Speech recognition in noise by individuals with mild hearing impairments

The study was designed to test the validity of the American Academy of Ophthalmology and Otolaryngology's (AAOO) 26-dB average hearing threshold level at 500, 1000, and 2000 Hz as a predictor of hearing handicap. To investigate this criterion the performance of a normal-hearing group was compared with that of two groups, categorized according to the AAOO [Trans. Am. Acad. Ophthal. Otolaryng. 63, 236-238 (1959)] guidelines as having no handicap. The latter groups, however, had significant hearing losses in the frequencies above 2000 Hz. Mean hearing threshold levels for 3000, 4000, and 6000 Hz were 54 dB for group II and 63 dB for group III. Two kinds of speech stimuli were presented at an A-weighted sound level of 60 dB in quiet and in three different levels of noise. The resulting speech recognition scores were significantly lower for the hearing-impaired groups than for the normal-hearing group on both kinds of speech materials and in all three noise conditions. Mean scores for group III were significantly lower than those of the normal-hearing group, even in the quiet condition. Speech recognition scores showed significantly better correlation with hearing levels for frequency combinations including frequencies above 2000 Hz than for the 500-, 1000-, and 2000-Hz combination. On the basis of these results the author recommends that the 26-dB fence should be somewhat lower, and that frequencies above 2000 Hz should be included in any scheme for evaluating hearing handicap.     

Energy-independent factors influencing noise-induced hearing loss in the chinchilla model.

The effects on hearing and the sensory cell population of four continuous, non-Gaussian noise exposures each having an A-weighted L(eq)=100 dB SPL were compared to the effects of an energy-equivalent Gaussian noise. The non-Gaussian noise conditions were characterized by the statistical metric, kurtosis (beta), computed on the unfiltered, beta(t), and the filtered, beta(f), time-domain signals. The chinchilla (n=58) was used as the animal model. Hearing thresholds were estimated using auditory-evoked potentials (AEP) recorded from the inferior colliculus and sensory cell populations were obtained from surface preparation histology. Despite equivalent exposure energies, the four non-Gaussian conditions produced considerably greater hearing and sensory cell loss than did the Gaussian condition. The magnitude of this excess trauma produced by the non-Gaussian noise was dependent on the frequency content, but not on the average energy content of the impacts which gave the noise its non-Gaussian character. These results indicate that beta(t) is an appropriate index of the increased hazard of exposure to non-Gaussian noises and that beta(f) may be useful in the prediction of the place-specific additional outer hair cell loss produced by non-Gaussian exposures. The results also suggest that energy-based metrics, while necessary for the prediction of noise-induced hearing loss, are not sufficient.     

The role of magnesium in the susceptibility of soldiers to noise-induced hearing loss

A number of studies have demonstrated that magnesium, administered prophylactically, can reduce the amount of hearing loss resulting from noise exposure. This study explored the possible role of naturally occurring body magnesium concentration in susceptibility of soldiers to noise-induced hearing loss. Participants were 68 adult males who had received extensive noise exposure over several years as a result of training with weapon systems. Each participant provided a pure-tone audiogram, blood sample, and noise-exposure history. A variety of pure-tone indices was correlated with serum magnesium levels as determined by atomic absorption spectroscopy. No significant correlations were observed between any audiometric index and body magnesium. The results of this study, therefore, do not support the hypothesis that there is a strong association between naturally occurring body magnesium and susceptibility to noise-induced hearing loss.     

Identification of vowels in quiet, noise, and reverberation: relationships with age and hearing loss

Vowel identification in quiet, noise, and reverberation was tested with 40 subjects who varied in age and hearing level. Stimuli were 15 English vowels spoken in a (b-t) context in a carrier sentence, which were degraded by reverberation or noise (a babble of 12 voices). Vowel identification scores were correlated with various measures of hearing loss and with age. The mean of four hearing levels at 0.5, 1, 2, and 4 kHz, termed HTL4, produced the highest correlation coefficients in all three listening conditions. The correlation with age was smaller than with HTL4 and significant only for the degraded vowels. Further analyses were performed for subjects assigned to four groups on the basis of the amount of hearing loss. In noise, performance of all four groups was significantly different, whereas, in both quiet and reverberation, only the group with the greatest hearing loss performed differently from the other groups. The relationship among hearing loss, age, and number and type of errors is discussed in light of acoustic cues available for vowel identification.     

