There is an alternative way of looking at the JND of intensity or sound level. Instead of asking how much the intensity of one given tone must be changed to give a JND effect, we ask:
What is the minimum intensity I2 that a second tone of the same frequency and phase must have to be noticed in the presence of the first one I1 whose intensity is kept constant? That minimum intensity I2 is called the threshold of masking. The original tone is called the masking tone and the additional tone is called the masked tone. Masking plays a key role in music.
The relation between the masking level ML (SL of tone threshold) and the JND of a sound level can be found using relation SL = 10log (I/) and taking into account the properties of logarithms. For e.g. for a JND of 0.2 db one obtains a ML that is 13 db less than the SL of the masking tone; a JND of 0.4 db corresponds to a ML that is 10 db below the masking tone.
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