Binaural Masking Level DifferenceBinaural Masking Level Difference
The masked threshold of a signal can sometimes be lower when listening with two ears rather than one; this is demonstrated by the phenomenon of the binaural masking level difference (BMLD). The BMLD can be summarised as follows; the detection of a signal in noise is improved when either the phase or level differences of the signal at the two ears are not the same as the masker. An implication of this is that the signal and masker appear to originate from different locations in space; hence, the BMLD appears to be related to the well-known 'cocktail party effect'.
In general, we can describe a particular stimulus using the symbols S (for signal) and N (for noise), each followed by a suffix to indicating the relative phase in the two ears; 'o' for same phase (so-called homophasic) and 'pi' for a 180 degree (pi radians) phase difference (so-called antiphasic). For example, NoSo means that the noise and signal both have the same phase in each ear, and NoSpi means that the noise has the same phase, but the signal is 180 degrees out of phase. Nu means that the noise is uncorrelated in the two ears, and the suffix m indicates monaural presentation (i.e. presentation to one ear only).
BMLDs are largest for low-frequency tones. This demonstration uses a tone frequency of 250 Hz.
Choose one of the presets from the menu (1). Now, move the slider in the CONDITION 1 box (2) until the tone is just masked by the noise, using the LISTEN button (3) to hear the stimulus. The masking level is shown in dB (4). Now do the same in the CONDITION 2 box (5); move the slider until the tone is just masked by the noise. The BMLD (6) is the difference between the masking level in the CONDITION 2 box and the masking level in the CONDITION 1 box.
Try each of the preset conditions and note the BMLD in each case. Compare your figure with the following, which are typical BMLDs for each interaural condition:
NoSo/NoSpi
15 dB
NoSo/NpiSo
13 dB
NmSm/NoSm
9 dB
NmSm/NpiSm
6 dB
NoSo/NuSo
4 dB
NoSo/NuSpi
3 dB
B.C.J. Moore (1997) An introduction to the psychology of hearing (fourth edition). Academic Press.
Produced by: Guy J. Brown
Release date: 6th November 1998.
Permissions: This demonstration may be used and modified freely by anyone. It may be distributed in unmodified form.