Harvey Dillon

The national acoustic laboratories (NAL) new procedure for selecting the gain and frequency response of a hearing aid

A new procedure is presented for selecting the gain and frequency response of a hearing aid from pure-tone thresholds. This was developed from research which showed that a previous procedure did not meet its aim of amplifying all frequency bands of speech to equal loudness but that frequency responses which did so were considerably more effective. Measurements of 30 sensorineurally hearing-impaired ears (27 subjects), together with data from other studies, were analyzed to determine the best formula for predicting the optimal frequency response, for each individual, from the audiogram. The analysis indicated that a flat audiogram would require a rising frequency response characteristic of about 8 dB/octave up to 1.25 kHz and thereafter a falling characteristic of about 2 dB/octave. Variations in audiogram slope required about one-third as much variation in response slope. Three frequency average (3FA) gain was calculated to equal the 3FA gain of the previous procedure. Forty-four subjects (67 aided ears) fitted by the new procedure were evaluated by paired comparison judgments of the intelligibility and pleasantness of speech. The prescribed frequency response was seldom inferior to, and usually better than, any of several variations having more, or less, low and/or high-frequency amplification. On the average, used gain was approximately equal to prescribed gain. It is concluded that the new formula should prescribe a near optimal frequency response with few exceptions.

An international comparison of long‐term average speech spectra

The long‐term average speech spectrum (LTASS) and some dynamic characteristics of speech were determined for 12 languages: English (several dialects), Swedish, Danish, German, French (Canadian), Japanese, Cantonese, Mandarin, Russian, Welsh, Singhalese, and Vietnamese. The LTASS only was also measured for Arabic. Speech samples (18) were recorded, using standardized equipment and procedures, in 15 localities for (usually) ten male and ten female talkers. All analyses were conducted at the National Acoustic Laboratories, Sydney. The LTASS was similar for all languages although there were many statistically significant differences. Such differences were small and not always consistent for male and female samples of the same language. For one‐third octave bands of speech, the maximum short‐term rms level was 10 dB above the maximum long‐term rms level, consistent across languages and frequency. A ‘‘universal’’ LTASS is suggested as being applicable, across languages, for many purposes including use in hearing aid prescription procedures and in the Articulation Index.

Speech recognition of hearing-impaired listeners: Predictions from audibility and the limited role of high-frequency amplification

Two experiments were conducted to examine the relationship between audibility and speech recognition for individuals with sensorineural hearing losses ranging from mild to profound degrees. Speech scores measured using filtered sentences were compared to predictions based on the Speech Intelligibility Index (SII). The SII greatly overpredicted performance at high sensation levels, and for many listeners, it underpredicted performance at low sensation levels. To improve predictive accuracy, the SII needed to be modified. Scaling the index by a multiplicative proficiency factor was found to be inappropriate, and alternative modifications were explored. The data were best fitted using a method that combined the standard level distortion factor (which accounted for decrease in speech intelligibility at high presentation levels based on measurements of normal-hearing people) with individual frequency-dependent proficiency. This method was evaluated using broadband sentences and nonsense syllables tests. Results indicate that audibility cannot adequately explain speech recognition of many hearing-impaired listeners. Considerable variations from audibility-based predictions remained, especially for people with severe losses listening at high sensation levels. The data suggest that, contrary to the basis of the SII, information contained in each frequency band is not strictly additive. The data also suggest that for people with severe or profound losses at the high frequencies, amplification should only achieve a low or zero sensation level at this region, contrary to the implications of the unmodified SII.

Optimal Outcome Measures, Research Priorities, and International Cooperation

&NA; The participants in the Eriksholm Workshop on “Measuring Outcomes in Audiological Rehabilitation Using Hearing Aids” debated three issues that are reported in this article. First, it was agreed that the characteristics of an optimal outcome measure vary as a function of the purpose of the measurement. Potential characteristics of outcome self‐report tools for four common goals of outcome measurement are briefly presented to illustrate this point. Second, 10 important research priorities in outcome measurement were identified and ranked. They are presented with brief discussion of the top five. Third, the concept of generating a brief universally applicable outcome measure was endorsed. This brief data set is intended to supplement existing outcome measures and to promote data combination and comparison across different social, cultural, and health‐care delivery systems. A set of seven core items is proposed for further study.

