If a person hears two sounds that fluctuate in intensity, the point at which he or she can detect the contrast in the sounds is referred to as the

difference threshold

Explanation:

Difference threshold refers to a situation in which two stimuli are being perceived simultaneously, but these stimuli do not have the same intensity and the lower concentration of one of these stimuli allows the contrast between their intensities to be perceived. An example of this can be seen in the question above, where a person can hear two sounds that fluctuate in intensity and the point at which he can detect the contrast in the sounds is known as the difference threshold.

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