Stream segregation of narrow-band noise bursts

Normally hearing adults heard rapid alternations of a pair of band-limited noise bursts that had flat spectra (in terms of equal-loudness weighting of components) and sharp band edges. The bursts differed in center frequency (CF), but were matched on overall intensity, on bandwidth (BW) on a logfrequency scale, and (roughly) on pitch strength. Listeners judged the ease with which the sequence could be held together perceptually in a single auditory stream (vs. forming separate high and low streams). Involuntary segregation was examined as a function of the following measures of frequency separation of the alternating noise bands: (1) the closest band edges, (2) the most remote band edges, (3) the CFs of the bands on a logarithmic scale, and (4) the BWs. Segregation was best predicted from the separation of the two CFs on a log-frequency scale (very strong effect). Increasing the BWs of the two alternating bursts (the same size, in log frequency, for both bands) also led to greater segregation (very weak effect).     

Is a common grouping mechanism involved in the phenomena of illusory continuity and stream segregation?

Two auditory phenomena--stream segregation and illusory continuity through a wide-band noise interruption--were studied to determine whether the same principles of perceptual organization applied to both. A cycle was formed of a repeating alternation of two short bursts of narrow-band noise (NBN), one centered at a high frequency (H) and the other at a low frequency (L), with shorter bursts of wide-band noise (WBN) inserted between successive NBNs (H WBN L WBN H WBN...). In some conditions, listeners could hear a single NBN moving up and down behind the WBN bursts, although there was no NBN present with the WBN. Listeners rated the strength of this illusory continuity. Center frequency separation, rate of onsets, and bandwidth of the NBNs were varied. Increases in values of all three variables decreased illusory continuity. Other listeners rated the stream segregation of the H and L bands when successive NBNs were separated either by WBN bursts (as above) or by silences. The same three acoustic variables were manipulated. Increases in all three variables decreased the perception of a single stream. The similar disruptive effects on illusory continuity and on the one-stream percept in the stream segregation task support the idea that both phenomena depend on a common preliminary process of linking together the parts of a sequence that have similar frequencies.     

Effects of the build-up and resetting of auditory stream segregation on temporal discrimination

The tendency to hear a tone sequence as 2 or more streams (segregated) builds up, but a sudden change in properties can reset the percept to 1 stream (integrated). This effect has not hitherto been explored using an objective measure of streaming. Stimuli comprised a 2.0-s fixed-frequency inducer followed by a 0.6-s test sequence of alternating pure tones (3 low [L]-high [H] cycles). Listeners compared intervals for which the test sequence was either isochronous or the H tones were slightly delayed. Resetting of segregation should make identifying the anisochronous interval easier. The HL frequency separation was varied (0-12 semitones), and properties of the inducer and test sequence were set to the same or different values. Inducer properties manipulated were frequency, number of onsets (several short bursts vs. one continuous tone), tone:silence ratio (short vs. extended bursts), level, and lateralization. All differences between the inducer and the L tones reduced temporal discrimination thresholds toward those for the no-inducer case, including properties shown previously not to affect segregation greatly. Overall, it is concluded that abrupt changes in a sequence cause resetting and improve subsequent temporal discrimination.     

Negating the effects of binaural cues: competition between auditory streaming and contralateral induction

When a pair of monaural pure tones, A and B, are repeatedly alternated in one ear, with noise bursts presented in synchrony with B in the other ear, the noise sometimes delateralizes B. This is presumably a case of Warren and Bashford's (1976) contralateral induction effect. However, the present experiment shows that the degree of contralateral induction is proportional to the separation in frequency between A and B. It was also found that the degree to which the noise bursts influenced B's timbre was proportional to the separation in frequency between A and B. The combined results suggest that cues that govern the sequential organization of sounds influence the use of binaural cues not only during the assignment of position to auditory events but during the assignment of timbre.     

