Binaural unmasking

Binaural unmasking is phenomenon of auditory perception where the brain combines information from the two ears in order to improve signal detection and identification in noise. The phenomenon, discovered by Ira Hirsh, is most commonly observed when there is a difference between the interaural phase of the signal and the interaural phase of the noise. When such a difference is present there is an improvement in masking threshold compared to a reference situation in which the interaural phases are the same, or when the stimulus has been presented monaurally. Those two cases usually give very similar thresholds. The size of the improvement is known as the "binaural masking level difference" (BMLD), or simply as the "masking level difference".

Binaural unmasking is most effective at low frequencies. The BMLD for pure tones in broadband noise reaches a maximum value of about 15 decibels (dB) at 250 Hz and progressively declines to 2-3 dB at 1500 Hz. The BMLD then stabilises at 2-3 dB for all higher frequencies, up to at least 4 kHz. Binaural unmasking can also be observed for narrowband masking noises, but the effect behaves differently: larger BMLDs can be observed and there is little evidence of a decline with increasing frequency.