The contribution of the near and far ear toward localization of sound in the sagittal plane

Eight listeners were required to locate a train of 4.5-kHz high-pass noise bursts emanating from loudspeakers positioned +/- 30, +/- 20, +/- 10, and 0 deg re: interaural axis. The vertical array of loudspeakers was placed at 45, 90, and 135 deg left of midline. The various experimental conditions incorporated binaural and monaural listening with the latter utilizing the ear nearest or ear farthest from the sound source. While performance excelled when listening with only the near ear, the contribution of the far ear was statistically significant when compared to localization performance when both ears were occluded. Based on head related transfer functions for stimuli whose bandwidth was 1.0 kHz, four spectral cues were selected as candidates for influencing location judgments. Two of them associated relative changes in energy across center frequencies (CFs) with vertical source positions. The other two associated an absolute minimum (maximum) energy for specific CFs with a vertical source position. All but one cue when measured for the near ear could account for localization proficiency. On the other hand, when listening with the far ear, maximum energy at a specific CF outperformed the remaining cues in accounting for localization proficiency.     

Consonant reception in noise by listeners with mild and moderate sensorineural hearing impairment

The goal of this study was to determine the extent to which the difficulty experienced by impaired listeners in understanding noisy speech can be explained on the basis of elevated tone-detection thresholds. Twenty-one impaired ears of 15 subjects, spanning a variety of audiometric configurations with average hearing losses to 75 dB, were tested for reception of consonants in a speech-spectrum noise. Speech level, noise level, and frequency-gain characteristic were varied to generate a range of listening conditions. Results for impaired listeners were compared to those of normal-hearing listeners tested under the same conditions with extra noise added to approximate the impaired listeners' detection thresholds. Results for impaired and normal listeners were also compared on the basis of articulation indices. Consonant recognition by this sample of impaired listeners was generally comparable to that of normal-hearing listeners with similar threshold shifts listening under the same conditions. When listening conditions were equated for articulation index, there was no clear dependence of consonant recognition on average hearing loss. Assuming that the primary consequence of the threshold simulation in normals is loss of audibility (as opposed to suprathreshold discrimination or resolution deficits), it is concluded that the primary source of difficulty in listening in noise for listeners with moderate or milder hearing impairments, aside from the noise itself, is the loss of audibility.     

Perception of intraphonemic differences by phoneticians, musicians, and inexperienced listeners

Several studies have shown that extensive training with synthetic speech sounds can result in substantial improvements in listeners' perception of intraphonemic differences. The purpose of the present study was to investigate the effects of listening experience on the perception of intraphonemic differences in the absence of specific training with the synthetic speech sounds being tested. Phonetically trained listeners, musicians, and untrained listeners were tested on a two-choice identification task, a three-choice identification task, and an ABX discrimination task using a synthetic [bi]-[phi] continuum and a synthetic [wei]-[rei] continuum. The three-choice identification task included the identification of stimuli with an "indefinite" or "ambiguous" quality in addition to clear instances of the opposing phonetic categories. Results included: (1) All three subject groups showed some ability to identify ambiguous stimuli; (2) phonetically trained listeners were better at identifying ambiguous stimuli than musicians and untrained listeners; (3) phonetically trained listeners performed better on the discrimination task than musicians and untrained listeners; (4) musicians and untrained listeners did not differ on any of the listening tasks; and (5) participation by the inexperienced listeners in a 10-week introductory phonetics course did not result in improvements in either the three-choice identification task or the discrimination task.     

Acceptable range of speech level in noisy sound fields for young adults and elderly persons

The acceptable range of speech level as a function of background noise level was investigated on the basis of word intelligibility scores and listening difficulty ratings. In the present study, the acceptable range is defined as the range that maximizes word intelligibility scores and simultaneously does not cause a significant increase in listening difficulty ratings from the minimum ratings. Listening tests with young adult and elderly listeners demonstrated the following. (1) The acceptable range of speech level for elderly listeners overlapped that for young listeners. (2) The lower limit of the acceptable speech level for both young and elderly listeners was 65 dB (A-weighted) for noise levels of 40 and 45 dB (A-weighted), a level with a speech-to-noise ratio of +15 dB for noise levels of 50 and 55 dB, and a level with a speech-to-noise ratio of +10 dB for noise levels from 60 to 70 dB. (3) The upper limit of the acceptable speech level for both young and elderly listeners was 80 dB for noise levels from 40 to 55 dB and 85 dB or above for noise levels from 55 to 70 dB.     

