Spectral and threshold effects on recognition of speech at higher-than-normal levels

To examine spectral and threshold effects for speech and noise at high levels, recognition of nonsense syllables was assessed for low-pass-filtered speech and speech-shaped maskers and high-pass-filtered speech and speech-shaped maskers at three speech levels, with signal-to-noise ratio held constant. Subjects were younger adults with normal hearing and older adults with normal hearing but significantly higher average quiet thresholds. A broadband masker was always present to minimize audibility differences between subject groups and across presentation levels. For subjects with lower thresholds, the declines in recognition of low-frequency syllables in low-frequency maskers were attributed to nonlinear growth of masking which reduced "effective" signal-to-noise ratio at high levels, whereas the decline for subjects with higher thresholds was not fully explained by nonlinear masking growth. For all subjects, masking growth did not entirely account for declines in recognition of high-frequency syllables in high-frequency maskers at high levels. Relative to younger subjects with normal hearing and lower quiet thresholds, older subjects with normal hearing and higher quiet thresholds had poorer consonant recognition in noise, especially for high-frequency speech in high-frequency maskers. Age-related effects on thresholds and task proficiency may be determining factors in the recognition of speech in noise at high levels.     

Tone detection and synthetic speech discrimination in band-reject noise by hearing-impaired listeners

Frequency resolution was evaluated for two normal-hearing and seven hearing-impaired subjects with moderate, flat sensorineural hearing loss by measuring percent correct detection of a 2000-Hz tone as the width of a notch in band-reject noise increased. The level of the tone was fixed for each subject at a criterion performance level in broadband noise. Discrimination of synthetic speech syllables that differed in spectral content in the 2000-Hz region was evaluated as a function of the notch width in the same band-reject noise. Recognition of natural speech consonant/vowel syllables in quiet was also tested; results were analyzed for percent correct performance and relative information transmitted for voicing and place features. In the hearing-impaired subjects, frequency resolution at 2000 Hz was significantly correlated with the discrimination of synthetic speech information in the 2000-Hz region and was not related to the recognition of natural speech nonsense syllables unless (a) the speech stimuli contained the vowel /i/ rather than /a/, and (b) the score reflected information transmitted for place of articulation rather than percent correct.     

Effects of low-pass filtering on the intelligibility of speech in quiet for people with and without dead regions at high frequencies

A dead region is a region of the cochlea where there are no functioning inner hair cells (IHCs) and/or neurons; it can be characterized in terms of the characteristic frequencies of the IHCs bordering that region. We examined the effect of high-frequency amplification on speech perception for subjects with high-frequency hearing loss with and without dead regions. The limits of any dead regions were defined by measuring psychophysical tuning curves and were confirmed using the TEN test described in Moore et al. [Br. J. Audiol. 34, 205-224 (2000)]. The speech stimuli were vowel-consonant-vowel (VCV) nonsense syllables, using one of three vowels (/i/, /a/, and /u/) and 21 different consonants. In a baseline condition, subjects were tested using broadband stimuli with a nominal input level of 65 dB SPL. Prior to presentation via Sennheiser HD580 earphones, the stimuli were subjected to the frequency-gain characteristic prescribed by the "Cambridge" formula, which is intended to give speech at 65 dB SPL the same overall loudness as for a normal listener, and to make the average loudness of the speech the same for each critical band over the frequency range important for speech intelligibility (in a listener without a dead region). The stimuli for all other conditions were initially subjected to this same frequency-gain characteristic. Then, the speech was low-pass filtered with various cutoff frequencies. For subjects without dead regions, performance generally improved progressively with increasing cutoff frequency. This indicates that they benefited from high-frequency information. For subjects with dead regions, two patterns of performance were observed. For most subjects, performance improved with increasing cutoff frequency until the cutoff frequency was somewhat above the estimated edge frequency of the dead region, but hardly changed with further increases. For a few subjects, performance initially improved with increasing cutoff frequency and then worsened with further increases, although the worsening was significant only for one subject. The results have important implications for the fitting of hearing aids.     

