However, binaural hearing provides additional advantages, including binaural summation and binaural squelch. As presented in two previous posts, input from two ears allows a listener to identify the location of a sound in space. The head shadow is critical to sound localization. As a result, the head shadow would increase the SNR to the left ear, meaning that the noise would not be as effective as a masker. Thus, the head shadow effect increases the signal-to-noise (SNR) because noise coming to a right ear would affect the right ear, but the head would cast an acoustic shadow to that sound reaching the left ear. Both of these involve sounds arriving at the two ears in different ways (time and intensity) due to the acoustic shadow created by the head. Spatial effects (spatial unmasking) of binaural hearing include the head shadow effect and the squelch effect. The improved SNR reflects a combination of elimination of the head shadow effect (allowing for sound localization) and binaural squelch. Listening with two ears benefits from the head shadow by focusing/attending to the ear with the better SNR.
This is especially true if one ear is closer to the noise source, and then adding a second, or contralateral ear, resulting in a better signal-to-noise ratio (SNR). Instead of nullifying any potential binaural advantage by creating a physical barrier between the ears, the barrier created by the head separating a signal of interest (speech as an example) from noise can create an advantage. Previous posts have listed factors contributing to the benefit of binaural hearing, with a major contributing factor being that of head shadow. This post will concentrate on loudness squelch (identified in the literature also as, and related to: release from masking, masking level difference, facilitation in noise, cocktail party effect, and auditory figure ground). Head diffraction has been discussed previously, and binaural redundancy is being reserved for a future post. This binaural advantage is believed to arise from a combination of head diffraction, redundancy, and binaural squelch effects. Studies have reported that listening with two ears is better than listening with one, especially in binaural understanding of speech in a noisy environment.
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