Characteristics Of Head-related Transfer Functions Research Articles

Properties of low-frequency head-related transfer functions in the barn owl (Tyto alba)

The barn owl (Tyto alba) possesses several specializations regarding auditory processing. The most conspicuous features are the directionally sensitive facial ruff and the asymmetrically arranged ears. The frequency-specific influence of these features on sound has consequences for sound localization that might differ between low and high frequencies. Whereas the high-frequency range (>3 kHz) is well investigated, less is known about the characteristics of head-related transfer functions for frequencies below 3 kHz. In the present study, we compared 1/3 octaveband-filtered transfer functions of barn owls with center frequencies ranging from 0.5 to 9 kHz. The range of interaural time differences was 600 micros at frequencies above 4 kHz, decreased to 505 micros at 3 kHz and increased again to about 615 micros at lower frequencies. The ranges for very low (0.5-1 kHz) and high frequencies (5-9 kHz) were not statistically different. Interaural level differences and monaural gains increased monotonically with increasing frequency. No systematic influence of the body temperature on the measured localization cues was observed. These data have implications for the mechanism underlying sound localization and we suggest that the barn owl's ears work as pressure receivers both in the high- and low-frequency ranges.

Numerical Analysis of the Effects of Pinna Shape and Position on the Characteristics of Head‐Related Transfer Functions

There are distinctive notches and peaks in Head‐Related Transfer Functions (HRTFs). Some of them are considered as important cues in the perception of the elevation angle and thus the roles of these peaks and notches should be clarified. It is known that the characteristics of HRTFs are deeply related to listener's anthropometry. It is thus naturally expected that frequencies of the peaks and notches also change according to the individuality of listeners' anthropometry. Therefore, in this study, effects of ear shapes and positions on frequency positions of the peaks and notches are examined by numerical analyses. The analysis was performed with boundary element method (BEM). A three‐dimensional model of a dummy‐head was constructed with a three‐dimensional laser scanner and HRTFs of the model were numerically computed with a BEM solver. The model was modified on some features as follows: 1) pinna position, 2) pinna size, 3) angle of pinna toward listener's head, and 4) existence of wrinkles of pinna. HRTFs of the modified models were also computed. From the comparison among the results of calculation, several systematic changes were found. For example, the frequency position of notches around 8 kHz is shifted toward lower frequency as the size of pinna increases.

Sound localization under conditions of covered ears on the horizontal plane

In this paper, we examine how covering one or both external ears affects sound localization on the horizontal plane. In our experiments, we covered subjects’ pinnae and external auditory canals with headphones, earphones, and earplugs, and conducted sound localization tests. Stimuli were presented from 12 different directions, and 12 subjects participated in the sound localization tests. The results indicate that covering one or both ears decreased their sound localization performance. Front-back confusion rates increased, particularly when covering both outer ears with open-air headphones or covering one ear with an intraconcha-type earphone or an earplug. Furthermore, incorrect answer rates were high when the sound source and the occluded ear that had an intraconcha-type earphone or an earplug were on the same side. We consider that the factors that cause poor performance can be clarified by comparing these results with characteristics of head-related transfer function.

Spatial Characteristics of Head-Related Transfer Function

The spatial characteristics of head-related transfer functions (HRTFs)are studied by a spatial Fourier analysis. A law of the HRTF spatialsampling in different elevation planes is obtained, and thecorresponding spatial interpolating method used to recover thecontinuous HRTF is proposed. The method is valid, for the average errorbetween the measured and interpolated HRTFs in the horizontal plane isabout 0.5%. These results provide guidance for the discrete spatialmeasurement of HRTFs.