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Title: Factors affecting speech recognition in noise and hearing loss in adults with a wide variety of auditory capabilities
Author: Athalye, Sheetal Purushottam
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2010
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Studies concerning speech recognition in noise constitute a very broad spectrum of work including aspects like the cocktail party effect or observing performance of individuals in different types of speech-signal or noise as well as benefit and improvement with hearing aids. Another important area that has received much attention is investigating the inter-relations among various auditory and non-auditory capabilities affecting speech intelligibility. Those studies have focussed on the relationship between auditory threshold (hearing sensitivity) and a number of suprathreshold abilities like speech recognition in quiet and noise, frequency resolution, temporal resolution and the non-auditory ability of cognition. There is considerable discrepancy regarding the relationship between speech recognition in noise and hearing threshold level. Some studies conclude that speech recognition performance in noise can be predicted solely from an individual’s hearing threshold level while others conclude that other supra-threshold factors such as frequency and/or temporal resolution must also play a role. Hearing loss involves more than deficits in recognising speech in noise, raising the question whether hearing impairment is a uni- or multi-dimensional construct. Moreover, different extents of hearing loss may display different relationships among measures of hearing ability, or different dimensionality. The present thesis attempts to address these three issues, by examining a wide range of hearing abilities in large samples of participants having a range of hearing ability from normal to moderate-severe impairment. The research extends previous work by including larger samples of participants, a wider range of measures of hearing ability and by differentiating among levels of hearing impairment. Method: Two large multi-centre studies were conducted, involving 103 and 128 participants respectively. A large battery of tests was devised and refined prior to the main studies and implemented on a common PC-based platform. The test domains included measurement of hearing sensitivity, speech recognition in quiet and noise, loudness perception, frequency resolution, temporal resolution, binaural hearing and localization, cognition and subjective measures like listening effort and self-report of hearing disability. Performance tests involved presentation of sounds via circum-aural earphones to one or both ears, as required, at intensities matched to individual hearing impairments to ensure audibility. Most tests involved measurements centred on a low frequency (500 Hz), high frequency (3000 Hz) and broadband. The second study included some refinements based on analysis of the first study. Analyses included multiple regression for prediction of speech recognition in stationary or fluctuating noise and factor analysis to explore the dimensionality of the data. Speech recognition performance was also compared with that predicted using the Speech Intelligibility Index (SII). iii Findings: Findings from regression analysis pooled across the two studies showed that speech recognition in noise can be predicted from a combination of hearing threshold at higher frequencies (3000/4000 Hz) and frequency resolution at low frequency (500 Hz). This supports previous studies that conclude that resolution is important in addition to hearing sensitivity. This was also confirmed by the fact that SII (representing sensitivity rather than resolution) underpredicted difficulties observed in hearing-impaired ears for speech recognition in noise. Speech recognition in stationary noise was predicted mainly by auditory threshold while speech recognition in fluctuating noise was predicted by a combination having a larger contribution from frequency resolution. In mild hearing losses (below 40 dB), speech recognition in noise was predicted mainly by hearing threshold, in moderate hearing losses (above 40 dB) it was predicted mainly by frequency resolution when combined for two studies. Thus it can be observed that the importance of auditory resolution (in this case frequency resolution) increases and the importance of the audiogram decreases as the degree of hearing loss increases, provided speech is presented at audible levels. However, for all degrees of hearing impairment included in the study, prediction based solely on hearing thresholds was not much worse than prediction based on a combination of thresholds and frequency resolution. Lastly, hearing impairment was shown to be multi-dimensional; main factors included hearing threshold, speech recognition in stationary and fluctuating noise, frequency and temporal resolution, binaural processing, loudness perception, cognition and self-reported hearing difficulties. A clinical test protocol for defining an individual auditory profile is suggested based on these findings. Conclusions: Speech recognition in noise depends on a combination of audibility of the speech components (hearing threshold) and frequency resolution. Models such as SII that do not include resolution tend to over-predict somewhat speech recognition performance in noise, especially for more severe hearing impairments. However, the over-prediction is not great. It follows that for clinical purposes there is not much to be gained from more complex psychoacoustic characterisation of sensorineural hearing impairment, when the purpose is to predict or explain difficulty understanding speech in noise. A conventional audiogram and possibly measurement of frequency resolution at 500 Hz is sufficient. However, if the purpose is to acquire a detailed individual auditory profile, the multidimensional nature of hearing loss should not be ignored. Findings from the present study show that, along with loss of sensitivity and reduced frequency resolution ability, binaural processing, loudness perception, cognition and self-report measures help to characterize this multi-dimensionality. Detailed studies should hence focus on these multiple dimensions of hearing loss and incorporate measuring a wide variety of different auditory capabilities, rather than inclusion of just a few, in order gain a complete picture of auditory functioning. Frequency resolution at low frequency (500 Hz) as a predictive factor for speech recognition in noise is a new finding. Few previous studies have included low-frequency measures of hearing, which may explain why it has not emerged previously. Yet this finding appears to be robust, as it was consistent across both of the present studies. It may relate to differentiation of vowel components of speech. The present work was unable to confirm the suggestion from previous studies that measures of temporal resolution help to predict speech recognition in fluctuating noise, possibly because few participants had extremely poor temporal resolution ability.
Supervisor: Lutman, Mark Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: RF Otorhinolaryngology