Effects of stimulus intensity on sound localization in the horizontal and upper-hemispheric median plane.

Sound lateralization ability with headphones is known to be affected by stimulus intensity. However, there have been few studies on sound localization with loudspeakers in free field from the standpoint of stimulus intensity. The aim of this study is to obtain basic data for the tests with normal hearing and hearing-impaired subjects, and for the fitting of a hearing aid. The author investigated the relationship between stimulus intensities and sound localization ability of the broadband noise and pure tone bursts delivered in the horizontal and upper-hemispheric median planes. Subjects were ten volunteers, 5 males and 5 females, at the age of 18-19 years, with normal hearing. The results showed well how subjects perceived directions at all stimulus intensities. The percentage of correct response (%correct) for broadband noise stimuli was maximum at the intensity from 40 to 70 dB SPL (sound pressure level), and was minimum at 0 to 20 dB SPL in both the horizontal plane and the upper-hemispheric median plane. At lower stimulus intensity, examinee could not judge the direction, even if they could hear the stimuli. For pure tone bursts stimuli in the upper-hemispheric median plane, %correct was low irrespective of stimulus intensity.