Signal detectability in the presence of monotic or dichotic noise bands of equal or unequal levels

The application of the power-spectrum model of masking to the detectability of a signal masked by dichotic noise was investigated in three experiments. In each experiment, the signal was a 2-kHz sinusoid of 400-msec duration, masked by either one or two 800-Hz wide bands of noise presented singly or in pairs. In Experiment 1, we compared the detectability of a diotic signal masked by dichotic noise with the detectability of a monaural signal masked by each of the noises separately. The spectrum level of the noise was 35 dB SPL. For dichotic presentations, the signal was sent to both ears while pairs of noise bands, one below and one above the signal frequency, were presented together, one band to each ear. Threshold levels with the dichotic stimuli were lower than or equal to the thresholds with either ear's stimulus on its own. Similar dichotic stimuli were used in Experiment 2, except that the signal frequency was nearer to one or the other of the bands of masking noise, and the noise had a spectrum level of 50 dB SPL. In Experiment 3, thresholds were obtained with two sets of symmetrically and asymmetrically placed notched-noise maskers. For one of these sets, the spectrum level of both noise bands was 35 dB SPL; for the other set, interaural intensity differences were introduced in the form of an inequality in the levels of the noise bands on either side of the signal. In one ear, the spectrum level of the lower frequency noise band was 35 dB SPL and the spectrum level of the higher frequency noise band was 25 dB SPL, whereas in the other ear, the allocation of noise level to noise band was reversed. The dichotic thresholds obtained with the unequal noise maskers could be predicted from the shapes of the auditory filters derived with equal noise maskers. The data from all three experiments suggest that threshold signal levels in the presence of interaural differences in masker intensity depend principally on the ear with the higher signal-to-masker ratio at the output of its auditory filter, a finding consistent with the power-spectrum model of masking.     

Effects of signal duration and masker duration on detectability under diotic and dichotic listening conditions

The detectability of a 500-Hz tone of either 32- or 256-msec duration in a broad-band 50-dB spectrum level noise was measured as a function of the duration of the noise. The noise was continuous or was gated 0, 125, or 250 msec before the onset of the signal. For the gated noise conditions the noise was terminated approximately 5 msec after termination of the signal. With a homophasic condition (NO SO), the three noise conditions led to approximately the same detectability as did the continuous masker. In an antiphasic condition (NO Sπ), detectability was poorest when signal and masker began together and improved as the delay between noise onset and signal onset increased. The difference between the simultaneous onset and the continuous noise condition was about 9 dB for the 32-msec signal and about 2 dB for the 256-msec signal. These results are compared to those reported by McFadden (1966).     

Effect of masker level on infants' detection of tones in noise

In adult listeners, the signal-to-noise ratio at masked threshold remains constant with increases in masker level over a wide range of stimulus conditions. This relationship was examined in 7-month-old infants by obtaining masked thresholds for .5- and 4-kHz tones presented in four levels of continuous masking noise. Adults were also tested for comparison. Masker spectrum levels ranged from 5 to 35 dB/Hz for .5-kHz tones, and from -5 to 25 dB/Hz for 4-kHz stimuli. Thresholds were determined for stimuli of both 10 and 100 msec in duration. The results indicated that infants' performance was more adult-like for 4-kHz stimuli. Although mean thresholds for both 10- and 100-msec, 4-kHz tones were approximately 7 dB higher in infants than in adults, E/N0 at threshold remained essentially constant over the 30-dB range of maskers employed. By contrast, infants' thresholds for .5-kHz tones were exceptionally high at lower levels of the masker. Threshold E/N0 decreased significantly as masker level increased from 5 to 35 dB/Hz, and this decrease was significantly greater for 10- than for 100-msec stimuli. Temporal summation of .5-kHz tones, measured as the difference between thresholds obtained at the two signal durations, was greater for infants than for adults at low levels of the masker. However, because infants' thresholds improved more rapidly with level for 10- than for 100-msec tones, age differences in temporal summation were no longer significant when masker spectrum level was 35 dB/Hz. These results suggest that the relationship between signal-to-noise ratio at masked threshold and level of the masker is dependent on both signal frequency and duration during infancy.     

