Independence of frequency channels in auditory temporal gap detection

The ability of listeners to detect a temporal gap in a 1600-Hz-wide noiseband (target) was studied as a function of the absence and presence of concurrent stimulation by a second 1600-Hz-wide noiseband (distractor) with a nonoverlapping spectrum. Gap detection thresholds for single noisebands centered on 1.0, 2.0, 4.0, and 5.0 kHz were in the range from 4 to 6 ms, and were comparable to those described in previous studies. Gap thresholds for the same target noisebands were only modestly improved by the presence of a synchronously gated gap in a second frequency band. Gap thresholds were unaffected by the presence of a continuous distractor that was either proximate or remote from the target frequency band. Gap thresholds for the target noiseband were elevated if the distractor noiseband also contained a gap which "roved" in time in temporal proximity to the target gap. This effect was most marked in inexperienced listeners. Between-channel gap thresholds, obtained using leading and trailing markers that differed in frequency, were high in all listeners, again consistent with previous findings. The data are discussed in terms of the levels of the auditory perceptual processing stream at which the listener can voluntarily access auditory events in distinct frequency channels.     

Effect of source spectrum on sound localization in an everyday reverberant room

Two experiments explored how frequency content impacts sound localization for sounds containing reverberant energy. Virtual sound sources from thirteen lateral angles and four distances were simulated in the frontal horizontal plane using binaural room impulse responses measured in an everyday office. Experiment 1 compared localization judgments for one-octave-wide noise centered at either 750 Hz (low) or 6000 Hz (high). For both band-limited noises, perceived lateral angle varied monotonically with source angle. For frontal sources, perceived locations were similar for low- and high-frequency noise; however, for lateral sources, localization was less accurate for low-frequency noise than for high-frequency noise. With increasing source distance, judgments of both noises became more biased toward the median plane, an effect that was greater for low-frequency noise than for high-frequency noise. In Experiment 2, simultaneous presentation of low- and high-frequency noises yielded performance that was less accurate than that for high-frequency noise, but equal to or better than for low-frequency noise. Results suggest that listeners perceptually weight low-frequency information heavily, even in reverberant conditions where high-frequency stimuli are localized more accurately. These findings show that listeners do not always optimally adjust how localization cues are integrated over frequency in reverberant settings.     

Measuring the critical band for speech

The current experiments were designed to measure the frequency resolution employed by listeners during the perception of everyday sentences. Speech bands having nearly vertical filter slopes and narrow bandwidths were sharply partitioned into various numbers of equal log- or ERBN-width subbands. The temporal envelope from each partition was used to amplitude modulate a corresponding band of low-noise noise, and the modulated carriers were combined and presented to normal-hearing listeners. Intelligibility increased and reached asymptote as the number of partitions increased. In the mid- and high-frequency regions of the speech spectrum, the partition bandwidth corresponding to asymptotic performance matched current estimates of psychophysical tuning across a number of conditions. These results indicate that, in these regions, the critical band for speech matches the critical band measured using traditional psychoacoustic methods and nonspeech stimuli. However, in the low-frequency region, partition bandwidths at asymptote were somewhat narrower than would be predicted based upon psychophysical tuning. It is concluded that, overall, current estimates of psychophysical tuning represent reasonably well the ability of listeners to extract spectral detail from running speech.     

Dynamic-range compression affects the lateral position of sounds

Dynamic-range compression acting independently at each ear in a bilateral hearing-aid or cochlear-implant fitting can alter interaural level differences (ILDs) potentially affecting spatial perception. The influence of compression on the lateral position of sounds was studied in normal-hearing listeners using virtual acoustic stimuli. In a lateralization task, listeners indicated the leftmost and rightmost extents of the auditory event and reported whether they heard (1) a single, stationary image, (2) a moving/gradually broadening image, or (3) a split image. Fast-acting compression significantly affected the perceived position of high-pass sounds. For sounds with abrupt onsets and offsets, compression shifted the entire image to a more central position. For sounds containing gradual onsets and offsets, including speech, compression increased the occurrence of moving and split images by up to 57 percentage points and increased the perceived lateral extent of the auditory event. The severity of the effects was reduced when undisturbed low-frequency binaural cues were made available. At high frequencies, listeners gave increased weight to ILDs relative to interaural time differences carried in the envelope when compression caused ILDs to change dynamically at low rates, although individual differences were apparent. Specific conditions are identified in which compression is likely to affect spatial perception.     

