Binaural Thresholds Research Articles

Some Determinants of Interaural Phase Effects

It has recently been shown that the binaural threshold for a pure tone presented against a background of noise depends upon the interaural phase differences of the tone and the noise. For example, the threshold of a low frequency tone, in phase at the two ears, is low when it is heard against noise that is out of phase at the two ears, but is high when it is heard against noise that is in phase. The masked threshold of a 250-cycle tone presented against a background of intense noise is in fact about 15 db lower under antiphasic conditions than it is under homophasic conditions. (Homophasic refers to the condition in which the tone and the noise have the same interaural phase difference. Antiphasic refers to the condition in which either tone or noise, but not both, is out of phase at the two ears.) When the tone is masked by another tone, fairly close in frequency but not so close that beats are detected, the dependence of masking on interaural phase does not appear. The present experiment attempts to determine what characteristics masking sound (“masker”) must have if it is to produce interaural phase effects. A 250-cycle tone was presented against four different kinds of background. The masking sounds were (1) pure tones, (2) regular pulses (125 p.p.s.), (3) random pulses (average 125 p.p.s.) and (4) random noise. Combinations of filters permitted the presentation of these maskers in frequency bands that were varied in width and center frequency. The results indicate that the threshold for the tone in the presence of a regular, periodic masker does not depend to any significant degree upon interaural phase relations. A necessary characteristic of the masker that produces interaural phase effects is randomness or irregularity. Both masking and the interaural phase effects increase as the frequency band of a random masker approaches the frequency of the tone. Band width alone does not contribute to the interaural phase effects except as widening a band tends to bring one cut-off nearer the frequency of the tone. Wide and narrow band maskers, both of which contain the frequency of the tone, produce the same effects.

Statistical bias in comparisons of monaural and binaural thresholds: binaural summation or binaural supplementation.

Statistical bias in comparisons of monaural and binaural thresholds: binaural summation or binaural supplementation.

The Influence of Interaural Phase on Interaural Summation and Inhibition

The difference between the binaural and monaural thresholds varies within fairly large limits. Although this binaural-monaural difference may not vary systematically as a function of frequency when pure-tone thresholds are measured in the quiet, the difference varies greatly, as does frequency, when pure tones are presented against a background of noise. Binaural summation, a term which has been used in the past to refer to the phenomenon in which the binaural threshold is lower than the monaural, obtains for masked thresholds only when the phase angles between the two earphones are opposite for the tones and the noise. If the tones and the noise have the same interaural phase angle the binaural threshold is higher than the monaural. This phenomenon, called interaural inhibition, as well as its antipode, interaural summation, is most marked at low frequencies and increases as the intensity of the masking noise is increased. The discussion considers some implications of these findings for the theory of masking and of interaural summation.

The Influence of Interaural Phase on Interaural Summation and Inhibition

The difference between the binaural and monaural thresholds varies within fairly large limits. Although this binaural-monaural difference does not vary as a function of frequency when pure-tone thresholds are measured in the quiet, the difference does vary with frequency when pure tones are presented against a background of noise. Binaural summation, a term which has been used in the past to refer to the phenomenon in which the binaural threshold is lower than the monaural, obtains for masked thresholds only when the phase angles between the the two earphones are opposite for the tones and the noise. If the tones and the noise have the same interaural phase angle the binaural threshold is higher than the monaural. This phenomenon, called interaural inhibition, as well as its antipode, interaural summation, is most marked at low frequencies and increases as the intensity of the masking noise is increased. The discussion considers some implications of these findings for the theory of masking and of interaural summation.

Monaural and Binaural Threshold Sensitivity for Tones and for White Noise

Monaural and binaural thresholds of ten listeners were determined for a 1000-cycle tone and for a white noise. For each of the observers, the binaural threshold was found to be significantly lower than the monaural threshold when the two ears were “equated” in sensitivity, i.e. when the difference in sensitivity between the ears of a given observer was effectively canceled by experimental procedures. Furthermore, the difference between the monaural and binaural thresholds was found to be significantly greater for the pure tone than the corresponding difference for the noise. Monaural and binaural thresholds were also determined with the two ears “equated” in sensitivity and with the two ears “mismatched” in sensitivity by fixed ratios. It was found that the difference between the binaural threshold and the threshold of the better ear decreased as the difference in the effective stimulation at the two ears was increased. The difference between the binaural threshold and the threshold of the better ear was found to be not statistically significant when the two ears were stimulated at sensation levels more than 6 db apart. The results of the present study fail to confirm the hypothesis that the auditory threshold is constant and equal to the sum of the effective acoustic powers at the two ears. For most of the observers, the difference between the monaural and binaural thresholds was found to be significantly less than would be predicted by this hypothesis.

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.

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.

The monaural threshold: the effect of subliminal and audible contralateral and ipsilateral stimuli

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.

The monaural threshold: effect of a subliminal contralateral stimulus

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.

A NOTE ON THE “BINAURAL THRESHOLD”

British Journal of Psychology. General SectionVolume 23, Issue 2 p. 148-151 A NOTE ON THE “BINAURAL THRESHOLD” F. H. GAGE, F. H. GAGE Physiology Institute, University College, Cardiff.Search for more papers by this author F. H. GAGE, F. H. GAGE Physiology Institute, University College, Cardiff.Search for more papers by this author First published: October 1932 https://doi.org/10.1111/j.2044-8295.1932.tb00656.xCitations: 2AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Citing Literature Volume23, Issue2October 1932Pages 148-151 RelatedInformation