Two types of auditory sequence perception

It has been suggested recently that there are two fundamentally distinct types of auditory sequence perception in man: (1) hofistic pattern recognition (HPR), operating for component item durations from a few milliseconds up to about 200 msec; and (2) direct identification of components and their order (Direct ICO), requiring verbal encoding of names for constituent sounds and requiring item durations roughly 200 msec and above for extended sequences. The present study, using only the very first judgments from 795 untrained participants presented with recycled three-item sequences, provided data consistent with this dichotomous formulation. In addition, it appeared that separate bursts of a noise band generated on-line were treated as different components in HPR and could not be used for sequence matching; “frozen” noise bursts having identical microstructure were treated as the same component and permitted HPR. On-fine noise bursts permitted Direct ICO, with naming based on long-term spectral characteristics of noise.     

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.     

Spectral restoration of speech: Intelligibility is increased by inserting noise in spectral gaps

In order to function effectively as a means of communication, speech must be intelligible under the noisy conditions encountered in everyday life. Two types of perceptual synthesis have been reported that can reduce or cancel the effects of masking by extraneous sounds: Phonemic restoration can enhance intelligibility when segments are replaced or masked by noise, and contralateral induction can prevent mislateralization by effectively restoring speech masked at one ear when it is heard in the other. The present study reports a third type of perceptual synthesis induced by noise: enhancement of intelligibility produced by adding noise to spectral gaps. In most of the experiments, the speech stimuli consisted of two widely separated narrow bands of speech (center frequencies of 370 and 6000 Hz, each band having high-pass and low-pass slopes of 115 dB/octave meeting at the center frequency). These very narrow bands effectively reduced the available information to frequency-limited patterns of amplitude fluctuation lacking information concerning formant structure and frequency transitions. When stochastic noise was introduced into the gap separating the two speech bands, intelligibility increased for “everyday” sentences, for sentences that varied in the transitional probability of keywords, and for monosyllabic word lists. Effects produced by systematically varying noise amplitude and noise bandwidth are reported, and the implications of some of the novel effects observed are discussed.     

The relation between auditory temporal interval processing and sequential stream segregation examined with stimulus laterality differences

In this study, we examine the effects of laterality differences between noise bursts on two objective measures of temporal interval processing (gap detection and temporal asymmetry detection) and one subjective measure of temporal organization (stream segregation). Noise bursts were lateralized by presentation to different ears or dichotic presentation with oppositely signed interaural level (ILD) or time (ITD) differences. Objective thresholds were strongly affected by ear-of-entry differences, were moderately affected by ILD differences, but were unaffected by ITD differences. Subjectively, A and B streams segregated well on the basis of ear-of-entry or ILD differences but segregated poorly on the basis of ITD differences. These results suggest that perceptual segregation may be driven more effectively by differential activation of the two ears (peripheral channeling) than by differences in perceived laterality.     

Spectral cues which influence monaural Localization in the horizontal plane

An extensive series of behavioral tests was carried out to determine what region, or regions, of the sound spectrum were critical for locating sounds monaurally in the horizontal plane. Seven subjects were requested to locate narrow bands of noise centered at different frequencies, combinations of these noise bands, low-pass, high-pass, and broadband noise. As observed in an earlier study, increasing bandwidth did not necessarily lead to improved localization performance until the band became broad, including, for example, all frequencies above 4.0 kHz. What seems to be happening is that listeners perceive narrow bands of noise originating from restricted places in the horizontal plane which may differ one from another depending on the frequency composition of the stimulus. In several instances, if two noise bands were presented simultaneously, the resulting stimulus was located with reasonable accuracy provided each component, when presented singly, was perceived as emanating from clearly separate azimuthal positions. If, however, two noise bands, which were perceived to originate from approximately the same azimuthal position when presented singly, were now presented simultaneously, the resulting stimulus still was perceived to originate from the same region of the horizontal plane. This, then, is a case where augmenting the spectral content of the stimulus does not bring about improved performance. We suggest that the expression of judgmental biases in the apparent location of a band of noise may prove useful for understanding why some stimuli of specified width and center frequency are localizable while others are not.     