Speech recognition in noise as a function of highpass-filter cutoff frequency for people with and without low-frequency cochlear dead regions

Regions in the cochlea with very few functioning inner hair cells and/or neurons are called "dead regions" (DRs). Previously, we measured the recognition of highpass-filtered nonsense syllables as a function of filter cutoff frequency for hearing-impaired people with and without low-frequency (apical) DRs [J. Acoust. Soc. Am. 122, 542-553 (2007)]. DRs were diagnosed using the TEN(HL) test, and psychophysical tuning curves were used to define the edge frequency (fe) more precisely. Stimuli were amplified differently for each ear, using the "Cambridge formula." The present study was similar, but the speech was presented in speech-shaped noise at a signal-to-noise ratio of 3 dB. For subjects with low-frequency hearing loss but without DRs, scores were high (65-80%) for low cutoff frequencies and worsened with increasing cutoff frequency above about 430 Hz. For subjects with low-frequency DRs, performance was poor (20-40%) for the lowest cutoff frequency, improved with increasing cutoff frequency up to about 0.56fe, and then worsened. As for speech in quiet, these results indicate that people with low-frequency DRs are able to make effective use of frequency components that fall in the range 0.56fe to fe, but that frequency components below 0.56fe have deleterious effects.     

Spectral weighting strategies for sentences measured by a correlational method

Listeners' ability to understand speech in adverse listening conditions is partially due to the redundant nature of speech. Natural redundancies are often lost or altered when speech is filtered, such as done in AI/SII experiments. It is important to study how listeners recognize speech when the speech signal is unfiltered and the entire broadband spectrum is present. A correlational method [R. A. Lutfi, J. Acoust. Soc. Am. 97, 1333-1334 (1995); V. M. Richards and S. Zhu, J. Acoust. Soc. Am. 95, 423-424 (1994)] has been used to determine how listeners use spectral cues to perceive nonsense syllables when the full speech spectrum is present [K. A. Doherty and C. W. Turner, J. Acoust. Soc. Am. 100, 3769-3773 (1996); C. W. Turner et al., J. Acoust. Soc. Am. 104, 1580-1585 (1998)]. The experiments in this study measured spectral-weighting strategies for more naturally occurring speech stimuli, specifically sentences, using a correlational method for normal-hearing listeners. Results indicate that listeners placed the greatest weight on spectral information within bands 2 and 5 (562-1113 and 2807-11,000 Hz), respectively. Spectral-weighting strategies for sentences were also compared to weighting strategies for nonsense syllables measured in a previous study (C. W. Turner et al., 1998). Spectral-weighting strategies for sentences were different from those reported for nonsense syllables.     

Speech recognition as a function of high-pass filter cutoff frequency for people with and without low-frequency cochlear dead regions

Regions in the cochlea with no (or very few) functioning inner hair cells and/or neurons are called "dead regions" (DRs). The recognition of high-pass filtered nonsense syllables was measured as a function of filter cutoff frequency for hearing-impaired people with and without low-frequency (apical) cochlear DRs. The diagnosis of any DR was made using the TEN(HL) test, and psychophysical tuning curves were used to define the edge frequency (f(e)) more precisely. Stimuli were amplified differently for each ear, using the "Cambridge formula." For subjects with low-frequency hearing loss but without DRs, scores were high (about 78%) for low cutoff frequencies, remained approximately constant for cutoff frequencies up to 862 Hz, and then worsened with increasing cutoff frequency. For subjects with low-frequency DRs, performance was typically poor for the lowest cutoff frequency (100 Hz), improved as the cutoff frequency was increased to about 0.57f(e), and worsened with further increases. These results indicate that people with low-frequency DRs are able to make effective use of frequency components that fall in the range 0.57f(e) to f(e), but that frequency components below 0.57f(e) have deleterious effects. The results have implications for the fitting of hearing aids to people with low-frequency DRs.     

