Interactions between Cognition and Hearing Aid Compression Release Time: Effects of Linguistic Context of Speech Test Materials on Speech-in-Noise Performance

Recent research has established a connection between hearing aid (HA) users’ cognition and speech recognition performance with short and long compression release times (RT). Contradictive findings prevent researchers from using cognition to predict RT prescription. We hypothesized that the linguistic context of speech recognition test materials was one of the factors that accounted for the inconsistency. The present study was designed to examine the relationship between HA users’ cognition and their aided speech recognition performance with short and long RTs using materials with various linguistic contexts. Thirty-four older HA users’ cognitive abilities were quantified using a reading span test. They were fitted with behind-the-ear style HAs with adjustable RT settings. Three speech recognition tests were used: the word-in-noise (WIN) test, the American four alternative auditory feature (AFAAF) test, and the Bamford-Kowal-Bench speech-in-noise (BKB-SIN) test. The results showed that HA users with high cognitive abilities performed better on the AFAAF and the BKB-SIN than those with low cognitive abilities when using short RT. None of the speech recognition tests produced significantly different performance between the two RTs for either cognitive group. These findings did not support our hypothesis. The results suggest that cognition might not be important in prescribing RT.

Application of Data Mining to a Large Hearing-Aid Manufacturer's Dataset to Identify Possible Benefits for Clinicians, Manufacturers, and Users

Modern hearing instruments contain logging technology to record data, such as the acoustic environments in which the device is being used and how the signal processing is consequently operating. Combined with patient data, such as the audiogram, this information gives a more comprehensive picture of the user and their relationship with the aid. Here, a relatively large, anonymized dataset (>300,000 devices, >150,000 wearers) from a hearing-aid manufacturer was data mined for connections between subsets of the logged varieties of data. Apart from replicating links that have previously been reported in labor-intensive studies, a link between device style (in-the-ear/behind-the-ear) and the sound levels of encountered environments was demonstrated, suggesting that some device types are more successful from a lifestyle perspective. Furthermore, the data also suggested links between the audiogram and the sound environments in which the aid was operated. Modeling the expected link between the environment and the microphone directionality settings revealed patterns of either abnormal fitting or where the aid was not operating correctly-factors that may indicate a failed fitting. Given the necessarily redacted nature of the dataset, the reported findings represent a proof-of-concept of the use of relatively large-scale data mining to guide and assess hearing-aid fitting procedures for possible benefits to the clinician, manufacturer, and patient.

Linear and nonlinear hearing aid fittings – 1. Patterns of benefit

We evaluated the benefits of fast-acting WDRC, slow-acting AVC, and linear reference fittings for speech intelligibility and reported disability, in a within-subject within-device masked crossover design on 50 listeners with SNHL. Five hearing aid fittings were implemented having two compression channels and seven frequency bands. Each listener sequentially experienced each fitting for a 10-week period. Outcome measures included speech intelligibility under diverse conditions and self-reported disability. At a group level, each nonlinear fitting was superior to the linear references for benefits in listening comfort, listener satisfaction, reported intelligibility and speech intelligibility. Slow-acting AVC outperformed the fast-acting WDRC fittings for listening comfort, while for reported and measured speech intelligibility the converse was true. For listener satisfaction there were no group differences between the nonlinear fittings. Analysis in terms of fittings for individual listeners revealed subsets with definite divergences from the group data and hence a need for candidature criteria. There are systematic differences between the benefits of nonlinear and linear fittings, and also within nonlinear fittings with fast versus slow time constants. The patterns of benefit and individual optima depend on the domain of outcome being assessed.

The effects of hearing aid compression parameters on the short-term dynamic range of continuous speech

PURPOSE

The purpose of this study was to evaluate and quantitatively model the independent and interactive effects of compression ratio, number of compression channels, and release time on the dynamic range of continuous speech.

METHOD

A CD of the Rainbow Passage (J. E. Bernthal & N. W. Bankson, 1993) was used. The hearing aid was a programmable, digital, wide dynamic range compression instrument. A fully crossed design and multiple regression analyses were used to evaluate and model the effects of release time (32, 128, and 1024 ms), compression ratio (1:1, 2:1, and 4:1), and number of compression channels (1, 2, and 4 channels) on the short-term octave-band dynamic range of speech. Dynamic range of speech was defined as the range between the 1% and 70% exceedance levels within each octave band.