Binaural summation of loudness: Noise and two-tone complexes

A series of six experiments used the method of magnitude estimation to assess how the two ears sum the loudness of stimuli with various spectra. The results showed that the binaural system sums loudnesses by at least two distinct sets of rules, one applicable to narrow-band stimuli (complete loudness summation), another to wide-band noises (partial summation, dependent on level). The main findings were: (1) Narrow-band noise (Vi-octave bands at 1,000 Hz) showed complete binaural loudness summation, like that previously reported for pure tones (Marks, 1978a). At all but low SPL, a monaural stimulus must be 10 dB greater than a binaural stimulus to be equally loud; a stimulus ratio of 10 dB corresponds to a loudness ratio of 2:1 on Stevens’ sone scale. (2) Wide-band noise (300-4,800 Hz) showed only partial summation, the subadditivity being confined largely to levels below about 60 dB SPL. This result obtained both with bands of white noise (flat spectrum) and pink noise (—3 dB/ octave). (3) Binaural summation of two-tone complexes depended slightly on frequency spacing. Narrow spacing (860 and 1,160 Hz) gave summation equal to about 10 dB, like that of narrowband noises and single tones, whereas wider spacing (675 and 1,475 Hz) gave less summation, equal to about 9 dB, and more like wide-band noise; however, a very wide spacing (300 and 4,800 Hz) gave summation like that of narrow-band noises and single pure tones.     

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.     

Spectral cues provided by the pinna for monaural localization in the horizontal plane

Six subjects located, monaurally, 1.0-kHz-wide noise bursts whose source originated on the side of the functioning ear and whose center frequency ranged from 4.0 through 9.0 kHz (Part 1). Irrespective of their actual locations, the stimuli appeared to migrate from the frontal sector of the arc toward the side as the center frequency was increased above 4.0 kHz. For some subjects, the sounds appeared again in front at the higher center frequencies. Comparable data were obtained with noise bursts 2.0 kHz in width. We referred to these constellations of location judgments, influenced by the frequency composition of the stimuli, as spatial referent maps. In Part 2, we measured, by means of a miniature microphone placed at the entrance of the external ear canal, the pinna amplification function for these same stimuli emanating from the same locations. The results showed a positive relation between the apparent location of noise bursts centered at 6.0 kHz and above and the relative amplification provided by the pinna. Localization performances by two subjects, chosen on the basis of their noncorresponding spatial referent maps, were examined for stimuli of wider bandwidths IPart 3). Their proficiency differed markedly from one another, which we accounted for in terms of different spatial referent maps that were associated with differences in the pinna amplification function.     

The linkage between stimulus frequency and covert peak areas as it relates to monaural localization.

Head-related transfer functions for differently centered narrow noise bands were obtained on 6 subjects. Derived from these measurements were covert peak areas (CPAs), defined as the spatial constellation of loudspeakers that generates maximal sound pressure at the entrance of the ear canal for specific bands of frequency. On the basis of previous data, we proposed that different frequency bands served as important spectral cues for monaural localization of sounds from different loci and that location judgments were directed toward the CPAs associated with the different bands. In the first study, the stimuli were bandpass filtered so that they contained only those frequencies whose associated CPAs occupied either the monaural listener's "upper" or "lower" spatial regions. Loudspeakers, separated by 15 degrees, were stationed in the left hemifield, ranging from 0 degree to 180 degrees azimuth and -45 degrees to 60 degrees elevation. Subjects reported the loudspeaker from which the sound appeared to originate. Judgments of the sound's elevation were in general accord with the CPAs associated with the different frequency segments. In the second study, monaural localization tests were administered in which different 2.0-kHz-wide frequency bands linked with specific CPAs were notch filtered from a 3.5-kHz highpass noise band. For the control condition, the highpass noise was unfiltered. The data demonstrated that filtering a frequency segment linked with specific CPAs resulted in significantly fewer location responses directed toward that particular spatial region. These results demonstrate in greater detail the relation between the directional filtering properties of the pinna and monaural localization of sound.     

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.     