Temporal gap resolution in listeners with high-frequency sensorineural hearing loss

Temporal gap resolution was measured in five normal-hearing listeners and five cochlear-impaired listeners, whose sensitivity losses were restricted to the frequency regions above 1000 Hz. The stimuli included a broadband noise and three octave band noises centered at 0.5, 1.0, and 4.0 kHz. Results for the normal-hearing subjects agree with previous findings and reveal that gap resolution improves progressively with an increase in signal frequency. Gap resolution in the impaired listeners was significantly poorer than normal for all signals including those that stimulated frequency regions with normal pure-tone sensitivity. Smallest gap thresholds for the impaired listeners were observed with the broadband signal at high levels. This result agrees with data from other experiments and confirms the importance of high-frequency signal audibility in gap detection. The octave band data reveal that resolution deficits can be quite large within restricted frequency regions, even those with minimal sensitivity loss.     

The just audible tonality of short exponential and Gaussian pure tone bursts

This paper reports on listening tests performed to investigate the just audible tonality (JAT) of decaying pure tone bursts. Both exponential and Gaussian functions are used to shape the envelopes of the tone bursts and critical band center frequencies between 150 and 7000 Hz are studied. Loudness compensation is implemented to compensate for the reduced loudness of short tone bursts and attack functions are used for minimizing clicks. By using the method of limits, a sequence of tone bursts with increasing decay times and constant frequencies is presented to the listeners at 0.9 s intervals. The first burst in the sequence which is perceived as being tonal is indicated by the listeners. When that happens, the decay times of the tone bursts decrease and the listeners are asked to select the first tone burst with no audible tonality. The listeners are allowed to freely define tonality. No reference is given. For frequencies above 3.4 kHz, the results indicate that tonality is just audible for tone burst lengths of approximately 2.6-3.0 ms. For the lowest stimuli frequencies, the corresponding burst length is approximately 20-23 ms.     

Measuring the threshold for speech reception by adaptive variation of the signal bandwidth. II. Hearing-impaired listeners

In a previous study [Noordhoek et al., J. Acoust. Soc. Am. 105, 2895-2902 (1999)], an adaptive test was developed to determine the speech-reception bandwidth threshold (SRBT), i.e., the width of a speech band around 1 kHz required for a 50% intelligibility score. In this test, the band-filtered speech is presented in complementary bandstop-filtered noise. In the present study, the performance of 34 hearing-impaired listeners was measured on this SRBT test and on more common SRT (speech-reception threshold) tests, namely the SRT in quiet, the standard SRT in noise (standard speech spectrum), and the spectrally adapted SRT in noise (fitted to the individual's dynamic range). The aim was to investigate to what extent the performance on these tests could be explained simply from audibility, as estimated with the SII (speech intelligibility index) model, or require the assumption of suprathreshold deficits. For most listeners, an elevated SRT in quiet or an elevated standard SRT in noise could be explained on the basis of audibility. For the spectrally adapted SRT in noise, and especially for the SRBT, the data of most listeners could not be explained from audibility, suggesting that the effects of suprathreshold deficits may be present. Possibly, such a deficit is an increased downward spread of masking.     

Discrimination of interaural temporal disparities by normal-hearing listeners and listeners with high-frequency sensorineural hearing loss

Thresholds of ongoing interaural time difference (ITD) were obtained from normal-hearing and hearing-impaired listeners who had high-frequency, sensorineural hearing loss. Several stimuli (a 500-Hz sinusoid, a narrow-band noise centered at 500 Hz, a sinusoidally amplitude-modulated 4000-Hz tone, and a narrow-band noise centered at 4000 Hz) and two criteria [equal sound-pressure level (Eq SPL) and equal sensation level (Eq SL)] for determining the level of stimuli presented to each listener were employed. The ITD thresholds and slopes of the psychometric functions were elevated for hearing-impaired listeners for the two high-frequency stimuli in comparison to: the listener's own low-frequency thresholds; and data obtained from normal-hearing listeners for stimuli presented with Eq SPL interaurally. The two groups of listeners required similar ITDs to reach threshold when stimuli were presented at Eq SLs to each ear. For low-frequency stimuli, the ITD thresholds of the hearing-impaired listener were generally slightly greater than those obtained from the normal-hearing listeners. Whether these stimuli were presented at either Eq SPL or Eq SL did not differentially affect the ITD thresholds across groups.     