RESULTS

As the compression ratio and number of channels increased, and as the release time decreased, the dynamic range of speech decreased. The effects of channels and release time increased as the compression ratio increased. In all conditions, the amount of effective compression for speech was less than the nominal compression ratio.

CONCLUSION

A multiple regression model is provided that predicts the effects of various combinations of compression parameters on the dynamic range of speech.

Temporal envelope changes of compression and speech rate: combined effects on recognition for older adults

PURPOSE

When understanding speech in complex listening situations, older adults with hearing loss face the double challenge of cochlear hearing loss and deficits of the aging auditory system. Wide-dynamic range compression (WDRC) is used in hearing aids as remediation for the loss of audibility associated with hearing loss. WDRC processing has the additional effect of altering the acoustics of the speech signal, particularly the temporal envelope. Older listeners are negatively affected by other types of temporal distortions, but this has not been found for the distortion of WDRC processing for simple signals. The purpose of this research was to determine the circumstances under which older adults might be negatively affected by WDRC processing and what compensatory mechanisms those listeners might be using for the listening conditions when speech recognition performance is not affected.

METHOD

Two groups of adults with mild to moderate hearing loss were tested: (a) young-old (62-74 years, n=11) and (b) old-old (75-88 years, n=14). The groups did not differ in hearing loss, cognition, working memory, or self-reported health status. Participants heard low-predictability sentences compressed at each of 4 compression settings. The effect of compression on the temporal envelope was quantified by the envelope difference index (EDI; T. W. Fortune, B. D. Woodruff, & D. A. Preves, 1994). The sentences were presented at three rates: (a) normal rate, (b) 50% time compressed, and (c) time restored.

RESULTS

There was no difference in performance between age groups, or any interactions involving age. There was a significant interaction between speech rate and EDI value; as the EDI value increased, representing higher amounts of temporal envelope distortion, speech recognition was significantly reduced. At the highest EDI value, this reduction was greater for the time-compressed than the normal rate condition. When time was restored to the time-compressed signals, speech recognition did not improve.

CONCLUSION

Temporal envelope changes were detrimental to recognition of low-context speech for older listeners once a certain threshold of distortion was reached, particularly for rapid rate speech. For this sample tested, the effect was not age related within the age range tested here. The results of the time-restored condition suggested that listeners were using acoustic redundancy to compensate for the negative effects of WDRC distortion in the normal rate condition.

Quantifying the effect of compression hearing aid release time on speech acoustics and intelligibility

Compression hearing aids have the inherent, and often adjustable, feature of release time from compression. Research to date does not provide a consensus on how to choose or set release time. The current study had 2 purposes: (a) a comprehensive evaluation of the acoustic effects of release time for a single-channel compression system in quiet and (b) an evaluation of the relation between the acoustic changes and speech recognition. The release times under study were 12, 100, and 800 ms. All of the stimuli were VC syllables from the Nonsense Syllable Task spoken by a female talker. The stimuli were processed through a hearing aid simulator at 3 input levels. Two acoustic measures were made on individual syllables: the envelope-difference index and CV ratio. These measurements allowed for quantification of the short-term amplitude characteristics of the speech signal and the changes to these amplitude characteristics caused by compression. The acoustic analyses revealed statistically significant effects among the 3 release times. The size of the effect was dependent on characteristics of the phoneme. Twelve listeners with moderate sensorineural hearing loss were tested for their speech recognition for the same stimuli. Although release time for this single-channel, 3:1 compression ratio system did not directly predict overall intelligibility for these nonsense syllables in quiet, the acoustic measurements reflecting the changes due to release time were significant predictors of phoneme recognition. Increased temporal-envelope distortion was predictive of reduced recognition for some individual phonemes, which is consistent with previous research on the importance of relative amplitude as a cue to syllable recognition for some phonemes.

Effects of compression on speech acoustics, intelligibility, and sound quality

The topic of compression has been discussed quite extensively in the last 20 years (eg, Braida et al., 1982; Dillon, 1996, 2000; Dreschler, 1992; Hickson, 1994; Kuk, 2000 and 2002; Kuk and Ludvigsen, 1999; Moore, 1990; Van Tasell, 1993; Venema, 2000; Verschuure et al., 1996; Walker and Dillon, 1982). However, the latest comprehensive update by this journal was published in 1996 (Kuk, 1996). Since that time, use of compression hearing aids has increased dramatically, from half of hearing aids dispensed only 5 years ago to four out of five hearing aids dispensed today (Strom, 2002b). Most of today's digital and digitally programmable hearing aids are compression devices (Strom, 2002a). It is probable that within a few years, very few patients will be fit with linear hearing aids. Furthermore, compression has increased in complexity, with greater numbers of parameters under the clinician's control. Ideally, these changes will translate to greater flexibility and precision in fitting and selection. However, they also increase the need for information about the effects of compression amplification on speech perception and speech quality. As evidenced by the large number of sessions at professional conferences on fitting compression hearing aids, clinicians continue to have questions about compression technology and when and how it should be used. How does compression work? Who are the best candidates for this technology? How should adjustable parameters be set to provide optimal speech recognition? What effect will compression have on speech quality? These and other questions continue to drive our interest in this technology. This article reviews the effects of compression on the speech signal and the implications for speech intelligibility, quality, and design of clinical procedures.