The linkage between stimulus frequency and covert peak areas as it relates to monaural localization

Head-related transfer functions for differently centered narrow noise bands were obtained on 6 subjects. Derived from these measurements were covert peak areas (CPAs), defined as the spatial constellation of loudspeakers that generates maximal sound pressure at the entrance of the ear canal for specific bands of frequency. On the basis of previous data, we proposed that different frequency bands served as important spectral cues for monaural localization of sounds from different loci and that location judgments were directed toward the CPAs associated with the different bands. In the first study, the stimuli were bandpass filtered so that they contained only those frequencies whose associated CPAs occupied either the monaural listener’s “upper” or “lower” spatial regions. Loudspeakers, separated by 15°, were stationed in the left hemifield, ranging from 0° to 180° azimuth and ?45° to 60° elevation. Subjects reported the loudspeaker from which the sound appeared to originate. Judgments of the sound’s elevation were in general accord with the CPAs associated with the different frequency segments. In the second study, monaural localization tests were administered in which different 2.0-kHz-wide frequency bands linked with specific CPAs were notch filtered from a 3.5-kHz highpass noise band. For the control condition, the highpass noise was unfiltered. The data demonstrated that filtering a frequency segment linked with specific CPAs resulted in significantly fewer location responses directed toward that particular spatial region. These results demonstrate in greater detail the relation between the directional filtering properties of the pinna and monaural localization of sound.     

Apparent distance of sounds recorded in echoic and anechoic chambers

With miniature microphones inserted into the external ear canals of a model and the sound source 90 degrees to left of midline, low-pass, and broadband noise bursts were picked up and recorded on magnetic tape. The bursts were generated in two highly contrasting acoustic environments: an anechoic and an echoic chamber. The taped sounds were played back monaurally and binaurally via headphones to 16 listeners seated in an acoustically neutral setting. They were instructed to estimate the distance of the stimuli. Apparent distances of bursts recorded in the echoic or reverberant chamber far exceeded those recorded in the anechoic chamber. It mattered not whether the sounds were presented monaurally or binaurally. What did influence distance estimates dramatically was the frequency composition of the stimuli. Low-pass sounds recorded in either acoustic environment were consistently judged to be further removed than high-pass sounds recorded in the same setting. They were also more likely to appear from behind the listener. In our moment-to-moment transaction with the acoustic environment, distant sounds generally have less acoustic energy in the higher audio frequency. We suggest that this lifetime of auditory experience influenced our listeners' scale of relative distance.     

Monaural and binaural temporal integration of noise bursts

Summary A comparison was made between monaural and binaural temporal integration of noise bursts at threshold. The data indicate partial integration, with approximately a 6 dB decrease in threshold per decade increase in noise burst duration for both conditions of stimulation (i.e., parallel functions) for durations ranging from 4 to 256 msec. When thresholds in dB are plotted as a function of log duration, the linear component accounts for 99% of the data indicating no essential change in the partial integration functions up to at least 256 msec. The intercept difference between the monaural and binaural integration functions is 2.5 dB.     

Auditory temporal summation in infants and adults: Effects of stimulus bandwidth and masking noise

A visually reinforced operant procedure was employed to determine the behavioral thresholds of 6- to 7-month-old infants and adults for stimuli of various bandwidths and durations. Experiment 1 compared absolute thresholds for broadband and 1/3-octavefiltered clicks and 300-msec noise bursts. For adult subjects, the difference in threshold for clicks and noise bursts was -quite comparable in the two bandwidth conditions, but infants’ click-noise threshold differences were significantly larger for broadband than for 1/3-octave stimuli. In Experiment 2, 2-point threshold-duration functions were compared for 4-kHz tones and octave-band noise bursts presented in backgrounds of quiet and continuous noise. Infants’ threshold-duration function for octave-band noise bursts was significantly steeper than the comparable adult function in quiet, but not in masking noise. These results suggest that young infants may have particular difficulty detecting low intensity broadband sounds when durations are very short.     

Critical-band effects in two-channel auditory signal detection

Two-channel auditory signal detection was investigated with 50-msec sinusoidal signals masked by binaurally uncorrelated noise. In the two-channel tasks, the signals in each earphone channel were presented with an independent probability during the single observation interval and the observers were required to detect the inputs in a single earphone (selective-attention condition) or in both earphones (divided-attention condition). When the signals in each earphone were within the same critical band (the assumed singled processing unit in frequency domain), there was a decrement in detection performance in both the selective- and divided-attention (i.e. dichotic) conditions compared with the monaural condition. However, when signals were separated in frequency by several critical bands, a decrement in dichotic performance, as compared with monaural performance, occurred only in the divided-attention condition. These findings are discussed in terms of their implications regarding models of multichannel signal processing and the definition of input channels in terms of earphones.