The monaural threshold: effect of a subliminal contralateral stimulus

Abstract

It has been assumed by some workers that the intensity of a note of any frequency in either ear at the binaural threshold is one-half of the threshold intensity in either ear when used alone, i.e. at the monaural threshold. Another form of the assumption is the statem ent that the monaural threshold intensity is 3.0 db. above the intensity in either ear at the binaural threshold. In the course of the investigation of the possibility of forming a loudness scale, Churcher (1935), and Fletcher and Munson (1933), have found that the intensity of a note when heard binaurally may be assumed to be onehalf of that of the same note heard monaurally when the note is judged to be equally loud under the two conditions of listening. In the present work, this fact has been further verified. The measurements were extended to the cases in which the subliminal note presented to the one ear w’as maintained at various levels below the monaural threshold, and the intensity of the note applied to the other ear to reach the “ binaural” threshold was determined. In this way it was possible to find the relation between the energy required at the monaural threshold and the total energy required at the “binaural” threshold determined under these conditions. The experiments were also carried out with notes of different frequencies in the two ears in an endeavour to investigate the variations of total energy in such cases. Two heterodyne oscillators were used to provide pure notes of the various frequencies used during the work. The first of these (oscillator A ) was already available, and has been described elsewhere (Shaxby and Gage 1932); the second (oscillator B ) was a commercial oscillator of similar frequency range, viz. 30-10,000 c./sec. A , if allowed to run for about an hour, was very stable in frequency, varying by only 1 or 2 c./sec. during the next 7 hr. B drifted for about 2 hr. fairly rapidly, and thereafter drifted quite steadily at the rate of 10-15 c./sec. every hour. A was therefore used as the frequency standard. It was adjusted to read correctly by the “zerobeat” method after the “settling down” period, and set to the particular frequency required; B was brought into unison with it. The actual frequency of B could be determined in this way at any time, and its drift checked.