Recognition of natural and time/intensity altered CVs by young and elderly subjects with normal hearing

The purpose of this study was to evaluate the efficiency of three acoustic modifications derived from clear speech for improving consonant recognition by young and elderly normal-hearing subjects. Percent-correct nonsense syllable recognition was measured for four stimulus sets: unmodified stimuli; stimuli with consonant duration increased by 100%; stimuli with consonant-vowel ratio increased by 10 dB; and stimuli with both consonant duration and consonant-vowel ratio increased. Analyses of overall nonsense syllable recognition, consonant feature recognition, and consonant confusion patterns demonstrated that the consonant-vowel ratio increase modification produced better performance than the other acoustic modifications by both subject groups. However, elderly subjects exhibited poorer performance than young subjects in most conditions. These results and their implications are discussed.     

Measures of auditory-visual integration in nonsense syllables and sentences.

For all but the most profoundly hearing-impaired (HI) individuals, auditory-visual (AV) speech has been shown consistently to afford more accurate recognition than auditory (A) or visual (V) speech. However, the amount of AV benefit achieved (i.e., the superiority of AV performance in relation to unimodal performance) can differ widely across HI individuals. To begin to explain these individual differences, several factors need to be considered. The most obvious of these are deficient A and V speech recognition skills. However, large differences in individuals' AV recognition scores persist even when unimodal skill levels are taken into account. These remaining differences might be attributable to differing efficiency in the operation of a perceptual process that integrates A and V speech information. There is at present no accepted measure of the putative integration process. In this study, several possible integration measures are compared using both congruent and discrepant AV nonsense syllable and sentence recognition tasks. Correlations were tested among the integration measures, and between each integration measure and independent measures of AV benefit for nonsense syllables and sentences in noise. Integration measures derived from tests using nonsense syllables were significantly correlated with each other; on these measures, HI subjects show generally high levels of integration ability. Integration measures derived from sentence recognition tests were also significantly correlated with each other, but were not significantly correlated with the measures derived from nonsense syllable tests. Similarly, the measures of AV benefit based on nonsense syllable recognition tests were found not to be significantly correlated with the benefit measures based on tests involving sentence materials. Finally, there were significant correlations between AV integration and benefit measures derived from the same class of speech materials, but nonsignificant correlations between integration and benefit measures derived from different classes of materials. These results suggest that the perceptual processes underlying AV benefit and the integration of A and V speech information might not operate in the same way on nonsense syllable and sentence input.     

Speech waveform envelope cues for consonant recognition

This study investigated the cues for consonant recognition that are available in the time-intensity envelope of speech. Twelve normal-hearing subjects listened to three sets of spectrally identical noise stimuli created by multiplying noise with the speech envelopes of 19(aCa) natural-speech nonsense syllables. The speech envelope for each of the three noise conditions was derived using a different low-pass filter cutoff (20, 200, and 2000 Hz). Average consonant identification performance was above chance for the three noise conditions and improved significantly with the increase in envelope bandwidth from 20-200 Hz. SINDSCAL multidimensional scaling analysis of the consonant confusions data identified three speech envelope features that divided the 19 consonants into four envelope feature groups ("envemes"). The enveme groups in combination with visually distinctive speech feature groupings ("visemes") can distinguish most of the 19 consonants. These results suggest that near-perfect consonant identification performance could be attained by subjects who receive only enveme and viseme information and no spectral information.     

Speech processing studies using an acoustic model of a multiple-channel cochlear implant

The speech perception of two multiple-channel cochlear implant patients was compared with that of three normally hearing listeners using an acoustic model of the implant for 22 different speech tests. The tests used included a minimal auditory capabilities battery, both closed-set and open-set word and sentence tests, speech tracking and a 12-consonant confusion study using nonsense syllables. The acoustic model represented electrical current pulses by bursts of noise and the effects of different electrodes were represented by using bandpass filters with different center frequencies. All subjects used a speech processor that coded the fundamental voicing frequency of speech as a pulse rate and the second formant frequency of speech as the electrode position in the cochlea, or the center frequency of the bandpass filter. Very good agreement was found for the two groups of subjects, indicating that the acoustic model is a useful tool for the development and evaluation of alternative cochlear implant speech processing strategies.     