Performances of human listeners and an automatic aural classifier in discriminating between sonar target echoes and clutter

Human listening tests were conducted to investigate if participants could distinguish between samples of target echoes and clutter obtained from a broadband active sonar experiment. For each echo, the listeners assigned a rating based on how confident they were that it was a target echo or clutter. The measure of performance was the area under the binormal receiver-operating-characteristic (ROC) curve, A(z). The mean performance was A(z)=0.95 ± 0.04 when signals were presented with their full available acoustic bandwidth of approximately 0-2 kHz. It was A(z)=0.77 ± 0.08 when the bandwidth was reduced to 0.5-2 kHz. The error bounds are stated as 95% confidence intervals. These results show that the listeners could definitely hear differences, but their performance was significantly degraded when the low-frequency signal information was removed. The performance of an automatic aural classifier was compared against this human-performance baseline. Results of statistical tests showed that it outperformed 2 of 13 listeners and 5 of 9 human listeners in the full-bandwidth and reduced-bandwidth tests, respectively, and performed similarly to the other listeners. Given its performance, the automatic aural classifier may prove beneficial to Navy sonar systems.     

Separation of concurrent broadband sound sources by human listeners

The effect of spatial separation on the ability of human listeners to resolve a pair of concurrent broadband sounds was examined. Stimuli were presented in a virtual auditory environment using individualized outer ear filter functions. Subjects were presented with two simultaneous noise bursts that were either spatially coincident or separated (horizontally or vertically), and responded as to whether they perceived one or two source locations. Testing was carried out at five reference locations on the audiovisual horizon (0 degrees, 22.5 degrees, 45 degrees, 67.5 degrees, and 90 degrees azimuth). Results from experiment 1 showed that at more lateral locations, a larger horizontal separation was required for the perception of two sounds. The reverse was true for vertical separation. Furthermore, it was observed that subjects were unable to separate stimulus pairs if they delivered the same interaural differences in time (ITD) and level (ILD). These findings suggested that the auditory system exploited differences in one or both of the binaural cues to resolve the sources, and could not use monaural spectral cues effectively for the task. In experiments 2 and 3, separation of concurrent noise sources was examined upon removal of low-frequency content (and ITDs), onset/offset ITDs, both of these in conjunction, and all ITD information. While onset and offset ITDs did not appear to play a major role, differences in ongoing ITDs were robust cues for separation under these conditions, including those in the envelopes of high-frequency channels.     

Discrimination of spectral density

Experiments were performed to determine the ability of human listeners to discriminate between a sound with a large number of spectral components in a band, of given characteristic frequency and bandwidth, and a sound with a smaller number of components in that band. A pseudorandom placement of the components within the band ensured that no two sounds were identical. The data suggested that discrimination is primarily based upon the perception of temporal fluctuations in the intensity of the sound and secondarily upon resolved structure in the spectrum, perceived as tone color. Experiments using clusters of complex harmonic sounds showed that listeners are able to use the information in upper harmonic bands to discriminate spectral density.     

Effects of background noise level on behavioral estimates of basilar-membrane compression

Hearing-impaired (HI) listeners often show poorer performance on psychoacoustic tasks than do normal-hearing (NH) listeners. Although some such deficits may reflect changes in suprathreshold sound processing, others may be due to stimulus audibility and the elevated absolute thresholds associated with hearing loss. Masking noise can be used to raise the thresholds of NH to equal the thresholds in quiet of HI listeners. However, such noise may have other effects, including changing peripheral response characteristics, such as the compressive input-output function of the basilar membrane in the normal cochlea. This study estimated compression behaviorally across a range of background noise levels in NH listeners at a 4 kHz signal frequency, using a growth of forward masking paradigm. For signals 5 dB or more above threshold in noise, no significant effect of broadband noise level was found on estimates of compression. This finding suggests that broadband noise does not significantly alter the compressive response of the basilar membrane to sounds that are presented well above their threshold in the noise. Similarities between the performance of HI listeners and NH listeners in threshold-equalizing noise are therefore unlikely to be due to a linearization of basilar-membrane responses to suprathreshold stimuli in the NH listeners.     

