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The level difference required to make a sine wave with noise as loud as a reference sine wave at the same frequency but without noise. The level of the reference sine wave for each subject was at five levels from threshold to 80% of the conservative estimate of the uncomfortable loudness level (denoted as levels 1 to 5). The error bars show the standard error about the mean at each frequency and level (100 Hz: five subjects, 1 kHz: four subjects, and 5 kHz: four subjects).
