Dichotic Loudness Summation for Pure Tones

The over-all loudness of two equally loud tones, one presented to the left ear, the other to the right ear, was measured as a function of their frequency separation. The two tones were presented simultaneously for 700 msec and matched in loudness to a 2000-Hz binaural tone. Matches were made at 20, 50, and 80 dB above the threshold for the binaural tone. The geometric mean of the dichotic tones was 2000 Hz; seven frequency separations were used, from 100 Hz (one-third of a critical band) to 4530 Hz (12 critical bands). Results from six subjects indicated that the loudness of the two tones presented dichotically is independent of frequency separation. Moreover, at all three sensation levels dichotic loudness summation is the same quantitatively as binaural loudness summation. On the average, a dichotic or binaural sound is about as loud as a monaural tone that is 8 dB more intense. [Work supported by a grant from the U. S. Public Health Service, National Institute of Neurological Diseases and Blindness.]