Masking Level Difference (Mld) - Interacoustics Affinity 2.0 Additional Information

2.0
Affinity Additional Information
1.7.6

Masking Level Difference (MLD)

Masking Level Difference refers to 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 auditory system's ability to perceive differences in
timing of a sound reaching the two ears. This helps to localize 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 an "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) are out of phase with each other when reaching the two ears. The three test conditions
for the MLD in the AC40 are:
•
SoNo: Signal and Noise are IN PHASE when reaching the two ears. (Homophasic condition)
• SπNo: Signal is OUT OF PHASE, noise is IN PHASE when reaching the two ears (Antiphasic
condition)
SoNπ: Signal is IN PHASE, noise is OUT OF PHASE when reaching the two ears (Antiphasic
•
condition)
The MLD is measured by presenting a low frequency pulsed tone with simultaneous presentation of the
corresponding narrow band noise, starting at an intensitiy of 60 or 65 dB dB to both ears. The first condition
should be to find the threshold for the homophasic condition (referred to as SoNo). 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 SπNo condition, that is, the condition where the signal is out of phase when
reaching the two ears but the noise is still in phase. .
Example: If the masked threshold for the SoNo condition is 60 dB, then we reverse the phase of the signal
by 180 degrees. The threshold for this phase reversed condition, called SπNo, improves to 44 dB. The
masking level difference from SoNo to SπNo is therefore 16 dB.
Most research that has been done on the MLD indicates that in general, if the MLD is less than 7 dB, 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 asymmetrical. Normal results for MLD is usually
around 12 dB (Brown & Musiek, Hearing Journal, Jan 2013).
Required Items:
•
2.0
The Affinity hardware
•
Licensed AC440 Audiometry module within your Affinity
•
Calibrated Headphones or insert phones
•
A patient response button
8
MLD requires an additional software license and is only available as an additional feature for the Affinity
8
2.0
hardware with MLD functionality
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