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.     

Pure tone audiograms and possible aminoglycoside-induced hearing loss in belugas (Delphinapterus leucas)

A behavioral response paradigm was used to measure pure-tone hearing sensitivities in two belugas (Delphinapterus leucas). Tests were conducted over a 20-month period at the Point Defiance Zoo and Aquarium, in Tacoma, WA. Subjects were two males, aged 8-10 and 9-11 during the course of the study. Subjects were born in an oceanarium and had been housed together for all of their lives. Hearing thresholds were measured using a modified up/down staircase procedure and acoustic response paradigm where subjects were trained to produce audible responses to test tones and to remain quiet otherwise. Test frequencies ranged from approximately 2 to 130 kHz. Best sensitivities ranged from approximately 40 to 50 dB re 1 microPa at 50-80 kHz and 30-35 kHz for the two subjects. Although both subjects possessed traditional "U-shaped" mammalian audiograms, one subject exhibited significant high-frequency hearing loss above 37 kHz compared to previously published data for belugas. Hearing loss in this subject was estimated to approach 90 dB for frequencies above 50 kHz. Similar ages, ancestry, and environmental conditions between subjects, but a history of ototoxic drug administration in only one subject, suggest that the observed hearing loss was a result of the aminoglycoside antibiotic amikacin.     

Sentence reception in noise from one versus two sources: effects of aging and hearing loss

Sentence reception thresholds (SRTs) and babble detection thresholds (BDTs) were measured for signals presented from loudspeakers located at 0 deg and 90 deg azimuth. In addition, the S/B ratios necessary to achieve SRT in the presence of a 12-talker babble were determined under three conditions in which the speech and babble were presented from the same or spatially separated loudspeakers. In the first experiment, normal-hearing subjects were tested on two occasions. The results were highly reliable, with standard errors of the test-retest differences of less than 1dB for all conditions. In the second experiment, SRTs, BDTs, and S/B ratios were measured for three normal-hearing groups (less than or equal to 39, 40-54, and greater than or equal to 55 years) and for a fourth group with presbycusis (greater than or equal to 55 years). Presbycusics had higher SRTs and BDTs than all other groups regardless of age, and the oldest normals had higher thresholds than younger subjects. Presbycusics required higher S/B ratios than any of the normal groups, and the oldest normal group needed a higher S/B ratio compared to the youngest group. All groups demonstrated a significant advantage in S/B ratio when the sentences and babble sources were spatially separated compared to when they were presented from the same loudspeaker. There was no significant difference in the magnitude of advantage due to spatial separation of speech and noise between the normal groups regardless of age, but the presbycusics had significantly smaller advantages than all normals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Perception of dissonance by people with normal hearing and sensorineural hearing loss

The purpose of this study was to determine whether the perceived sensory dissonance of pairs of pure tones (PT dyads) or pairs of harmonic complex tones (HC dyads) is altered due to sensorineural hearing loss. Four normal-hearing (NH) and four hearing-impaired (HI) listeners judged the sensory dissonance of PT dyads geometrically centered at 500 and 2000 Hz, and of HC dyads with fundamental frequencies geometrically centered at 500 Hz. The frequency separation of the members of the dyads varied from 0 Hz to just over an octave. In addition, frequency selectivity was assessed at 500 and 2000 Hz for each listener. Maximum dissonance was perceived at frequency separations smaller than the auditory filter bandwidth for both groups of listners, but maximum dissonance for HI listeners occurred at a greater proportion of their bandwidths at 500 Hz than at 2000 Hz. Further, their auditory filter bandwidths at 500 Hz were significantly wider than those of the NH listeners. For both the PT and HC dyads, curves displaying dissonance as a function of frequency separation were more compressed for the HI listeners, possibly reflecting less contrast between their perceptions of consonance and dissonance compared with the NH listeners.     