Temporal integration, frequency resolution, and off-frequency listening in normal-hearing and cochlear-impaired listeners

Temporal integration for a 1000-Hz signal was determined for normal-hearing and cochlear hearing-impaired listeners in quiet and in masking noise of variable bandwidth. Critical ratio and 3-dB critical band measures of frequency resolution were derived from the masking data. Temporal integration for the normal-hearing listeners was markedly reduced in narrow-band noise, when contrasted with temporal integration in quiet or in wideband noise. The effect of noise bandwidth on temporal integration was smaller for the hearing-impaired group. Hearing-impaired subjects showed both reduced temporal integration and reduced frequency resolution for the 200-ms signal. However, a direct relation between temporal integration and frequency resolution was not indicated. Frequency resolution for the normal-hearing listeners did not differ from that of the hearing-impaired listeners for the 20-ms signal. It was suggested that some of the frequency resolution and temporal integration differences between normal-hearing and hearing-impaired listeners could be accounted for by off-frequency listening.     

Impacts of age on memory for auditory intensity

It is hypothesized that older listeners are more likely than younger listeners to be impaired when asked to make intensity judgments about target tones embedded in rapidly presented auditory sequences. This study examined this hypothesis by asking listeners ranging in age from 19 to 74?yr to make judgments of intensity based on narrowband noise bursts varying in frequency and intensity. In two experiments, listeners made intensity judgments of target bursts alone or embedded in sequences of bursts. In the first experiment, one of four fixed sequences was presented and had to be identified. In the second experiment, pre- or post-trial bursts acted as cues that identified the frequency of the target burst in the sequence. In both experiments, intensity discrimination thresholds for single bursts were good predictors of performance with sequences and were little affected by age. Significant negative relationships between age and accuracy were observed when single sequences had to be identified or a post-trial cue was used, but no age effects were apparent when a pre-trial cue was used. These data are interpreted as being consistent with previous suggestions that the aging process results in a decline in auditory memory capacity and/or internally generated selective attention.     

Auditory localization in the vertical plane: accuracy and constraint on bodily movement

In two preliminary experiments, listeners were instructed to limit increasingly the movement of their heads and/or bodies while attempting to localize narrow bands of noise centered on 2.3 or 8.3 kHz. With increasing constraint on movement, the high-frequency band was incorrectly perceived as elevated above the horizon. The low-frequency band, when actually elevated above the horizon, was not so regularly perceived incorrectly as being below the horizon, a finding inconsistent with a previous report. A third experiment, which more closely replicated the task conditions and strategies of the previous study, did tend to reveal the anomalous low-frequency error. The error is explicable as a default response to which listeners whose sensitivity to the vertical dimension, in general, appears imperfect are prone. From various reports, it emerges that about 25% of presumed normally hearing people exhibit this insensitivity.     

Relationship between listening difficulty rating and objective measures in reverberant and noisy sound fields for young adults and elderly persons

Listening difficulty ratings [Morimoto et al., J. Acoust. Soc. Am. 116, 1607-1613 (2004)] were obtained for 20 young adult listeners and 34 elderly listeners in reverberant and noisy sound fields simulated in an anechoic room. The listening difficulty ratings were compared with acoustical objective measures. The results and analyses showed the following: (i) The correlation between listening difficulty ratings and the revised speech transmission index (STI(r)), and that for the useful-detrimental ratio (U(50)) were high, regardless of the age of the listeners. (ii) STI(r) and U(50) need to be increased by 0.12 and 4.2 dB, respectively, to equalize the listening difficulty ratings for the elderly listeners with those for the young listeners. (iii) The estimation accuracies for STI(r) and U(50) can be improved by calculating them with the L(eq) of background noise linearly increased by 4 to 10 dB, which depends on the age of the listeners and the objective measures. However, the improvement was not statistically significant for the elderly listeners.     

Perceptual weighting of individual and concurrent cues for sentence intelligibility: frequency, envelope, and fine structure

The speech signal may be divided into frequency bands, each containing temporal properties of the envelope and fine structure. For maximal speech understanding, listeners must allocate their perceptual resources to the most informative acoustic properties. Understanding this perceptual weighting is essential for the design of assistive listening devices that need to preserve these important speech cues. This study measured the perceptual weighting of young normal-hearing listeners for the envelope and fine structure in each of three frequency bands for sentence materials. Perceptual weights were obtained under two listening contexts: (1) when each acoustic property was presented individually and (2) when multiple acoustic properties were available concurrently. The processing method was designed to vary the availability of each acoustic property independently by adding noise at different levels. Perceptual weights were determined by correlating a listener's performance with the availability of each acoustic property on a trial-by-trial basis. Results demonstrated that weights were (1) equal when acoustic properties were presented individually and (2) biased toward envelope and mid-frequency information when multiple properties were available. Results suggest a complex interaction between the available acoustic properties and the listening context in determining how best to allocate perceptual resources when listening to speech in noise.     

