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Auditory Masking

Auditory masking refers to the situation in which one sound reduces a listener's ability to detect or discriminate another sound. We are all familiar with situations in which what we are trying to hear is obscured by the presence of other sounds. For example, we may have difficulty hearing a friend speak at a rock concert, or we may have difficulty hearing a knock on the door when using a vacuum cleaner. These are examples of auditory masking.

In the psychophysical literature, the sound that is masking another sound is called the masker, and the sound that a listener is trying to detect is called the signal. Masking is often quantified by determining the smallest detectable sound intensity (threshold intensity) for the signal in the presence of the masker or sometimes the masker intensity required to render the signal undetectable. Detection depends in complex ways on the relations between the physical characteristics of the masker and signal. Important parameters include sound intensity, presentation time, frequency, and binaural presentation (the way the sounds are presented to the two ears).

Parametric Effects

Sound Intensity

Weber's law states that the smallest detectable change in a quantity is proportional to the value of that quantity before the change. Weber's law is common across sensory systems and, for hearing, can be restated as the general rule that the smallest detectable change in sound intensity is approximately proportional to the baseline intensity. So, as masker intensity increases, so must the intensity of the signal at threshold. Sound intensity is usually expressed as sound level in decibels (dB), that is, on a logarithmic scale. Equal multiples in intensity correspond to equal intervals in dB. Expressed in dB, the signal-to-masker ratio at threshold is roughly constant, having a value in ideal listening conditions of about −10 dB for maskers and signals with identical frequency composition. This means that we can just detect a signal with a level 10 dB less (10 times less intense) than the masker level when the two sounds occupy the same frequency region (although see section on frequency selectivity).

Temporal Factors

Masking is maximal when the masker and the signal are presented at the same time (simultaneous masking). However, masking may also be caused by a masker presented either just before (forward masking) or just after (backward masking) the signal. Backward masking is a weak effect in trained listeners and only causes an increase in the threshold within 20 milliseconds or so of the onset of the masker. Forward masking, however, can persist for over 100 milliseconds after the offset of the masker.

Frequency Selectivity

The basilar membrane in the cochlea separates out the different frequency components of sounds. Each place on the membrane is tuned to a particular “characteristic” frequency and, for a pure tone, sensitivity decreases as the frequency of the tone is moved away from the characteristic frequency. The frequency selective properties of the ear mean that masking is greatest when sounds occupy the same frequency region. When sounds occupy remote frequency regions, the sounds are separated on the basilar membrane, and little masking may occur. This enables us to “hear out” sounds of different frequencies played at the same time (e.g., different musical instruments in an orchestra).

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