Localization in speech mixtures by listeners with hearing loss

The ability of listeners with bilateral sensorineural hearing loss to localize a speech source in a multitalker mixture was measured. Five simultaneous words spoken by different talkers were presented over loudspeakers in a small room, and listeners localized one target word. Errors were significantly larger in this group compared to a control group with normal hearing. Localization of the target presented alone was not different between groups. The results suggest that hearing loss does not impair spatial hearing per se, but degrades the spatial representation of multiple simultaneous sounds.     

Consonant recognition in quiet and in noise with aging among normal hearing listeners

Consonant recognition in quiet and in noise was investigated as a function of age for essentially normal hearing listeners 21-68 years old, using the nonsense syllable test (NST) [Resnick et al., J. Acoust. Soc. Am. Suppl. 1 58, S114 (1975)]. The subjects audited the materials in quiet and at S/N ratios of +10 and +5 dB at their most comfortable listening levels (MCLs). The MCLs approximated conversational speech levels and were not significantly different between the age groups. The effects of age group, S/N condition (quiet, S/N +10, S/N +5) and NST subsets, and the S/N condition X subset interaction were all significant. Interactions involving the age factor were nonsignificant. Confusion matrices were similar across age groups, including the directions of errors between the most frequently confused phonemes. Also, the older subjects experienced performance decrements on the same features that were least accurately recognized by the younger subjects. The findings suggest that essentially normal older persons listening in quiet and in noise experience decreased consonant recognition ability, but that the nature of their phoneme confusions is similar to that of younger individuals. Even though the older subjects met the same selection criteria as did younger ones, there was an expected shift upward in auditory thresholds with age within these limits. Sensitivity at 8000 Hz was correlated with NST scores in noise when controlling for age, but the correlation between performance in noise and age was nonsignificant when controlling for the 8000-Hz threshold. These associations seem to implicate the phenomena underlying the increased 8000-Hz thresholds in the speech recognition problems of the elderly, and appear to support the concept of peripheral auditory deterioration with aging even among those with essentially normal hearing.     

Speech production in noise with and without hearing protection

People working in noisy environments often complain of difficulty communicating when they wear hearing protection. It was hypothesized that part of the workers' communication difficulties stem from changes in speech production that occur when hearing protectors are worn. To address this possibility, overall and one-third-octave-band SPL measurements were obtained for 16 men and 16 women as they produced connected speech while wearing foam, flange, or no earplugs (open ears) in quiet and in pink noise at 60, 70, 80, 90, and 100 dB SPL. The attenuation and the occlusion effect produced by the earplugs were measured. The Speech Intelligibility Index (SII) was also calculated for each condition. The talkers produced lower overall speech levels, speech-to-noise ratios, and SII values, and less high-frequency speech energy, when they wore earplugs compared with the open-ear condition. Small differences in the speech measures between the talkers wearing foam and flange earplugs were observed. Overall, the results of the study indicate that talkers wearing earplugs (and consequently their listeners) are at a disadvantage when communicating in noise.     

Providing low- and mid-frequency speech information to listeners with sensorineural hearing loss.

The present study examined the benefits of providing amplified speech to the low- and mid-frequency regions of listeners with various degrees of sensorineural hearing loss. Nonsense syllables were low-pass filtered at various cutoff frequencies and consonant recognition was measured as the bandwidth of the signal was increased. In addition, error patterns were analyzed to determine the types of speech cues that were, or were not, transmitted to the listeners. For speech frequencies of 2800 Hz and below, a positive benefit of amplified speech was observed in every case, although the benefit provided was very often less than that observed in normal-hearing listeners who received the same increase in speech audibility. There was no dependence of this benefit upon the degree of hearing loss. Error patterns suggested that the primary difficulty that hearing-impaired individuals have in using amplified speech is due to their poor ability to perceive the place of articulation of consonants, followed by a reduced ability to perceive manner information.     