Effect of multiple speechlike maskers on binaural speech recognition in normal and impaired hearing.

Speech-reception thresholds (SRT) were measured for 17 normal-hearing and 17 hearing-impaired listeners in conditions simulating free-field situations with between one and six interfering talkers. The stimuli, speech and noise with identical long-term average spectra, were recorded with a KEMAR manikin in an anechoic room and presented to the subjects through headphones. The noise was modulated using the envelope fluctuations of the speech. Several conditions were simulated with the speaker always in front of the listener and the maskers either also in front, or positioned in a symmetrical or asymmetrical configuration around the listener. Results show that the hearing impaired have significantly poorer performance than the normal hearing in all conditions. The mean SRT differences between the groups range from 4.2-10 dB. It appears that the modulations in the masker act as an important cue for the normal-hearing listeners, who experience up to 5-dB release from masking, while being hardly beneficial for the hearing impaired listeners. The gain occurring when maskers are moved from the frontal position to positions around the listener varies from 1.5 to 8 dB for the normal hearing, and from 1 to 6.5 dB for the hearing impaired. It depends strongly on the number of maskers and their positions, but less on hearing impairment. The difference between the SRTs for binaural and best-ear listening (the "cocktail party effect") is approximately 3 dB in all conditions for both the normal-hearing and the hearing-impaired listeners.     

Speech perception,localization, and lateralization with bilateral cochlear implants

Five bilateral cochlear implant users were tested for their localization abilities and speech understanding in noise, for both monaural and binaural listening conditions. They also participated in lateralization tasks to assess the impact of variations in interaural time delays (ITDs) and interaural level differences (ILDs) for electrical pulse trains under direct computer control. The localization task used pink noise bursts presented from an eight-loudspeaker array spanning an arc of approximately 108 degrees in front of the listeners at ear level (0-degree elevation). Subjects showed large benefits from bilateral device use compared to either side alone. Typical root-mean-square (rms) averaged errors across all eight loudspeakers in the array were about 10 degrees for bilateral device use and ranged from 20 degrees to 60 degrees using either ear alone. Speech reception thresholds (SRTs) were measured for sentences presented from directly in front of the listeners (0 degrees) in spectrally matching speech-weighted noise at either 0 degrees, +90 degrees or -90 degrees for four subjects out of five tested who could perform the task. For noise to either side, bilateral device use showed a substantial benefit over unilateral device use when noise was ipsilateral to the unilateral device. This was primarily because of monaural head-shadow effects, which resulted in robust SRT improvements (P<0.001) of about 4 to 5 dB when ipsilateral and contralateral noise positions were compared. The additional benefit of using both ears compared to the shadowed ear (i.e., binaural unmasking) was only 1 or 2 dB and less robust (P = 0.04). Results from the lateralization studies showed consistently good sensitivity to ILDs; better than the smallest level adjustment available in the implants (0.17 dB) for some subjects. Sensitivity to ITDs was moderate on the other hand, typically of the order of 100 micros. ITD sensitivity deteriorated rapidly when stimulation rates for unmodulated pulse-trains increased above a few hundred Hz but at 800 pps showed sensitivity comparable to 50-pps pulse-trains when a 50-Hz modulation was applied. In our opinion, these results clearly demonstrate important benefits are available from bilateral implantation, both for localizing sounds (in quiet) and for listening in noise when signal and noise sources are spatially separated. The data do indicate, however, that effects of interaural timing cues are weaker than those from interaural level cues and according to our psychophysical findings rely on the availability of low-rate information below a few hundred Hz.     

Some difference limens for the perception of breathiness

Perception of breathy voice quality appears to be cued by changes in the vowel spectrum. These changes are related to alterations in the intensity of aspiration noise and spectral slope of the harmonic energy [Shrivastav and Sapienza, J. Acoust. Soc. Am., 114 (4), 2217-2224 (2003)]. Ten young-adult listeners with normal hearing were tested using an adaptive listening task to determine the smallest change in signal-to-noise ratio that resulted in a change in breathiness. Six vowel continua, three female and three male, were generated using a Klatt synthesizer and served as stimuli. Results showed that listeners needed as much as 20-dB increase in aspiration noise to perceive a change in breathiness against a relatively normal voice. In contrast, listeners needed approximately an 11-dB increase in aspiration noise to discriminate breathiness against a severely breathy voice. The difference limens for breathiness were observed to vary across the six talkers. Voices having aspiration noise that was predominantly in the high frequencies had smaller difference limens. No significant differences for male and female voice were observed.     