Comparing models of the combined-stimulation advantage for speech recognition

The "combined-stimulation advantage" refers to an improvement in speech recognition when cochlear-implant or vocoded stimulation is supplemented by low-frequency acoustic information. Previous studies have been interpreted as evidence for "super-additive" or "synergistic" effects in the combination of low-frequency and electric or vocoded speech information by human listeners. However, this conclusion was based on predictions of performance obtained using a suboptimal high-threshold model of information combination. The present study shows that a different model, based on Gaussian signal detection theory, can predict surprisingly large combined-stimulation advantages, even when performance with either information source alone is close to chance, without involving any synergistic interaction. A reanalysis of published data using this model reveals that previous results, which have been interpreted as evidence for super-additive effects in perception of combined speech stimuli, are actually consistent with a more parsimonious explanation, according to which the combined-stimulation advantage reflects an optimal combination of two independent sources of information. The present results do not rule out the possible existence of synergistic effects in combined stimulation; however, they emphasize the possibility that the combined-stimulation advantages observed in some studies can be explained simply by non-interactive combination of two information sources.     

Effects of cochlear hearing loss on perceptual grouping cues in competing-vowel perception

This study compared how normal-hearing listeners (NH) and listeners with moderate to moderately severe cochlear hearing loss (HI) use and combine information within and across frequency regions in the perceptual separation of competing vowels with fundamental frequency differences (deltaF0) ranging from 0 to 9 semitones. Following the procedure of Culling and Darwin [J. Acoust. Soc. Am. 93, 3454-3467 (1993)], eight NH listeners and eight HI listeners identified competing vowels with either a consistent or inconsistent harmonic structure. Vowels were amplified to assure audibility for HI listeners. The contribution of frequency region depended on the value of deltaF0 between the competing vowels. When deltaF0 was small, both groups of listeners effectively utilized deltaF0 cues in the low-frequency region. In contrast, HI listeners derived significantly less benefit than NH listeners from deltaF0 cues conveyed by the high-frequency region at small deltaF0's. At larger deltaF0's, both groups combined deltaF0 cues from the low and high formant-frequency regions. Cochlear impairment appears to negatively impact the ability to use F0 cues for within-formant grouping in the high-frequency region. However, cochlear loss does not appear to disrupt the ability to use within-formant F0 cues in the low-frequency region or to group F0 cues across formant regions.     

Comodulation masking release in speech identification with real and simulated cochlear-implant hearing

For normal-hearing (NH) listeners, masker energy outside the spectral region of a target signal can improve target detection and identification, a phenomenon referred to as comodulation masking release (CMR). This study examined whether, for cochlear implant (CI) listeners and for NH listeners presented with a "noise vocoded" CI simulation, speech identification in modulated noise is improved by a co-modulated flanking band. In Experiment 1, NH listeners identified noise-vocoded speech in a background of on-target noise with or without a flanking narrow band of noise outside the spectral region of the target. The on-target noise and flanker were either 16-Hz square-wave modulated with the same phase or were unmodulated; the speech was taken from a closed-set corpus. Performance was better in modulated than in unmodulated noise, and this difference was slightly greater when the comodulated flanker was present, consistent with a small CMR of about 1.7 dB for noise-vocoded speech. Experiment 2, which tested CI listeners using the same speech materials, found no advantage for modulated versus unmodulated maskers and no CMR. Thus although NH listeners can benefit from CMR even for speech signals with reduced spectro-temporal detail, no CMR was observed for CI users.     

Bimodal listeners are not sensitive to interaural time differences in unmodulated low-frequency stimuli (L)

Sensitivity to interaural time differences (ITDs) with unmodulated low-frequency stimuli was assessed in bimodal listeners who had previously shown to be good performers in ITD experiments. Two types of stimuli were used: (1) an acoustic sinusoid combined with an electric transposed signal and (2) an acoustic sinusoid combined with an electric clicktrain. No or very low sensitivity to ITD was found for these stimuli, even though subjects were highly trained on the task and were intensively tested in multiple test sessions. In previous studies with users of a cochlear implant (CI) and a contralateral hearing aid (HA) (bimodal listeners), sensitivity was shown to ITD with modulated stimuli with frequency content between 600 and 3600 Hz. The outcomes of the current study imply that in speech processing design for users of a CI in combination with a HA on the contralateral side, the emphasis should be more on providing salient envelope ITD cues than on preserving fine-timing ITD cues present in acoustic signals.     

