The difference between monaural and binaural thresholds.

Although there is considerable agreement on the increase in apparent loudness of auditory stimuli presented binaurally as compared with monaural stimulation at supraliminal intensities, less agreement is found concerning binaural summation at liminal or subliminal intensities. Experimenters in the past have reported the binaural threshold to be (1) the same as that of the “better” ear, (2) lower than that of the “better” ear, or (3) lower than that of the “average” ear. The present paper shows the binaural threshold to be approximately 3 db lower than the “equated” monaural threshold for both pure tones and speech. “Normal” summation occurs when the stimuli presented to both ears are physically equal and the binaural threshold is 1.5 db lower than that of the “better” ear and 4 db lower than that of the “average” ear (average value of 14 frequencies tested). “Maximum” binaural summation occurs when the stimuli presented are functionally equal, i.e. when the stimulus intensities differ according to the difference between right and left monaural thresholds. For pure tones the “maximum” binaural threshold is 3.6 db below the thus equated monaural threshold (average of 14 frequencies tested). The comparable binaural threshold for speech intelligibility is approximately 3 db lower than the equated monaural threshold.