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Masking
Masking, is a property of he human ear, whereby tones are not heard due to the current presence (spectral masking) or recent presence (temporal masking) of other, usually louder, tones.
Temporal Masking
The following figure shows how a "mask tone" reduces the ear's sensitivity to any tone that commences after the "mask tone", stops playing. For about 1.5 milliseconds after the 60 dB (clearly audible, but not loud) "mask tone" stops playing, the ear fails to hear a tone at the samelevel or, or a lower level. Then, tones just below the level of the "mask tone" start being audible. As the time after the "mask tone" stops playing increases, the ear starts to restore its sensitivity. After about 5 milliseconds, tones up to 20 dB down from the "mask tone" are audible. After 200 milliseconds, tones over 45 dB down may be heard. Eventually, the normal threshold of hearing that is around 0 dB, is restored.
Spectral Masking
The following figure shows how a "mask tone" reduces the ear's sensitivity to any tone that is at a lower level and at a nearby freqency. As shown, a 1 kHz tone at 60 dB reduces the ear's sensitivity at nearby frequencies.
The lower curve is the normal threshold of hearing, with no "mask tone" present. The upper curve shows how the 60 dB 1 kHz tone reduces the ear's sensitivity to the point where a the threshold for hearing a 2 KHz tone is about 10 dB higher than what it would be with no mask tone present. Masking is far higher, about 40 dB, for frequencies in the 1/3 octave band centered at 1 KHz. In other words, a tone in the 1/3 octave range around a 60 dB, 1 kHz tone has to be at least 40 dB above the threshold of hearing with no mask tone present.
Masking makes levels of noise and distortion that are well above the normal threshold of hearing remain inaudible. For example, the second harmonic of a 60 dB, 1 KHz tone would be audible, if 55 dB down, were it not for masking. Because of masking by the 1 kHz fundamental, it has to be about 10 dB louder, or about 45 dB down, in order to be heard. You can see this reflected in the chart of the audible threshold of THD.
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