Monaural Thresholds Research Articles

Continuity effects with alternately sounded noise and tone signals as a function of manner of presentation.

An experiment is reported on continuity effects produced in a longer-duration, less-intense noise signal alternating with a shorter-duration, higher-intensity tonal signal. The perceived continuity in the noise signal is demonstrated to be significantly affected by the following: (1) the frequency of the tonal signal (200, 400, 1000, 2000, and 4000 cps), (2) the duration of the noise signal (70, 250, and 950 msec), and (3) the manner of presentation (monaural versus dichotic). Monaural thresholds were found to be larger than dichotic continuity thresholds. Continuity effects were demonstrated under dichotic presentation. That is, the noise signal was reported as continuous when interrupted for periods of time that would be clearly perceived if no tonal signal were interpolated the interruptions of the noise. A two factor theory is forwarded to explain the dichotic and monaural continuity effects. The monaural effects are discussed in terms of rate of decay of auditory sensation. The dichotic effects are discussed in terms of facilitation of ongoing neural discharge somewhat more central than the first-order neurons.

V. Experiment III: Monaural thresholds for bone-conducted and air-conducted signals masked by air-conducted noises

V. Experiment III: Monaural thresholds for bone-conducted and air-conducted signals masked by air-conducted noises

V. Experiment III: Monaural thresholds for bone-conducted and air-conducted signals masked by air-conducted noises

V. Experiment III: Monaural thresholds for bone-conducted and air-conducted signals masked by air-conducted noises

Effects of Noise on Dichotic Temporal Resolution

The effect of broadband (125–8000 cps) background noise on the dichotic temporal interval between two clicks required to report them as two was investigated. As noise level is increased up to 30 dB SL (re monaural threshold), the dichotic temporal interval required for a report of two (Δt) is decreased. For noise levels from 30–50 dB SL, the dichotic temporal interval for a report of two increases. The extent of this reversal seems related to mask level, although the effect of noise in decreasing Δt is not related to signal-to-noise ratio. With narrow-band low-pass noise, the Δt required for a report of two continues to decrease as noise level increases up to 50 dB SL, whereas a narrow-band high-pass noise shows the same reversal as does the broadband noise. Increasing the click level without adding any background noise decreases the Δt required for a report of two. Additional experiments that elucidate these findings are presented.

Effect of Interaural Phase on the Binaural Absolute Threshold for Tone

Earlier workers have found that where monaural thresholds are equal, or where inequalities are compensated for, the binaural threshold is about 3 db lower than the monaural. Licklider in Steven's Handbook suggests that this implies a kind of power summation. Since the powers being summated are going to different ears, this summation cannot be acoustical. The summation, therefore, appears to be neural, and we should expect some of the temporal phenomena of neural summation to apply. Delaying the sound to one ear, for instance, should make conditions less favorable for summation and should raise the binaural threshold. In the present experiment a 2-msec delay (or advance) was produced by reversing the phase of a 250-cps tone. Monaural and binaural thresholds (both homophasic and antiphasic) were determined for four subjects, and the best ear was taken as reference. The results indicate that the threshold for the antiphasic condition instead of being elevated relative to the homophasic is actually lower. This suggests that we are dealing, not with neural summation, but with a masking phenomenon—that the absolute threshold is really a masked threshold. [This Work was partially supported by the Bureau of Ships.]

Auditory Sensitization

This experiment studied the lowering of the absolute auditory threshold that follows low-tone stimulation. Monaural thresholds were determined continuously with a recording audiometer before and after the ear was exposed to a pure tone. The frequency, intensity, and duration of both the exposure tones and the test tones were varied. Auditory sensitization was found to be a general effect among seventeen subjects. The magnitude of the effect was investigated intensively with three subjects. Their results demonstrated that sensitization produced by a pure tone is nonspecific: a given tone can sensitize the auditory system to a relatively wide range of test frequencies. Conversely, a relatively wide range of exposure frequencies can sensitize the auditory system to a given test tone. This nonspecificity is consistent with what we know, through microelectrode studies, about the spread of energy on the basilar membrane and the spread of excitation in the auditory nervous system. Contralateral sensitization effects were found.

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.