The effects of hearing loss on the contribution of high- and low-frequency speech information to speech understanding. II. Sloping hearing loss

The speech understanding of persons with sloping high-frequency (HF) hearing impairment (HI) was compared to normal hearing (NH) controls and previous research on persons with "flat" losses [Hornsby and Ricketts (2003). J. Acoust. Soc. Am. 113, 1706-1717] to examine how hearing loss configuration affects the contribution of speech information in various frequency regions. Speech understanding was assessed at multiple low- and high-pass filter cutoff frequencies. Crossover frequencies, defined as the cutoff frequencies at which low- and high-pass filtering yielded equivalent performance, were significantly lower for the sloping HI, compared to NH, group suggesting that HF HI limits the utility of HF speech information. Speech intelligibility index calculations suggest this limited utility was not due simply to reduced audibility but also to the negative effects of high presentation levels and a poorer-than-normal use of speech information in the frequency region with the greatest hearing loss (the HF regions). This deficit was comparable, however, to that seen in low-frequency regions of persons with similar HF thresholds and "flat" hearing losses suggesting that sensorineural HI results in a "uniform," rather than frequency-specific, deficit in speech understanding, at least for persons with HF thresholds up to 60-80 dB HL.     

Susceptibility to intraspeech spread of masking in listeners with sensorineural hearing loss

Previous research with speechlike signals has suggested that upward spread of masking from the first formant (F 1) may interfere with the identification of place of articulation information signaled by changes in the upper formants. This suggestion was tested by presenting two-formant stop consonant--vowel syllables varying along a/ba--/da/--/ga/ continuum to hearing-impaired listeners grouped according to etiological basis of the disorder. The syllables were presented monaurally at 80 dB and 100 dB SPL when formant amplitudes were equal and when F 1 amplitude was reduced by 6, 12, and 18 dB. Noise-on-tone masking patterns were also generated using narrow bands of noise at 80 and 100 dB SPL to assess the extent of upward spread of masking. Upward spread of masking could be demonstrated in both speech and nonspeech tasks, irrespective of the subject's age, audiometric configuration, or etiology of hearing impairment. Attenuation of F 1 had different effects on phonetic identification in different subject groups: While listeners with noise-induced hearing loss showed substantial improvement in identifying place of articulation, upward spread of masking did not consistently account for poor place identification in other types of sensorineural hearing impairment.     

The effects of age and cochlear hearing loss on temporal fine structure sensitivity, frequency selectivity, and speech reception in noise

Temporal fine structure (TFS) sensitivity, frequency selectivity, and speech reception in noise were measured for young normal-hearing (NHY), old normal-hearing (NHO), and hearing-impaired (HI) subjects. Two measures of TFS sensitivity were used: the "TFS-LF test" (interaural phase difference discrimination) and the "TFS2 test" (discrimination of harmonic and frequency-shifted tones). These measures were not significantly correlated with frequency selectivity (after partialing out the effect of audiometric threshold), suggesting that insensitivity to TFS cannot be wholly explained by a broadening of auditory filters. The results of the two tests of TFS sensitivity were significantly but modestly correlated, suggesting that performance of the tests may be partly influenced by different factors. The NHO group performed significantly more poorly than the NHY group for both measures of TFS sensitivity, but not frequency selectivity, suggesting that TFS sensitivity declines with age in the absence of elevated audiometric thresholds or broadened auditory filters. When the effect of mean audiometric threshold was partialed out, speech reception thresholds in modulated noise were correlated with TFS2 scores, but not measures of frequency selectivity or TFS-LF test scores, suggesting that a reduction in sensitivity to TFS can partly account for the speech perception difficulties experienced by hearing-impaired subjects.     

