Elevation localization and head-related transfer function analysis at low frequencies

Abstract

Monaural spectral features due to pinna diffraction are the primary cues for elevation. Because these features appear above 3 kHz where the wavelength becomes comparable to pinna size, it is generally believed that accurate elevation estimation requires wideband sources. However, psychoacoustic tests show that subjects can estimate elevation for low-frequency sources. In the experiments reported, random noise bursts low-pass filtered to 3 kHz were processed with individualized head-related transfer functions (HRTFs), and six subjects were asked to report the elevation angle around four cones of confusion. The accuracy in estimating elevation was degraded when compared to a baseline test with wideband stimuli. The reduction in performance was a function of azimuth and was highest in the median plane. However, when the source was located away from the median plane, subjects were able to estimate elevation, often with surprisingly good accuracy. Analysis of the HRTFs reveals the existence of elevation-dependent features at low frequencies. The physical origin of the low-frequency features is attributed primarily to head diffraction and torso reflections. It is shown that simple geometrical approximations and models of the head and torso explain these low-frequency features and the corresponding elevations cues.

Influence of aging on human sound localization

“…The auditory spatial cues involved in sound localization include interaural time (and/or phase) and intensity differences (ITDs and IIDs, respectively) in azimuth and pinna/headgenerated spectral cues in elevation that also assist front-back discrimination (Algazi et al 2001;Asano et al 1990;Middlebrooks 1992;Musicant and Butler 1984;Oldfield and Parker 1984b;Rayleigh 1907;Wightman and Kistler 1989b). For pure tones, ITDs are accompanied by interaural phase differences (IPDs) that are reliable only up to ϳ1.5 kHz in humans due to phase ambiguity coupled with declining neuronal temporal coding (phase-locking) at higher frequencies (Dreyer and Delgutte 2006;Johnson 1980;Joris and Yin 1992;Klumpp and Eady 1956;Zwislocki and Feldman 1956).…”

Sound Source and Loudspeaker Base Angle Dependency of Phantom Image Elevation Effect

“…On the other hand, the role of low frequencies in vertical localization has also been reported by Morimoto et al [7]. Gardner [8] and Algazi et al [9] showed that torso reflections produce spectral notches in the HRTF below 3 kHz, which are additional localization cues for an elevated source.…”

“…0, for a sound source in the median plane, a torso reflection produces a spectral notch in the HRTF below 3 kHz when it is combined with the direct sound at the ear [8,9]. Algazi et al [9] showed that such a notch varies in frequency as a function of torso reflection delay, which is elevation-dependent. The torso reflection delay tends to increase as the elevation angle increases, and reaches its maximum when the source is in the "above" region.…”

“…Long term average spectra (LTAS) of the natural sound sources used for the experiment; the frame length was 4096 samples with a Hanning window and 50% overlap, the FFT point was 4096, and a 1/6-octave Gaussian smoothing was applied to the resulting spectral magnitude. that corresponds to the perceived position is selected (e.g., Morimoto et al [7], Algazi et al [9]). However, from a pilot test it was recognized to be a challenging and timeconsuming task to precisely localize perceived image position in the median plane, especially when the image appeared above or behind the listener.…”

Application of sound source separation methods to advanced spatial audio systems.

“…However humans are still able to localize sound in the median plane by what is known as monaural cues, which are related to the spectral changes introduced by the outer ears (i.e. pinnae) at high frequencies and other body structures like the torso at low frequencies [111]. Some studies have shown that these cues help listeners with complete hearing loss in one ear to localize the azimuth direction of a source with relatively high accuracy.…”