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When a narrow band of noise, of constant level (the signal) is masked by a wider band of noise (the masker), the amount of noise required to drown out the signal often depends significantly upon the interaural phase relations of signal and masker. Thus, more masking noise is required if the signal is 180° out of phase at the two ears than if it is in phase: provided that the masker remains in phase. The out-of-phase signal, in other words, shows an advantage in audibility. The advantage is very striking for a signal near 250 cps; and falls off progressively toward 1400 cps, above which it is small. At all frequencies between 250 and 6000 cps, the level of masking, for a signal in phase at the ears, remains relatively constant. An important property of physical stimuli that produce an audibility-advantage is an interaural phase-deviation caused by the signal. When the signal is added to a corresponding masking component, the combined pressure-wave, on the average, leads in one ear and lags in the other. The resulting phase-deviation depends upon the relative levels and phase-positions of the masking and masked components. It is maximum when these components are 90° apart. The interaural time-deviation thus produced depends also upon frequency. If the frequency is increased, the level of the masking component must be decreased in order to hold the time-deviation, associated with the different frequencies, constant. The present experiments showed that, with the out-of-phase signal, the decline in level of masking, as a function of frequency, is just such as to hold the interaural time-deviation (for the 90° position) constant at about 125 microseconds. Tonal signals tend to show a decreasing critical time-deviation with increasing frequency.

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