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

Area of Science:

• Auditory perception
• Psychoacoustics
• Acoustics

Background:

• A small peak near 1000 Hz in the threshold for detecting interaural level difference (ILD), known as the Grantham effect, has been consistently observed in human listeners across multiple laboratories.
• This phenomenon suggests a specific auditory processing anomaly in this frequency region.

Purpose of the Study:

• To review existing literature on the Grantham effect.
• To propose and investigate a novel explanation for the Grantham effect.
• To determine the physical basis and perceptual implications of the observed anomaly.

Main Methods:

• Review of eight previous studies on the Grantham effect.
• Analysis of perceptual localization of tones and narrowband noise at 1000 Hz.
• Free-field front/back measurements using a KEMAR manikin to quantify physical front-back level differences.
• Investigation of listener strategies in discounting auditory localization cues.

Main Results:

• The Grantham effect is a reproducible finding in auditory experiments.
• A proposed explanation links the effect to "in-back" perceptual localization caused by anomalous physical front-back level differences.
• KEMAR manikin measurements confirmed a unique and widespread physical anomaly in front-back level differences around 1000 Hz.
• Listeners may learn to de-emphasize left-right localization cues at 1000 Hz due to less reliable spatial information from sounds originating behind the listener.

Conclusions:

• The Grantham effect likely originates from a combination of physical front-back level anomalies and learned perceptual strategies.
• The anomalous physical cues at 1000 Hz lead to unique "in-back" localization, prompting listeners to rely less on binaural cues in this frequency range.
• Understanding this effect provides insights into the complex mechanisms of human auditory localization and spatial hearing.