Should children who use cochlear implants wear hearing aids in the opposite ear

Objective The aim of this study was to investigate 1) whether a hearing aid needs to be adjusted differently depending on whether a child wears a cochlear implant or another hearing aid in the contralateral ear; 2) whether the use of a hearing aid and a cochlear implant in opposite ears leads to binaural interference; and 3) whether the use of a hearing aid and a cochlear implant in opposite ears leads to binaural benefits in speech perception, localization, and communicative functioning in real life. Design Sixteen children participated in this study. All children used a Nucleus 22 or Nucleus 24 cochlear implant system programmed with the SPEAK strategy in one ear. The hearing aid amplification requirements in the nonimplanted ear of these children were determined using two procedures. A paired comparison technique was used to identify the frequency response that was best for speech intelligibility in quiet, and a loudness balancing technique was used to match the loudness of speech in the ear with a hearing aid to that with a cochlear implant. Eleven of the 16 children participated in the investigation of binaural effects. Performance in speech perception, localization, and communicative functioning was assessed under four aided conditions: cochlear implant with hearing aid as worn, cochlear implant alone, hearing aid alone, and cochlear implant with hearing aid adjusted according to individual requirements. Results Fifteen of the 16 children whose amplification requirements were determined preferred a hearing aid frequency response that was within ±6 dB/octave of the NAL-RP prescription. On average, the children required 6 dB more gain than prescribed to balance the loudness of the implanted ear for a speech signal presented at 65 dB SPL. For all 11 children whose performance was evaluated for investigating binaural effects, there was no indication of significantly poorer performance under bilaterally aided conditions compared with unilaterally aided conditions. On average, there were significant benefits in speech perception, localization, and aural/oral function when the children used cochlear implants with adjusted hearing aids than when they used cochlear implants alone. All individuals showed benefits in at least one of the measures. Conclusions Hearing aids for children who also use cochlear implants can be selected using the NAL-RP prescription. Adjustment of hearing aid gain to match loudness in the implanted ear can facilitate integration of signals from both ears, leading to better speech perception. Given that there are binaural advantages from using cochlear implants with hearing aids in opposite ears, clinicians should advise parents and other professionals about these potential advantages, and facilitate bilateral amplification by adjusting hearing aids after stable cochlear implant MAPs are established.

Tutorial Compression? Yes, But for Low or High Frequencies, for Low or High Intensities, and with What Response Times?

&NA; Several rationales for using compression in hearing aids are outlined. These rationales comprise discomfort avoidance, loudness normalization, noise reduction, short term signal dynamic range reduction, empirically determined compression, and long‐term signal dynamic range reduction. The compression systems needed to implement each of these differ greatly, and these differences can be viewed as differences in the frequency range undergoing most compression, the intensity range undergoing most compression, and the speed at which the compressor(s) operate. A classification system along these lines is introduced and examples of currently available hearing aids falling into each category are given. The effects of each type of compression on speech intelligibility is investigated via a review of published research. The results of this indicate that, for speech in quiet at a comfortable level, no compression scheme yet tested offers better intelligibility than individually selected linear amplification. If input level is then decreased and the aid wearer is prevented from adjusting the volume control, many types of compression provide intelligibility superior to that available from linear amplification. In broadband noise, only one system, containing wideband compression followed by fast acting high‐frequency compression, has so far been shown to provide significant intelligibility advantages.

Outcomes of early- and late-identified children at 3 years of age: findings from a prospective population-based study.

Objective: To address the question of whether, on a population level, early detection and amplification improve outcomes of children with hearing impairment. Design: All families of children who were born between 2002 and 2007, and who presented for hearing services below 3 years of age at Australian Hearing pediatric centers in New South Wales, Victoria, and Southern Queensland were invited to participate in a prospective study on outcomes. Children’s speech, language, functional, and social outcomes were assessed at 3 years of age, using a battery of age-appropriate tests. Demographic information relating to the child, family, and educational intervention was solicited through the use of custom-designed questionnaires. Audiological data were collected from the national database of Australian Hearing and records held at educational intervention agencies for children. Regression analysis was used to investigate the effects of each of 15 predictor variables, including age of amplification, on outcomes. Results: Four hundred and fifty-one children enrolled in the study, 56% of whom received their first hearing aid fitting before 6 months of age. On the basis of clinical records, 44 children (10%) were diagnosed with auditory neuropathy spectrum disorder. There were 107 children (24%) reported to have additional disabilities. At 3 years of age, 317 children (70%) were hearing aid users and 134 children (30%) used cochlear implants. On the basis of parent reports, about 71% used an aural/oral mode of communication, and about 79% used English as the spoken language at home. Children’s performance scores on standardized tests administered at 3 years of age were used in a factor analysis to derive a global development factor score. On average, the global score of hearing-impaired children was more than 1 SD below the mean of normal-hearing children at the same age. Regression analysis revealed that five factors, including female gender, absence of additional disabilities, less severe hearing loss, higher maternal education, and (for children with cochlear implants) earlier age of switch-on were associated with better outcomes at the 5% significance level. Whereas the effect of age of hearing aid fitting on child outcomes was weak, a younger age at cochlear implant switch-on was significantly associated with better outcomes for children with cochlear implants at 3 years of age. Conclusions: Fifty-six percent of the 451 children were fitted with hearing aids before 6 months of age. At 3 years of age, 134 children used cochlear implants and the remaining children used hearing aids. On average, outcomes were well below population norms. Significant predictors of child outcomes include: presence/absence of additional disabilities, severity of hearing loss, gender, maternal education, together with age of switch-on for children with cochlear implants.

