Front-back sound level differences for human heads and the 1000-Hz interaural level anomaly

Data from headphone experiments with human listeners indicate a small peak near 1000 Hz in the threshold for the detection of interaural level difference (Grantham effect). The peak has been observed in at least six different laboratories. This article discusses this peak in three parts. The first part reviews eight articles on the peak effect. The second part begins a new explanation for the effect. It proposes that the peak originates through the unique "in-back" perceptual localization of tones or narrowband noise in the 1000-Hz region, evidently caused by an anomalous physical front-back level difference. The angular extent of this physical anomaly is here determined through extensive free-field front/back measurements with a KEMAR manikin, indicating that the anomalous level difference is unique and extends over a wide range of source azimuths at two elevations. The third part suggests that listeners have learned to discount the left-right localization cues in the 1000-Hz frequency region in favor of more reliable frequency regions because individual sounds that appear in back are much less well localized in the left-right dimension than sounds that appear in front. These facts may be the origin of the Grantham Effect.