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Using the experimental technique developed during earlier work on the binaural threshold (Hughes 1938), the effect on the monaural threshold of subliminal notes has now been investigated. It was hoped that the experiments, when carried out with two notes of various frequency differences, would yield information on the extent to which the cochlear resonators postulated by the resonance theory could be influenced by notes of frequencies different from their natural frequencies, and thus permit the determination of the amount of damping to which they were subject. It was anticipated that the threshold of a note of given frequency would be lowered by the simultaneous application to the ear of a second subliminal note provided that the two frequencies were not too far apart. In practice it was found that no matter how different the frequencies, a process of central summation produced such lowering of threshold. The purely physical question of what range of resonators in the cochlea responds to a given exciting tone cannot therefore be investigated in this way. The lowering of threshold of one ear due to simultaneous subliminal stimulation of the other (Hughes 1938) made it of interest to investigate the case when the note applied to the second ear is audible. Previous workers (e.g. Harvey Fletcher 1929) have shown that when a “ masking tone” is well above threshold no change of contralateral threshold is produced. The experimental results described below bear this out in a general way, but the presence of a weak audible tone causes so much difficulty in determining the contralateral threshold that it is not possible to find any critical intensity at which summation ceases. When, however, the two notes were in unison a rise in this threshold occurred, the explanation of which is discussed later.

It is shown that, in common with that for other sensory stimuli, the threshold of audition rises as the time of presentation of the stimulus is decreased. Investigation of the law relating change of threshold to period of stimulation suggests that it has a form similar to that found in the case of other sensory mechanisms.

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.

It is well known that if a pure tone is presented binaurally so that its intensity is the same in each ear, and there is no phase difference between the notes at the ears, the resulting sensation is that of a source of sound located in the median plane and situated is that of a source of sound located in the median plane and situated either within the skull or outside it. If a difference of phase between the notes is gradually introduced without altering their intensities, and the frequency of the tone is, say, 800 cyc. /sec., then the source appears to move laterally towards the side of the ear in which the phase is made to lead. According to some observers, the movement is on a horizontal circular arc centred between the ears, while others consider it to be along a line joining the ears. There is general agreement among workers on localization that considerable difficulty exists in the detection of such movements at frequencies above 800 cyc. /sec., and that their extent is more limited, but they disagree as to the frequency above which the lateral motion ceases to be noted. Stewart (1920) obtained movement at 1280 cyc. /sec., but not 1536 cyc. /sec.; Banister (1925) found movements at 1040 and 1345 cyc. /sec., but anomalous in character. On the other hand, von Hornbostel and Wertheimer (1920) state that at about 800 cyc. /sec., the lateral movement no longer extends 90° left and right, but decreases as the frequency increases, and should reach zero at 17,000 cyc. /sec. Halverson (1927) found that observable right and left effects appear to be present up to the upper auditory limit. Below 1400 cyc. /sec. judgments of direction are fairly consistent, between 1400 and 3000 cyc. /sec. there is no particular difficulty if allowance is made for the more rapid onset of fatigue at these frequencies. Above 3000 cyc. /sec. lateral effects are still observable, though median localization is extremely difficult to achieve. Hartley (1919) gave theoretical curves showing the phase difference produced at the ears by sources of various frequencies at different distances from the head. According to these, above 650 cyc. /sec. the maximum movement of the apparent source would be less than 90°, though at 1860 cyc. /sec. there should still be about 27° of movement on either side of the median plane. These curves have been checked experimentally by Firestone (1930), who found for three frequencies the phase difference and amplitude ratio of the sounds entering the ears of a man-shaped wax dummy from a source at varying azimuths around the head and at different distances from it. The phase differences found were in good agreement with the values calculated by Hartley. Various methods have been used to present the notes to the ears and to vary the phase difference between them. In the experiments of Halverson the sounds were produced by a tuning fork and led to the ears through tubes, in one of which was incorporated a sliding section to vary its length and thus the phase difference at the ears. An objection to this is the possibility of resonances in the tubes, different on both sides on account of their different lengths, so that intensity changes might occur as well as changes oh phase. Stewart, and Banister, used notes generated electrically and presented to the ears by telephones. Continuous phase changes could be made with their generators, but the purity of the resulting notes was doubtful.