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Summary

The Grantham effect, a peak in detecting interaural level differences near 1000 Hz, may stem from unique "in-back" sound localization. This anomaly is linked to physical front-back level differences and learned auditory cue discounting.

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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.