Harmonic and melodic octave templates

For normal-hearing adult listeners, two simultaneous pure tones with a frequency ratio close to 2/1 may perceptually fuse into a single sound, which shows that such listeners are sensitive to "octave harmony." Many adult listeners are also able to consistently adjust two successive pure tones "one octave apart," which shows that they possess melodic octave templates. According to Terhardt [J. Acoust. Soc. Am. 55, 1061-1069 (1974)], melodic octave templates and the perception of octave harmony originate from a common learning process taking place in early life. In the two experiments reported here, subjects performed repeated octave adjustments for pairs of simultaneous and successive tone bursts. Both tones were presented monaurally, at 45 or 65 dB SPL. The frequency of the lower tone (fref) was an independent variable, while the frequency of the higher tone was adjustable within a 500-cent range. In some conditions, when the two tones were presented simultaneously, they were sinusoidally frequency modulated in a coherent manner, at a rate of 2 or 4 Hz; the aim of this frequency modulation was to force the subjects to adopt a synthetic listening strategy, i.e., to base their adjustments on perceived harmony. For fref values ranging from 270-2000 Hz, subjects performed consistent adjustments when the tones were presented successively: fref had little effect on the adjustments' variability. However, in the same frequency range, the variability of the harmonic adjustments markedly increased with fref; for the highest fref values, it was much greater than the variability of the melodic adjustments. The results suggest that, in adult listeners, the perception of octave harmony disappears at frequencies for which melodic octaves are still accurately perceived.     

Quantifying the intelligibility of speech in noise for non-native listeners

When listening to languages learned at a later age, speech intelligibility is generally lower than when listening to one's native language. The main purpose of this study is to quantify speech intelligibility in noise for specific populations of non-native listeners, only broadly addressing the underlying perceptual and linguistic processing. An easy method is sought to extend these quantitative findings to other listener populations. Dutch subjects listening to Germans and English speech, ranging from reasonable to excellent proficiency in these languages, were found to require a 1-7 dB better speech-to-noise ratio to obtain 50% sentence intelligibility than native listeners. Also, the psychometric function for sentence recognition in noise was found to be shallower for non-native than for native listeners (worst-case slope around the 50% point of 7.5%/dB, compared to 12.6%/dB for native listeners). Differences between native and non-native speech intelligibility are largely predicted by linguistic entropy estimates as derived from a letter guessing task. Less effective use of context effects (especially semantic redundancy) explains the reduced speech intelligibility for non-native listeners. While measuring speech intelligibility for many different populations of listeners (languages, linguistic experience) may be prohibitively time consuming, obtaining predictions of non-native intelligibility from linguistic entropy may help to extend the results of this study to other listener populations.     

Upward spread of masking, hearing loss, and speech recognition in young and elderly listeners

Upward spreading of masking, measured in terms of absolute masked threshold, is greater in hearing-impaired listeners than in listeners with normal hearing. The purpose of this study was to make further observations on upward-masked thresholds and speech recognition in noise in elderly listeners. Two age groups were used: One group consisted of listeners who were more than 60 years old, and the second group consisted of listeners who were less than 36 years old. Both groups had listeners with normal hearing as well as listeners with mild to moderate sensorineural loss. The masking paradigm consisted of a continuous low-pass-filtered (1000-Hz) noise, which was mixed with the output of a self-tracking, sweep-frequency Bekesy audiometer. Thresholds were measured in quiet and with maskers at 70 and 90 dB SPL. The upward-masked thresholds were similar for young and elderly hearing-impaired listeners. A few elderly listeners had lower upward-masked thresholds compared with the young control group; however, their on-frequency masked thresholds were nearly identical to the control group. A significant correlation was found between upward-masked thresholds and the Speech Perception in Noise (SPIN) test in elderly listeners.     