Binaural masking of tones by tones: Effects of uncertainty regarding the phase angle of addition of signal to masker

This paper compares performance in tone-on-tone coherent binaural masking for conditions with and without listener knowledge concerning the phase angle of addition of the signal to the masker. Uncertainty was introduced by randomizing the four phase angles: 0, 45, 90, and 135 deg. Stimuli were 125-msec tone bursts at 250 Hz. The results showed a clear decrement in detection performance under the uncertainty condition at 135 deg, with weaker evidence for decrements at other phase angle values. This supports the assumption that the decision process was a function of the phase angle in previous studies of binaural tone-on-tone masking.     

Some nonmasking auditory postsignal effects

Observers were asked to detect a 20-msec segment of a sinusoidal signal masked by a band-limited white noise. A postsignal decrease in the spectrum level of the noise within the critical band of the signal enhanced the detectability of that signal if the decrease occurred within approximately 25 msec following signal termination. Postsignal decreases outside the critical band of the signal, and decreases within the critical band delayed longer than approximately 40 msec, reduced the detectability of the signal for decrease delays up to between approximately 150 and 400 msec, depending on the spectral characteristics of the decrease. Comparisons with typical backward masking results indicate probable common factors of short-term temporal summation and longer term attention.     

Detection of periodically and randomly interrupted noise signals

The detectability of a masked, interrupted, band-limited (150-2,400 Hz) noise signal was studied as a function of whether the interruption process was periodic or random. A standard monaural two-interval, temporal forced-choice detection procedure was employed both with and without a low-level contralateral cue (MDCC). The cue was an independent noise interrupted synchronously with the signal. Detection performance was equal under all conditions and was equivalent to the detection of an uninterrupted noise signal of equal power. Furthermore, like speech waveforms, the two types of interruption processes were discriminable only at levels at least 10–12 dB above the level required for detection.     

Binocular vision enhances phase discrimination by filtering the background

Previous studies have shown that the detectability of a noise-masked target can be enhanced under stereoscopic viewing when the target’s interocular disparity differs from that of the noise. This enhanced detectability can be accounted for by a model postulating that the binocular system linearly sums the left-eye and right-eye views of a visual scene. This model also predicts enhanced phase discrimination under specifiable interocular disparities of target and noise. Two experiments were conducted in which subjects were asked to discriminate between two luminance patterns (target and foil) that differed only in phase. The target patterns were constructed by summating two vertical sinusoidal gratings in which the phase difference between the higher and the lower spatial frequency gratings was 45°. The foils contained the same two component frequencies, with a phase difference of ?45°. Thus, targets and foils were mirror images of one another. The ability of subjects to Discriminate between these stereoscopically viewed mirror-image patterns was investigated under two sets of interocular disparities: those that, according to our model, would unmask one or both spatial frequency components, and those that would leave both components masked by the noise. Phase discrimination was enhanced only when both component frequencies of the target and foil were unmasked. The implications of these findings for template-matching and phase-discrimination models of pattern discrimination are considered.     

The detection of a tone added to a narrow band of noise: The energy model revisited

In an effort to determine whether cues related to changes in energy contribute to the detection of a tone added to a narrow band of noise, we examined the effect of level variation on detection thresholds. In the first experiment, the level of each waveform was randomly varied on each presentation. Level variation had only marginal effects on performance. In addition, detection thresholds were obtained using bands of noise with equal energy across intervals. Neither increasing nor decreasing the variance of the noise-alone and tone-plus-noise energy difference distributions altered the detectability of a tone added to noise. Thus, the changes in energy that are concomitant with the addition of the tone are not the sole cue for the detection of the tone. In a second experiment, three psychometric functions were measured. One function was determined using no level variation, one was measured in the presence of level variation, and one was measured in the context of level variation, but for trials without level variation. The context of level variation slightly reduced detectability. In a third experiment, we compared detectability in three conditions: no level variation, across-trial level variation, and across-interval level variation. The thresholds obtained in the absence of level variation were superior to those measured in the presence of level variation, regardless of the manner in which the level variation was incorporated.     

Binocular vision enhances phase discrimination by filtering the background.

