Interindividual differences in directional loudness

Abstract

Sensibility of loudness to sound direction was measured in a two-intervals, 2AFC interleaved-adaptive procedure. We measured the level (at the center of the listener head) of a test sound matching the loudness of a reference fixed at 0° and 65 dB SPL (at the center of the listener head). Azimuths of the test sound varied from 0 to 180° by steps of 15°. Elevations were 15°, 30°, 45°, 60°, and 90°. Both test and reference sounds were wide-band noise. The difference between the levels of reference and test sounds at the point of loudness equality defined the Directional Loudness Sensibility (DLS). A group of 24 listeners showed up large differences in DLS. For the majority, the highest DLSs were around -4 dB. For a few of them DLSs were as large as -9 dB. Some listeners did not exhibit any DLS and could even achieve positive DLSs. The sound pressure levels, measured at the entrance of the ear canal, were related to the DLSs to see whether the interindividual differences in DLS were exclusively due to the filtering by the head or if extra-acoustics factors (such as binaural loudness constancy) might be involved in directional loudness sensitivity.