Effect of sound level on virtual and free-field localization of brief sounds in the anterior median plane

Abstract

The detection of high-frequency spectral notches has been shown to be worse at 70–80 dB sound pressure level (SPL) than at higher levels up to 100 dB SPL. The performance improvement at levels higher than 70–80 dB SPL has been related to an ‘ideal observer’ comparison of population auditory nerve spike trains to stimuli with and without high-frequency spectral notches. Insofar as vertical localization partly relies on information provided by pinna-based high-frequency spectral notches, we hypothesized that localization would be worse at 70–80 dB SPL than at higher levels. Results from a first experiment using a virtual localization set-up and non-individualized head-related transfer functions (HRTFs) were consistent with this hypothesis, but a second experiment using a free-field set-up showed that vertical localization deteriorates monotonically with increasing level up to 100 dB SPL. These results suggest that listeners use different cues when localizing sound sources in virtual and free-field conditions. In addition, they confirm that the worsening in vertical localization with increasing level continues beyond 70–80 dB SPL, the highest levels tested by previous studies. Further, they suggest that vertical localization, unlike high-frequency spectral notch detection, does not rely on an ‘ideal observer’ analysis of auditory nerve spike trains.