Previous studies have shown that the detectability of a noise-masked target can be enhanced under stereoscopic viewing when the target's interocular disparity differs from that of the noise. This enhanced detectability can be accounted for by a model postulating that the binocular system linearly sums the left-eye and right-eye views of a visual scene. This model also predicts enhanced phase discrimination under specifiable interocular disparities of target and noise. Two experiments were conducted in which subjects were asked to discriminate between two luminance patterns (target and foil) that differed only in phase. The target patterns were constructed by summating two vertical sinusoidal gratings in which the phase difference between the higher and the lower spatial frequency gratings was 45 degrees. The foils contained the same two component frequencies, with a phase difference of -45 degrees. Thus, targets and foils were mirror images of one another. The ability of subjects to discriminate between these stereoscopically viewed mirror-image patterns was investigated under two sets of interocular disparities: those that, according to our model, would unmask one or both spatial frequency components, and those that would leave both components masked by the noise. Phase discrimination was enhanced only when both component frequencies of the target and foil were unmasked. The implications of these findings for template-matching and phase-discrimination models of pattern discrimination are considered.     

Lateralization of narrow-band noise by blind and sighted listeners

The effects of varying interaural time delay (ITD) and interaural intensity difference (IID) were measured in normal-hearing sighted and congenitally blind subjects as a function of eleven frequencies and at sound pressure levels of 70 and 90 dB, and at a sensation level of 25 dB (sensation level refers to the pressure level of the sound above its threshold for the individual subject). Using an 'acoustic' pointing paradigm, the subject varied the IID of a 500 Hz narrow-band (100 Hz) noise (the 'pointer') to coincide with the apparent lateral position of a 'target' ITD stimulus. ITDs of 0, +/-200, and +/-400 micros were obtained through total waveform delays of narrow-band noise, including envelope and fine structure. For both groups, the results of this experiment confirm the traditional view of binaural hearing for like stimuli: non-zero ITDs produce little perceived lateral displacement away from 0 IID at frequencies above 1250 Hz. To the extent that greater magnitude of lateralization for a given ITD, presentation level, and center frequency can be equated with superior localization abilities, blind listeners appear at least comparable and even somewhat better than sighted subjects, especially when attending to signals in the periphery. The present findings suggest that blind listeners are fully able to utilize the cues for spatial hearing, and that vision is not a mandatory prerequisite for the calibration of human spatial hearing.     

Effect of masker level on infants’ detection of tones in noise

In adult listeners, the signal-to-noise ratio at masked threshold remains constant with increases in masker level over a wide range of stimulus conditions. This relationship was examined in 7-month-old infants by obtaining masked thresholds for .5- and 4-kHz tones presented in four levels of continuous masking noise. Adults were also tested for comparison. Masker spectrum levels ranged from 5 to 35 dB/Hz for .5-kHz tones, and from ?5 to 25 dB/Hz for 4-kHz stimuli. Thresholds were determined for stimuli of both 10 and 100 msec in duration. The results indicated that infants’ performance was more adultlike for 4-kHz stimuli. Although mean thresholds for both 10- and 100-msec, 4-kHz tones were approximately 7 dB higher in infants than in adults, E/N₀ at threshold remained essentially constant over the 30-dB range of maskers employed. By contrast, infants’ thresholds for .5-kHz tones were exceptionally high at lower levels of the masker. Threshold E/N0 decreased significantly as masker level increased from 5 to 35 dB/Hz, and this decrease was significantly greater for 10- than for 100-msec stimuli. Temporal summation of .5-kHz tones, measured as the difference between thresholds obtained at the two signal durations, was greater for infants than for adults at low levels of the masker. However, because infants’ thresholds improved more rapidly with level for 10- than for 100-msec tones, age differences in temporal summation were no longer significant when masker spectrum level was 35 dB/Hz. These results suggest that the relationship between signal-to-noise ratio at masked threshold and level of the masker is dependent on both signal frequency and duration during infancy.     

