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If the noise is random, sampling it over time will produce a normal distribution , with its mean at the average noise level (shown below in blue; note that cyan is the presence of both blue and green distributions). If there is a recurring signal present, its energy will be added to the background; sampling signals over time would also produce a normal distribution of energy, but with a higher mean (shown in green below). The intensity difference ( d') between these means reflects the effect of the signal on the observer; a large difference results from a high sensitivity to the signal, while a small difference shows a low sensitivity.