Dissociation of perceptual judgments of "what" and "where" in an ambiguous auditory scene

Whenever an acoustic scene contains a mixture of sources, listeners must segregate the mixture in order to compute source content and/or location. Some past studies have explored whether perceived location depends on which sound elements are perceived within a source. However, no direct comparisons have been made of "what" and "where" judgments for the same sound mixtures using the same listeners. The current study tested if the sound elements making up an auditory object predict that object's perceived location. Listeners were presented with an auditory scene containing competing "target" and "captor" sources, each of which could logically contain a "promiscuous" tone complex. In separate blocks, the same listeners matched the perceived spectro-temporal content ("what") and location ("where") of the target. Generally, as the captor intensity decreased, the promiscuous complex contributed more to both what and where judgments of the target. However judgments did not agree either quantitatively or qualitatively. For some subjects, the promiscuous complex consistently contributed more to the spectro-temporal content of the target than to its location while for some it consistently contributed more to target location. These results show a dissociation between the perceived spectro-temporal content of an auditory object and where that object is perceived.     

Speechreading supplemented with frequency-selective sound-pressure information

The benefit of supplementing speechreading with frequency-selective sound-pressure information was studied by auditorily presenting this information to normal-hearing listeners. The sound-pressure levels in one or two frequency bands of the speech signal with center frequencies of 500, 1600, and 3160 Hz, respectively, and with 1- or 1/3-oct bandwidth were used to amplitude-modulate pure-tone carriers with frequencies equal to the center frequencies of the filter bands. Short sentences were presented to 18 normal-hearing listeners under the conditions of speechreading-only and speechreading combined with the sound-pressure information. The mean number of correctly perceived syllables increased from 22.8% for speechreading-only to 65.7% when sound-pressure information was supplied in a single 1-oct band at 500 Hz and to 86.7% with two 1-oct bands at 500 and 3160 Hz, respectively. The latter signal scored only 26.7% correct syllables without accompanying visual information.     

Accuracy, latency, and listener-search behavior in localization in the horizontal and vertical planes

Thirty-six listeners localized continuous filtered noise bursts centered on 2.3 or 8.3 kHz under normal listening conditions or while wearing earmuffs. The noise bursts were from any one of 20 loudspeakers, 18 degrees apart, visible to the listeners, and arranged in the horizontal and vertical planes. Listeners were free to move, while remaining seated, throughout all trials. The noise bursts were terminated by the listeners. Measures of accuracy and latency showed that earmuff listening had a significant effect whereas, overall, signal frequency had no significant effect. There was, however, an apparent downward shift of the 2.3-kHz signals presented above the midline in the vertical plane. Analysis of video records of listeners' behavior revealed a strong tendency for initial orientation by head or eye movement to correlate with final response, even when both were inaccurate. The paradigm adopted in this latter aspect of the study illustrates an "ecological" approach to the study of auditory phenomena.     

A psychophysical pitch function determined by absolute magnitude estimation and its relation to the musical pitch scale

A psychophysical pitch function, describing the relation of perceived magnitude of pitch to the frequency of a pure tone, was determined by absolute magnitude estimation. Pitch estimates were made by listeners with relative pitch and by absolute pitch possessors for 27 tones spanning a frequency range of 31.5-12,500 Hz in 1/3 octave steps. Results show that the pitch function, plotted in log-log coordinates, is steeper below 200 Hz than at higher frequencies. It is hypothesized that the pitch function's bend may reflect the diversity of neurophysiological mechanisms of pitch encoding in frequency ranges below and above 200 Hz. The variation of the function's slope implies that pitch distances between tones with the same frequency ratios are perceived as larger below 200 Hz than at higher frequencies. It is argued that this implication may apply only to a purely sensory concept of pitch distance and cannot be extended to the perception of musical intervals, a phenomenon governed by musical cognitive principles. The results also show that pitch functions obtained for listeners with relative and absolute pitch have a similar shape, which means that quantitative pitch relations determined for both groups of listeners do not differ appreciably along the frequency scale.