The perceptual onset of musical tones

The perceptual onset of a musical tone can be defined as the moment in time at which the stimulus is first perceived. In the present experiments, a simple threshold model for the perceptual onset was applied. A paradigm was used in which a sequence of tones had to be adjusted in such a way that the onsets were perceived at equally spaced moments in time. In Experiment 1, the threshold model was applied in a design in which the rise times of the tones were varied. We concluded that the perceptual onsets of the tones can, indeed, be defined as the times at which the envelopes pass a relative threshold of 15 dB below the maximum level of the tones (82 dB). In Experiment 2, the maximum levels of the tones were varied from 37 to 77 dB. The results show that there is a shift in the relative threshold, but that this shift is small relative to the shift in the stimulus level. In Experiment 3, the effect of level above masked threshold on the perceptual onset was investigated in more detail by varying the level of a background noise. The results show that the relative threshold decreases with increasing level above masked threshold. The results from our experiments strongly suggest that the relative threshold is linearly dependent on the level above masked or absolute threshold and that a 7-dB increment of this level results in a 1-dB relative threshold decrement. The threshold model is compared with a current temporal integration model for the perceptual onset of tones. It is shown that our data cannot be adequately explained by temporal integration. Our experimental results suggest that adaptation of the hearing mechanism to a certain relative stimulus level is responsible for perceptual onset. The applicability of our threshold model in various realistic musical situations is discussed.     

The effect of white-noise mask level on sinewave contrast detection thresholds and the critical-band-masking model

It is known that visual noise added to sinusoidal gratings changes the typical U-shaped threshold curve which becomes flat in log-log scale for frequencies below 10c/deg when gratings are masked with white noise of high power spectral density level. These results have been explained using the critical-band-masking (CBM) model by supposing a visual filter-bank of constant relative bandwidth. However, some psychophysical and biological data support the idea of variable octave bandwidth. The CBM model has been used here to explain the progressive change of threshold curves with the noise mask level and to estimate the bandwidth of visual filters. Bayesian staircases were used in a 2IFC paradigm to measure contrast thresholds of horizontal sinusoidal gratings (0.25-8 c/deg) within a fixed Gaussian window and masked with one-dimensional, static, broadband white noise with each of five power density levels. Raw data showed that the contrast threshold curve progressively shifts upward and flattens out as the mask noise level increases. Theoretical thresholds from the CBM model were fitted simultaneously to the data at all five noise levels using visual filters with log-Gaussian gain functions. If we assume a fixed-channel detection model, the best fit was obtained when the octave bandwidth of visual filters decreases as a function of peak spatial frequency.     

Lateralization of an auditory signal in correlated noise and in uncorrelated noise as a function of signal frequency

Listeners lateralized a monaural signal presented against a continuous background of perfectly correlated noise (NO) or of uncorrelated noise (NU). Measures of signal detectability were also secured in separate tests. Psychometric functions (percent correct vs signal energy) were determined for each task. For a tonal signal of either low or high frequency, a listener requires only slightly greater signal energy (about 1 dB) in order to lateralize as well as he can detect when the noise is uncorrelated (NU). When the noise is perfectly correlated (NO), the slope of the psychometric function for lateralization depends upon signal frequency. With 250 Hz, the slope of the psychometric function for lateralization is much smaller than that for detection. With 1,000 Hz, the function for lateralization is steeper than that for 250 Hz, but the slope is still less than that of the function for detection for 1,000 Hz. With 2,000 Hz, the function for lateralization has about the same slope as that for detection.     

Asymmetry of masking between noise and tone

A pure tone was used to mask narrow and wide bands of noise centered on the frequency of the tone. In a given experimental session, the sound-pressure level (SPL) of the tone was held constant and loudness balances were obtained between a masked and unmasked noise band of equal width. These results are compared to earlier measures of the partial masking of tone by noise. The comparison shows that noise masks a tone more effectively than the tone masks the noise. Although the effect of the tone on a critical band of noise is greater than its effect on either an octave-band noise or wide-band noise, it is considerably smaller than the effect of the noise on the tone. Decreasing the noise bandwidth still further to a subcritical width reduces the asymmetry of masking somewhat, but a difference at high intensities of about 20 dB between the masking effects of an equally intense noise and tone remains. Whether the masker is a tone or noise, masking ceases when the effective energy of the masked and masking stimuli is the same.     