The effects of multichannel compression/expansion amplification on the intelligibility of nonsense syllables in noise

The effects of six-channel compression and expansion amplification on the intelligibility of nonsense syllables embedded in speech spectrum noise were examined for four hearing-impaired subjects. For one condition (linear) the stimulus was given six-channel amplification with frequency shaping to suit the subject's hearing loss. The other condition (nonlinear) was the same except that low level inputs, to any given channel, received expansion amplification and high level inputs received compression. For each condition, each subject received the nonsense syllables at three different input levels, representing low, average, and high intensity speech. The results of this study, like those of most other studies of multichannel compression, are mainly negative. Nonlinear processing (mainly expansion) of low intensity speech resulted in a significant degradation of speech intelligibility for two subjects and in no improvement for the others. One subject showed a significant improvement in intelligibility for the nonlinearly processed average intensity speech and another subject showed significant improvement for the high intensity input (mainly compression). Clearly, nonlinear processing is beneficial for some subjects, under some listening conditions, but further research is needed to identify the relevent characteristics of such subjects. An acoustic analysis of selected items revealed that the failure of expansion to improve intelligibility was primarily due to the very low intensity consonants /e/ and /k/, in final position, being presented at an even lower intensity in the expansion condition than in the linear condition. Expansion may be worth further investigation with different parameters. Several other problems caused by the multichannel processing were also revealed. These included alteration of spectral shapes and band interaction effects. Ways of overcoming these problems, and of capitalizing on the likely advantages of multichannel amplification, are currently being investigated.     

Quantitative evaluation of lexical status,word frequency,and neighborhood density as context effects in spoken word recognition

Listeners identified a phonetically balanced set of consonant-vowel-consonant (CVC) words and nonsense syllables in noise at four signal-to-noise ratios. The identification scores for phonemes and syllables were analyzed using the j-factor model [Boothroyd and Nittrouer, J. Acoust. Soc. Am. 84, 101-114 (1988)], which measures the perceptual independence of the parts of a whole. Results indicate that nonsense CVC syllables are perceived as having three independent phonemes, while words show j = 2.34 independent units. Among the words, high-frequency words are perceived as having significantly fewer independent units than low-frequency words. Words with dense phonetic neighborhoods are perceived as having 0.5 more independent units than words with sparse neighborhoods. The neighborhood effect in these data is due almost entirely to density as determined by the initial consonant and vowel, demonstrated in analyses by subjects and items, and correlation analyses of syllable recognition with the neighborhood activation model [Luce and Pisoni, Ear Hear. 19, 1-36 (1998)]. The j factors are interpreted as measuring increased efficiency of the perception of word-final consonants of words in sparse neighborhoods during spoken word recognition.     

Stop-consonant recognition for normal-hearing listeners and listeners with high-frequency hearing loss. II: Articulation index predictions

Articulation index (AI) theory was used to evaluate stop-consonant recognition of normal-hearing listeners and listeners with high-frequency hearing loss. From results reported in a companion article [Dubno et al., J. Acoust. Soc. Am. 85, 347-354 (1989)], a transfer function relating the AI to stop-consonant recognition was established, and a frequency importance function was determined for the nine stop-consonant-vowel syllables used as test stimuli. The calculations included the rms and peak levels of the speech that had been measured in 1/3 octave bands; the internal noise was estimated from the thresholds for each subject. The AI model was then used to predict performance for the hearing-impaired listeners. A majority of the AI predictions for the hearing-impaired subjects fell within +/- 2 standard deviations of the normal-hearing listeners' results. However, as observed in previous data, the AI tended to overestimate performance of the hearing-impaired listeners. The accuracy of the predictions decreased with the magnitude of high-frequency hearing loss. Thus, with the exception of performance for listeners with severe high-frequency hearing loss, the results suggest that poorer speech recognition among hearing-impaired listeners results from reduced audibility within critical spectral regions of the speech stimuli.     

