The acoustical bright spot and mislocalization of tones by human listeners

Abstract

Listeners attempted to localize 1500-Hz sine tones presented in free field from a loudspeaker array, spanning azimuths from 0°͑straight ahead͒ to 90°͑extreme right͒. During this task, the tone levels and phases were measured in the listeners' ear canals. Because of the acoustical bright spot, measured interaural level differences ͑ILD͒ were non-monotonic functions of azimuth with a maximum near 55°. In a source-identification task, listeners' localization decisions closely tracked the non-monotonic ILD, and thus became inaccurate at large azimuths. When listeners received training and feedback, their accuracy improved only slightly. In an azimuth-discrimination task, listeners decided whether a first sound was to the left or to the right of a second. The discrimination results also reflected the confusion caused by the non-monotonic ILD, and they could be predicted approximately by a listener's identification results. When the sine tones were amplitude modulated or replaced by narrow bands of noise, interaural time difference ͑ITD͒ cues greatly reduced the confusion for most listeners, but not for all. Recognizing the important role of the bright spot requires a reevaluation of the transition between the low-frequency region for localization ͑mainly ITD͒ and the high-frequency region ͑mainly ILD͒.

Computing interaural differences through finite element modeling of idealized human heads

“…For the ILD, the spherical head model produces a peak as a function of azimuth because of the acoustical bright spot for a source at the extreme left or right side (azimuth of 90 degrees with respect to the forward direction). Such a peak is seen in measurements on human listeners, and it has a strong perceptual effect (Macaulay et al, 2010). However, the spherical head model also has serious deficiencies that become apparent upon detailed comparison with measured interaural differences.…”

Auditory Spatial Perception: Auditory Localization

“…Due to the physics of wave diffraction around the head (Kuhn, 1977;, the maximum IID appears not at 90° but at a smaller angle, making the relationship between the IID and azimuth angle non-monotonic. However, the higher the frequency, the higher the IID and the larger the angle at which the IID reaches its maximum (Macaulay et al, 2010). Thus as frequency increases, the angle of maximum IID approaches 90° and the non-monotonicity is gradually reduced.…”

“…Thus as frequency increases, the angle of maximum IID approaches 90° and the non-monotonicity is gradually reduced. The non-monotonic behavior of the IID does cause large localization uncertainty for mid-high frequency tones (1000-1600 Hz) that arrive from locations more than 30-40° off the midline (Firestone, 1930;Macaulay et al, 2010;Mills, 1958;Nordlund, 1962ab).…”

“…Although categorical localization was the predominant localization methodology in older studies (e.g., Bergman, 1957;Bienvenue and Siegenthaler, 1974;Kuyper and de Boer, 1969), it is still commonly used today (Abel and Banerjee, 1996;Inoue, 2001, Macaulay et al, 2010Van Hoesel and Clark, 1999;Vause and Grantham, 1999). For example, the Source Azimuth Identification in Noise Test (SAINT) uses categorical judgments with a circular array of 12 loudspeakers (Vermiglio et al, 1998) and a standard system for testing the localization ability of cochlear implant users is categorical with 8 loudspeakers distributed in a symmetric manner in the horizontal plane in front of the listener with 15.5° of separation (Tyler and Witt, 2004).…”

Onset- and offset-specific effects in interaural level difference discrimination

“…In the free field, these various cues tend to agree in corresponding to a single, veridical, sound-source direction, but that relationship may be disrupted by acoustical effects including diffraction around the head (Macauley et al, 2010), echoes (Rakerd and Hartmann, 1985), and reverberation. In such cases, accurate localization requires either selection of the more reliable cues or an appropriately weighted combination of discrepant cues.…”