Vowel perception by noise masked normal-hearing young adults

This study examined vowel perception by young normal-hearing (YNH) adults, in various listening conditions designed to simulate mild-to-moderate sloping sensorineural hearing loss. YNH listeners were individually age- and gender-matched to young hearing-impaired (YHI) listeners tested in a previous study [Richie et al., J. Acoust. Soc. Am. 114, 2923-2933 (2003)]. YNH listeners were tested in three conditions designed to create equal audibility with the YHI listeners; a low signal level with and without a simulated hearing loss, and a high signal level with a simulated hearing loss. Listeners discriminated changes in synthetic vowel tokens /I e epsilon alpha ae/ when Fl or F2 varied in frequency. Comparison of YNH with YHI results failed to reveal significant differences between groups in terms of performance on vowel discrimination, in conditions of similar audibility by using both noise masking to elevate the hearing thresholds of the YNH and applying frequency-specific gain to the YHI listeners. Further, analysis of learning curves suggests that while the YHI listeners completed an average of 46% more test blocks than YNH listeners, the YHI achieved a level of discrimination similar to that of the YNH within the same number of blocks. Apparently, when age and gender are closely matched between young hearing-impaired and normal-hearing adults, performance on vowel tasks may be explained by audibility alone.     

Speech intelligibility in cochlear implant simulations: Effects of carrier type, interfering noise, and subject experience

Channel vocoders using either tone or band-limited noise carriers have been used in experiments to simulate cochlear implant processing in normal-hearing listeners. Previous results from these experiments have suggested that the two vocoder types produce speech of nearly equal intelligibility in quiet conditions. The purpose of this study was to further compare the performance of tone and noise-band vocoders in both quiet and noisy listening conditions. In each of four experiments, normal-hearing subjects were better able to identify tone-vocoded sentences and vowel-consonant-vowel syllables than noise-vocoded sentences and syllables, both in quiet and in the presence of either speech-spectrum noise or two-talker babble. An analysis of consonant confusions for listening in both quiet and speech-spectrum noise revealed significantly different error patterns that were related to each vocoder's ability to produce tone or noise output that accurately reflected the consonant's manner of articulation. Subject experience was also shown to influence intelligibility. Simulations using a computational model of modulation detection suggest that the noise vocoder's disadvantage is in part due to the intrinsic temporal fluctuations of its carriers, which can interfere with temporal fluctuations that convey speech recognition cues.     

Informational masking and musical training

The relationship between musical training and informational masking was studied for 24 young adult listeners with normal hearing. The listeners were divided into two groups based on musical training. In one group, the listeners had little or no musical training; the other group was comprised of highly trained, currently active musicians. The hypothesis was that musicians may be less susceptible to informational masking, which is thought to reflect central, rather than peripheral, limitations on the processing of sound. Masked thresholds were measured in two conditions, similar to those used by Kidd et al. [J. Acoust. Soc. Am. 95, 3475-3480 (1994)]. In both conditions the signal was comprised of a series of repeated tone bursts at 1 kHz. The masker was comprised of a series of multitone bursts, gated with the signal. In one condition the frequencies of the masker were selected randomly for each burst; in the other condition the masker frequencies were selected randomly for the first burst of each interval and then remained constant throughout the interval. The difference in thresholds between the two conditions was taken as a measure of informational masking. Frequency selectivity, using the notched-noise method, was also estimated in the two groups. The results showed no difference in frequency selectivity between the two groups, but showed a large and significant difference in the amount of informational masking between musically trained and untrained listeners. This informational masking task, which requires no knowledge specific to musical training (such as note or interval names) and is generally not susceptible to systematic short- or medium-term training effects, may provide a basis for further studies of analytic listening abilities in different populations.     

Molecular analysis of the effect of relative tone level on multitone pattern discrimination

Molecular psychophysics attempts to model the observer's response to stimuli as they vary from trial to trial. The approach has gained popularity in multitone pattern discrimination studies as a means of estimating the relative reliance or decision weight listeners give to different tones in the pattern. Various factors affecting decision weights have been examined, but one largely ignored is the relative level of tones in the pattern. In the present study listeners detected a level-increment in a sequence of 5, 100-ms, 2.0-kHz tone bursts alternating in level between 40 and 80 dB SPL. The level increment was made largest on the 40-dB tones, yet despite this all four highly-practiced listeners gave near exclusive weight to the 80-dB tones. The effect was the same when the tones were replaced by bursts of broadband Gaussian noise alternating in level. It was reduced only when the level differences were made <10 dB, and it was entirely reversed only when the low-level tones alternated with louder bursts of Gaussian noise. The results are discussed in terms of the effects of both sensory and perceptual factors on estimates of decision weights.     

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.