account for the broad acoustic properties of the human head assume the head surface is completely rigid.…”

“…The mathematical formulation of this problem is reviewed in many texts and is not repeated here, e.g., Ref. 3. Given a 0.124 m radius rigid sphere uniformly covered in the synthetic hair material, the pressure exterior to the surface can be calculated using the normal acoustic impedance.…”

“…The traditional use of such models assumes that the head surface is completely rigid in nature. 1,2 However, the surface impedance of the head affects both the binaural localization cues, 3 and the head-related transfer function ͑HRTF͒. [4][5][6] Given the appropriate impedance data, recent mathematical treatments of the sphere scattering problem facilitate an anthropometric investigation into the contribution of hair to the auditory percept.…”