The Masking Level Difference (MLD) is the improvement in detecting a tone or speech in noise when the phase of the tone or the noise is reversed by 180 degrees. It aims to assess central auditory function and is specifically sensitive to brainstem lesions, but peripheral changes (like a hearing loss) may also affect the MLD.
The MLD is a low frequency phenomenon, related to the ability of the auditory system to perceive differences in timing of a sound reaching the two ears. This helps to localise low frequency sounds that reach the ears at different times due to the longer wave length.
The MLD is usually referred to as the difference of improvement in dB between a “homophasic” (in-phase) condition and an “antiphasic” (out of phase) condition. Homophasic means that both the signal and the noise are in phase with each other when reaching the two ears. Antiphasic means that either the signal or the noise (not both) is out of phase with each other when reaching the two ears.
The three test conditions for the MLD are:
The MLD is measured by presenting a low frequency pulsed tone with simultaneous presentation of the corresponding narrow band noise, starting at an intensity of 60 or 65dB to both ears. The first condition should be to find the threshold for the homophasic condition (referred to as S0N0). The next step is to measure the antiphasic condition, either presenting the tone out of phase or the noise out of phase and the masked threshold is determined again. If the brainstem is functioning normally, there will be an improvement in the masked threshold from the homophasic condition to the antiphasic condition. The condition that will yield the greatest MLD is the SπN0 condition, that is, the condition where the signal is out of phase when reaching the two ears but the noise is still in phase.
Make a masked threshold search. The threshold will typically be equal to the noise in the S0N0 condition.
The masked threshold for the S0N0 condition is 60dB and we reverse the phase of the signal by 180 degrees the threshold for this phase reversed condition (SπN0) improves to 44dB. Then the masking level difference (MLD) from SoNo to SπN0 is 16dB.
Most research having been done on the MLD indicates that in general, if the MLD is less than 7dB, this will indicate a problem with the brainstem and binaural interaction. There will also be a decrease in the hearing MLD if there is a peripheral hearing loss or if the hearing aid is asymmetrical. A normal result for MLD is usually around 12dB (Brown & Musiek, 2013).
Brown, M., Musiek, F. (2013). The Fundamentals of MLD for Assessing Auditory Function. Hearing Journal