Integration efficiency for speech perception within and across sensory modalities by normal-hearing and hearing-impaired individuals

In face-to-face speech communication, the listener extracts and integrates information from the acoustic and optic speech signals. Integration occurs within the auditory modality (i.e., across the acoustic frequency spectrum) and across sensory modalities (i.e., across the acoustic and optic signals). The difficulties experienced by some hearing-impaired listeners in understanding speech could be attributed to losses in the extraction of speech information, the integration of speech cues, or both. The present study evaluated the ability of normal-hearing and hearing-impaired listeners to integrate speech information within and across sensory modalities in order to determine the degree to which integration efficiency may be a factor in the performance of hearing-impaired listeners. Auditory-visual nonsense syllables consisting of eighteen medial consonants surrounded by the vowel [a] were processed into four nonoverlapping acoustic filter bands between 300 and 6000 Hz. A variety of one, two, three, and four filter-band combinations were presented for identification in auditory-only and auditory-visual conditions: A visual-only condition was also included. Integration efficiency was evaluated using a model of optimal integration. Results showed that normal-hearing and hearing-impaired listeners integrated information across the auditory and visual sensory modalities with a high degree of efficiency, independent of differences in auditory capabilities. However, across-frequency integration for auditory-only input was less efficient for hearing-impaired listeners. These individuals exhibited particular difficulty extracting information from the highest frequency band (4762-6000 Hz) when speech information was presented concurrently in the next lower-frequency band (1890-2381 Hz). Results suggest that integration of speech information within the auditory modality, but not across auditory and visual modalities, affects speech understanding in hearing-impaired listeners.     

Individual differences in the processing of speech and nonspeech sounds by normal-hearing listeners.

While a large portion of the variance among listeners in speech recognition is associated with the audibility of components of the speech waveform, it is not possible to predict individual differences in the accuracy of speech processing strictly from the audiogram. This has suggested that some of the variance may be associated with individual differences in spectral or temporal resolving power, or acuity. Psychoacoustic measures of spectral-temporal acuity with nonspeech stimuli have been shown, however, to correlate only weakly (or not at all) with speech processing. In a replication and extension of an earlier study [Watson et al., J. Acoust. Soc. Am. Suppl. 1 71. S73 (1982)] 93 normal-hearing college students were tested on speech perception tasks (nonsense syllables, words, and sentences in a noise background) and on six spectral-temporal discrimination tasks using simple and complex nonspeech sounds. Factor analysis showed that the abilities that explain performance on the nonspeech tasks are quite distinct from those that account for performance on the speech tasks. Performance was significantly correlated among speech tasks and among nonspeech tasks. Either, (a) auditory spectral-temporal acuity for nonspeech sounds is orthogonal to speech processing abilities, or (b) the appropriate tasks or types of nonspeech stimuli that challenge the abilities required for speech recognition have yet to be identified.     

Integral processing of phonemes: evidence for a phonetic mode of perception

To investigate the extent and locus of integral processing in speech perception, a speeded classification task was utilized with a set of noise-tone analogs of the fricative-vowel syllables (fae), (integral of ae), (fu), and (integral of u). Unlike the stimuli used in previous studies of selective perception of syllables, these stimuli did not contain consonant-vowel transitions. Subjects were asked to classify on the basis of one of the two syllable components. Some subjects were told that the stimuli were computer generated noise-tone sequences. These subjects processed the noise and tone separably. Irrelevant variation of the noise did not affect reaction times (RTs) for the classification of the tone, and vice versa. Other subjects were instructed to treat the stimuli as speech. For these subjects, irrelevant variation of the fricative increased RTs for the classification of the vowel, and vice versa. A second experiment employed naturally spoken fricative-vowel syllables with the same task. Classification RTs showed a pattern of integrality in that irrelevant variation of either component increased RTs to the other. These results indicate that knowledge of coarticulation (or its acoustic consequences) is a basic element of speech perception. Furthermore, the use of this knowledge in phonetic coding is mandatory, even in situations where the stimuli do not contain coarticulatory information.     