Masking-level differences and the form of the psychometric function

Functions showing the relation between-the detectability and the energy of a signal were determined for various interaural phase conditions. The empirical relation between d′ and signal energy E is approximately d′=m(E/N₀)^k where m and k are constants for a particular function. The data show that k is fairly constant for a particular O and that m depends upon the interaural condition. That is, the various psychometrie functions for a given O all had the same form, independent of location. Therefore, the magnitude of the MLD=(10/k)log(m_i/m_r), where m_i is the constant for condition i, and m_r corresponds to a reference condition. Consequently, an MLD is relatively independent of the level of performance that is chosen for the determination of that MLD.     

An investigation of the interaural time difference threshold for speech

The interaural time difference threshold for speech has been reported to be approximately 35 μsec (Cherry & Sayers, 1956), a value substantially larger than the 6 ~sec reported for broadband noise signals by other experimenters (e.g., Tobias & Zerlin, 1959). In the two studies just mentioned, however, different subjects and psychoacoustical methods were employed; thus, it is unclear whether larger interaural time differences are needed to lateralize speech signals. The purpose of this experiment, therefore, was to compare lateralization performance for speech and nonspeech stimuli. Interaural time difference thresholds were obtained for speech, speech spectrum noise, speech multiplied noise, and 200-, 500-, and 1,000-Hz sinusoids for the same subjects using a 2 IFC experimental paradigm. Under these conditions, speech and speech-multiplied noise yielded essentially the same interaural time difference thresholds.     

Tone-on-tone binaural masking with an antiphasic masker

One pure tone (500 Hz) was used to mask another pure tone of the same frequency and duration. The signal and masker were presented in three binaural stimulus configurations, Mo-So, Mo-Sπ, and Mπ -So. The Mo-So condition is a diotic condition; the Mo-Sβ condition is a dichotic condition in which the masker is homophasic and the signal is antiphasic; and the Mπ-So condition is a dichotic condition in which the masker is antiphasic and the signal homophasic. The signal-to-masker ratio required for detection was measured in each condition as a function of the signal-plus-masker phase angle, α. The data showed that the difference in detection between the Mo-Sπ and Mπ-So conditions varied between 0 dB when α=0 deg and 11 dB when α=90 deg. The difference in detection between the Mo-Sπ and Mπ-So conditions is due to the Os’ sensitivities to the interaural phase difference present in the Mo-Sπ and Mπ-So conditions. The results are similar to those obtained in investigations involving lateralization. The difference between detection in either the Mo-Sπ or Mπ-So condition and that in the Mo-So condition (the MLD) was variable due to differences in the Os’ sensitivities in the Mo-So condition.     

Persistence and integration: Two consequences of a sliding integrator

The detection of a silent interval, or gap, placed in the temporal center of a gated noise burst was investigated. The gated noise masker ranged from 2 to 400 msec in duration. For long noises, the duration, Δ, of the just-detectable gap remained fixed at about 2.8 msec. Progressively shortening the duration of the noise did not affect Δ until the duration was approximately 20 msec; thereafter, decreasing the noise duration improved detectability of the gap. In a second experiment, continuous noise filled the temporal gap, although the decibel difference between the noise in the gap and the noise surrounding the gap was always at least 5 dB. The level of noise filling the gap did not greatly affect Δ. The third experiment was similar to the first, except that the signal was a click rather than a gap. The results for both gaps and clicks were fitted by a model assuming a sliding integrator.     

Discrimination of time intervals marked by brief acoustic pulses of various intensities and spectra

Experienced observers were asked to identify, in a four-level 2AFC situation, the longer of two unfilled time intervals, each of which was marked by a pair of 20-msec acoustic pulses. When all the markers were identical, high-level (186-dB SPL) bursts of coherently gated sinusoids or bursts of band-limited Gaussian noise, a change in the spectrum of the markers generally did not affect performance. On the other hand, for 1-kHz tone-burst markers, intensity decreases below 25 dB SL were accompanied by sizable deterioration of the discrimination performance, especially at short (25-msec) base intervals. Similarly large changes in performance were observed also when the two tonal markers of each interval were made very dissimilar from each other, either in frequency (frequency difference larger than 1 octave) or in intensity (level of the first marker at least 45 dB below the level of the second marker). Time-difference thresholds in these two latter cases were found to be nonmonotonically related to the base interval, the minima occurring between 40- and 80-msec onset separations.