Simultaneous Masking - Mechanisms of Masking

Mechanisms of Masking

There are many different mechanisms of masking, one being suppression. This is when there is a reduction of a response to a signal due to the presence of another. This happens because the original neural activity caused by the first signal is reduced by the neural activity of the other sound.

Combination tones are products of a signal and a masker. This happens when the two sounds interact causing new sound, which can be more audible than the original signal. This is caused by the non linear distortion that happens in the ear. For example, the combination tone of two maskers can be a better masker than the two original maskers alone.

The sounds interact in many ways depending on the difference in frequency between the two sounds. The most important two are cubic difference tones and quadratic difference tones.

Cubic difference tones are calculated by the sum

F1 – F2

(F1 being the first frequency, F2 the second) These are audible most of the time and especially when the level of the original tone is low. Hence they have a greater effect on psychoacoustic tuning curves than quadratic difference tones.

Quadratic difference tones are the result of

F2 – F1

This happens at relatively high levels hence have a lesser effect on psychoacoustic tuning curves.

Combination tones can interact with primary tones resulting in secondary combination tones due to being like their original primary tones in nature, stimulus like. An example of this is

3F1 – 2F2

Secondary combination tones are again similar to the combination tones of the primary tone.