Intelligibility of speech in noise at high presentation levels: effects of hearing loss and frequency region

These experiments examined how high presentation levels influence speech recognition for high- and low-frequency stimuli in noise. Normally hearing (NH) and hearing-impaired (HI) listeners were tested. In Experiment 1, high- and low-frequency bandwidths yielding 70%-correct word recognition in quiet were determined at levels associated with broadband speech at 75 dB SPL. In Experiment 2, broadband and band-limited sentences (based on passbands measured in Experiment 1) were presented at this level in speech-shaped noise filtered to the same frequency bandwidths as targets. Noise levels were adjusted to produce approximately 30%-correct word recognition. Frequency bandwidths and signal-to-noise ratios supporting criterion performance in Experiment 2 were tested at 75, 87.5, and 100 dB SPL in Experiment 3. Performance tended to decrease as levels increased. For NH listeners, this "rollover" effect was greater for high-frequency and broadband materials than for low-frequency stimuli. For HI listeners, the 75- to 87.5-dB increase improved signal audibility for high-frequency stimuli and rollover was not observed. However, the 87.5- to 100-dB increase produced qualitatively similar results for both groups: scores decreased most for high-frequency stimuli and least for low-frequency materials. Predictions of speech intelligibility by quantitative methods such as the Speech Intelligibility Index may be improved if rollover effects are modeled as frequency dependent.     

Contribution of low-frequency acoustic information to Chinese speech recognition in cochlear implant simulations

Chinese sentence recognition strongly relates to the reception of tonal information. For cochlear implant (CI) users with residual acoustic hearing, tonal information may be enhanced by restoring low-frequency acoustic cues in the nonimplanted ear. The present study investigated the contribution of low-frequency acoustic information to Chinese speech recognition in Mandarin-speaking normal-hearing subjects listening to acoustic simulations of bilaterally combined electric and acoustic hearing. Subjects listened to a 6-channel CI simulation in one ear and low-pass filtered speech in the other ear. Chinese tone, phoneme, and sentence recognition were measured in steady-state, speech-shaped noise, as a function of the cutoff frequency for low-pass filtered speech. Results showed that low-frequency acoustic information below 500 Hz contributed most strongly to tone recognition, while low-frequency acoustic information above 500 Hz contributed most strongly to phoneme recognition. For Chinese sentences, speech reception thresholds (SRTs) improved with increasing amounts of low-frequency acoustic information, and significantly improved when low-frequency acoustic information above 500 Hz was preserved. SRTs were not significantly affected by the degree of spectral overlap between the CI simulation and low-pass filtered speech. These results suggest that, for CI patients with residual acoustic hearing, preserving low-frequency acoustic information can improve Chinese speech recognition in noise.     

The responses of models of "high-spontaneous" auditory-nerve fibers in a damaged cochlea to speech syllables in noise

The responses of four high-spontaneous fibers from a damaged cat cochlea responding to naturally uttered consonant-vowel (CV) syllables [m], [p], and [t], each with [a], [i], and [u] in four different levels of noise were simulated using a two-stage computer model. At the lowest noise level [+30 dB signal-to-noise (S/N) ratio], the responses of the models of the three fibers from a heavily damaged portion of the cochlea [characteristic frequencies (CFs) from 1.6 to 2.14 kHz] showed quite different response patterns from those of fibers in normal cochleas: There was little response to the noise alone, the consonant portions of the syllables evoked small-amplitude wide-bandwidth complexes, and the vowel-segment response synchrony was often masked by low-frequency components, especially the first formant. At the next level of noise (S/N = 20 dB), spectral information regarding the murmur segments of the [m] syllables was essentially lost. At the highest noise levels used (S/N = +10 and 0 dB), the noise was almost totally disruptive of coding of the spectral peaks of the consonant portions of the stop CVs. Possible implications of the results with regard to the understanding of speech by hearing-impaired listeners are discussed.     

Frequency importance functions for a feature recognition test material

The relative importance of different parts of the auditory spectrum to recognition of the Diagnostic Rhyme Test (DRT) and its six speech feature subtests was determined. Three normal hearing subjects were tested twice in each of 70 experimental conditions. The analytical procedures of French and Steinberg [J. Acoust. Soc. Am. 19, 90-119 (1947)] were applied to the data to derive frequency importance functions for each of the DRT subtests and the test as a whole over the frequency range 178-8912 Hz. For the DRT as a whole, the low frequencies were found to be more important than is the case for nonsense syllables. Importance functions for the feature subtests also differed from those for nonsense syllables and from each other as well. These results suggest that test materials loaded with different proportions of particular phonemes have different frequency importance functions. Comparison of the results with those from other studies suggests that importance functions depend to a degree on the available response options as well.