The effect of hair on human sound localisation cues

Bradley Treeby

    Research output: ThesisDoctoral Thesis

    344 Downloads (Pure)

    Abstract

    The acoustic scattering properties of the human head and torso match well with those of simple geometric shapes. Consequently, analytical scattering models can be utilised to account for the sound localisation cues introduced by these features. The traditional use of such models assumes that the head surface is completely rigid in nature. This thesis is concerned with modelling and understanding the effect of terminal scalp hair (i.e., a non-rigid head surface) on the auditory localisation cues. The head is modelled as a sphere, and the acoustical characteristics of hair are modelled using a locally-reactive equivalent impedance parameter. This allows the scattering boundary to be defined on the inner rigid surface of the head. The boundary assumptions are validated experimentally, through impedance measurement at oblique incidence and analysis of the near-field scattering pattern of a uniformly covered sphere. The impedance properties of human hair are also discussed, including trends with variations in sample thickness, bulk density, and fibre diameter. A general solution for the scattering of sound by a sphere with an arbitrarily distributed, locally reactive surface impedance is then presented. From this, an analytical solution is derived for a surface boundary that is evenly divided into two uniformly distributed hemispheres. For this boundary condition, cross-coupling is shown to exist between incoming and scattered wave modes of equi-order when the degrees are non-equal and opposite in parity. The overall effect of impedance on the resultant scattering characteristics is discussed in detail, both for uniform and for hemispherically divided surface boundaries. Finally, the analytical formulation and the impedance characteristics of hair are collectively utilised to investigate the effect of hair on human auditory localisation cues. The hair is shown to produce asymmetric perturbations to both the monaural and binaural cues. These asymmetries may help to resolve localisation confusions between sound stimuli positioned in the front and rear hemi-fields. The cue changes in the azimuth plane are characterised by two predominant features and remain consistent regardless of the decomposition baseline (i.e., the inclusion of a pinna offset, neck, etc). Experimental comparisons using a synthetic hair material show a good agreement with simulated results.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2007

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