Forward- and simultaneous-masked thresholds in bandlimited maskers in subjects with normal hearing and cochlear hearing loss

Forward- and simultaneous-masked thresholds were measured at 0.5 and 2.0 kHz in bandpass maskers as a function of masker bandwidth and in a broadband masker with the goal of estimating psychophysical suppression. Suppression was operationally defined in two ways: (1) as a change in forward-masked threshold as a function of masker bandwidth, and (2) as a change in effective masker level with increased masker bandwidth, taking into account the nonlinear growth of forward masking. Subjects were younger adults with normal hearing and older adults with cochlear hearing loss. Thresholds decreased as a function of masker bandwidth in forward masking, which was attributed to effects of suppression; thresholds remained constant or increased slightly with increasing masker bandwidth in simultaneous masking. For subjects with normal hearing, slightly larger estimates of suppression were obtained at 2.0 kHz rather than at 0.5 kHz. For hearing-impaired subjects, suppression was reduced in regions of hearing loss. The magnitude of suppression was strongly correlated with the absolute threshold at the signal frequency, but did not vary with thresholds at frequencies remote from the signal. The results suggest that measuring forward-masked thresholds in bandlimited and broadband maskers may be an efficient psychophysical method for estimating suppression.     

Psychometric functions for discrimination of two-component complex tones in listeners with normal hearing and listeners with hearing loss

This study compared the ability of 5 listeners with normal hearing and 12 listeners with moderate to moderately severe sensorineural hearing loss to discriminate complementary two-component complex tones (TCCTs). The TCCTs consist of two pure tone components (f1 and f2) which differ in frequency by delta f (Hz) and in level by delta L (dB). In one of the complementary tones, the level of the component f1 is greater than the level of component f2 by the increment delta L; in the other tone, the level of component f2 exceeds that of component f1 by delta L. Five stimulus conditions were included in this study: fc = 1000 Hz, delta L = 3 dB; fc = 1000 Hz, delta L = 1 dB; fc = 2000 Hz, delta L = 3 dB; fc = 2000 Hz, delta L = 1 dB; and fc = 4000 Hz, delta L = 3 dB. In listeners with normal hearing, discrimination of complementary TCCTs (with a fixed delta L and a variable delta f) is described by an inverted U-shaped psychometric function in which discrimination improves as delta f increases, is (nearly) perfect for a range of delta f's, and then decreases again as delta f increases. In contrast, group psychometric functions for listeners with hearing loss are shifted to the right such that above chance performance occurs at larger values of delta f than in listeners with normal hearing. Group psychometric functions for listeners with hearing loss do not show a decrease in performance at the largest values of delta f included in this study. Decreased TCCT discrimination is evident when listeners with hearing loss are compared to listeners with normal hearing at both equal SPLs and at equal sensation levels. In both groups of listeners, TCCT discrimination is significantly worse at high center frequencies. Results from normal-hearing listeners are generally consistent with a temporal model of TCCT discrimination. Listeners with hearing loss may have deficits in using phase locking in the TCCT discrimination task and so may rely more on place cues in TCCT discrimination.     

Speech intelligibility of normal listeners and persons with impaired hearing in traffic noise

Speech intelligibility (PB words) in traffic-like noise was investigated in a laboratory situation simulating three common listening situations, indoors at 1 and 4 m and outdoors at 1 m. The maximum noise levels still permitting 75% intelligibility of PB words in these three listening situations were also defined. A total of 269 persons were examined. Forty-six had normal hearing, 90 a presbycusis-type hearing loss, 95 a noise-induced hearing loss and 38 a conductive hearing loss. In the indoor situation the majority of the groups with impaired hearing retained good speech intelligibility in 40 dB(A) masking noise. Lowering the noise level to less than 40 dB(A) resulted in a minor, usually insignificant, improvement in speech intelligibility. Listeners with normal hearing maintained good speech intelligibility in the outdoor listening situation at noise levels up to 60 dB(A), without lip-reading (i.e., using non-auditory information). For groups with impaired hearing due to age and/or noise, representing 8% of the population in Sweden, the noise level outdoors had to be lowered to less than 50 dB(A), in order to achieve good speech intelligibility at 1 m without lip-reading.