Comparison of hearing aids over the 20th century

OBJECTIVE

To measure hearing aid performance using circuitry representative of the major eras of technological advancement during the 20th century.

DESIGN

Twenty subjects with audiometric profiles consistent with hearing aid candidacy were fit with each of seven hearing aids. No directional microphones were used and binaural benefit was not assessed. Each hearing aid was fit to the strategy or fitting scheme of the era, or that which was intended by the presenting manufacturer. Electroacoustic and/or real ear measures of gain, output, bandwidth, and distortion were obtained. Objective outcome measures assessing speech perception in backgrounds of noise were obtained. Subjective outcome measures of sound quality and ease of listening were obtained in the laboratory and in real life settings.

RESULTS

Electroacoustic and real ear measures indicate that gain and bandwidth have increased, and output and distortion have decreased with current electronic aids. Speech perception ability across the different outcome measures showed significantly poorer performance with the body and linear hearing aids when input levels were high; when input levels were low, outcome measures with hearing aids using a dynamic range compression were not negatively affected. At the most adverse signal to noise ratios, none of the hearing aids was shown to be superior. Measured bandwidth did not correlate highly with speech perception ability for any of the objective outcome measures used. For the subjective measures of sound quality done in a blinded manner, no significant differences were found across different listening situations for current hearing aids.

CONCLUSIONS

The two most important factors for aided speech perception appear to be the audibility and distortion of the signal. No current compression scheme proved superior with the outcome measures used in this investigation.

Accuracy of predicted ear canal speech levels using the VIOLA input/output-based fitting strategy

OBJECTIVE

The Visual Input/Output Locator Algorithm (VIOLA) is a software-assisted method for prescribing amplification targets and selecting a hearing aid to match the targets. Although the procedure calls for selection and fitting of hearing aids in terms of their pure-tone input/output functions in a coupler, it is assumed that a hearing aid that matches the coupler prescription targets will produce specific amplified speech levels in the patient's ear canal. This investigation evaluated the validity of that assumption.

DESIGN

Six hearing aids were evaluated. They were representative of linear and compression processing as well as single- and 2-channel designs. The "subject" was a KEMAR manikin with realistic assumed hearing loss and loudness perception characteristics. Each hearing aid was configured to match the subject's VIOLA prescription as closely as possible. Predicted ear canal speech levels were determined using the prescription rules and modified by the differences between coupler prescription targets and coupler performance of the actual hearing aids. With the subject wearing each hearing aid coupled to an unvented earmold, continuous speech was presented in the sound field and measured, after amplification, in the ear canal. The match between observed and predicted levels of amplified speech indicated the validity of the VIOLA assumptions under examination.

RESULTS

The match between predicted and observed levels was good for soft speech input levels. As speech input levels increased, the differences between observed and predicted levels also increased, with the largest differences seen for loud speech inputs. When differences were seen between observed and predicted levels, they were always in the direction of lower than predicted ear canal levels. The differences between observed and predicted levels were attributed to the effects of limiting, effects of compression ratio in wide range compression, the individual subject's field-to-microphone transfer function, and the subject's individual real-ear-to-coupler level difference.

CONCLUSIONS

Ear canal speech levels were reasonably close to predicted values, and the deviations from predicted levels were plausibly accounted for by consideration of hearing aid performance. Thus, the approach used by the VIOLA procedure holds considerable promise for extending clinical control over the complex and interactive parameters of nonlinear hearing aids. The results of this study indicate that selection and fitting of hearing aids using the current VIOLA procedure usually will result in the generation of lower than predicted ear canal speech levels, especially for loud speech inputs. However, the accuracy of the procedure could be improved substantially by modification of the software to account for the effects of limiting and those of the compression ratio in systems with compression thresholds lower than the level of unamplified loud speech.