Maximizing effective audibility in hearing aid fitting

Objective This paper examines why more audibility is not always better than less audibility if hearing-impaired people are to best understand speech. Design We used speech perception data from 14 normally hearing and 40 hearing-impaired people to quantify the contribution of audibility to speech intelligibility. The quantification revealed that the effectiveness of audibility decreased with hearing loss, and the decrement was greater at high frequencies than at lower frequencies. To apply the Speech Intelligibility Index (SII) model to predict speech intelligibility for hearing-impaired people, we modified the model to take account of effective audibility rather than physical audibility. Results The modified SII model provided an adequate description of speech performance of people with a wide range of hearing threshold levels. We applied the model to the evaluation of two prescriptions for a sloping audiogram at prescribed levels and at equated loudness levels to demonstrate the necessity of considering loudness and effective audibility in prescribing amplification. Effective audibility is defined as audibility corrected for the effects of level distortion and hearing loss desensitization, and this paper proposes a method of estimating effective audibility from hearing threshold level at different frequencies. Conclusions The practical implication of considering effective audibility in prescribing hearing aids is that for a given listening level, less gain is provided at frequencies where the hearing is most impaired to allow more gain at frequencies where audibility is most useful. In developing the NAL-NL1 prescription for nonlinear hearing aids, we adopted the modified SII model together with a loudness model to derive optimal gain-frequency response characteristics that maximize predicted speech intelligibility for people with different degrees of hearing losses.

The NAL-NL2 prescription procedure

NAL-NL2 is the second generation of prescription procedures from The National Acoustic Laboratories (NAL) for fitting wide dynamic range compression (WDRC) instruments. Like its predecessor NALNL1 (Dillon, 1999), NAL-NL2 aims at making speech intelligible and overall loudness comfortable. This aim is mainly driven by a belief that these factors are most important for hearing aid users, but is also driven by the fact that less information is available about how to adjust gain to optimise other parameters that affect prescription such as localisation, tonal quality, detection of environmental sounds, and naturalness. In both formulas, the objective is achieved by combining a speech intelligibility model and a loudness model in an adaptive computer- controlled optimisation process. Adjustments have further been made to the theoretical component of NAL-NL2 that are directed by empirical data collected during the past decade with NAL-NL1. In this paper, the data underlying NAL-NL2 and the derivation procedure are presented, and the main differences from NAL-NL1 are outlined.

Development of the Listening in Spatialized Noise-Sentences Test (LISN-S)

Objective: The goals of this research were to develop and evaluate a new version of the Listening in Spatialized Noise Test (LISN®; Cameron, Dillon & Newall, 2006a) by incorporating a simplified and more objective response protocol to make the test suitable for assessing the ability of children as young as 5 yr to understand speech in background noise. The LISN-Sentences test (LISN-S; Cameron & Dillon, Reference Note 1) produces a three-dimensional auditory environment under headphones and is presented by using a personal computer. A simple repetition response protocol is used to determine speech reception thresholds (SRTs) for sentences presented in competing speech under various conditions. In four LISN-S conditions, the maskers are manipulated with respect to location (0° versus ±90° azimuth) and vocal quality of the speaker(s) of the stories (same as, or different than, the speaker of the target sentences). Performance is measured as two SRT measures and three “advantage” measures. These advantage measures represent the benefit in decibels gained when either talker, spatial, or both talker and spatial cues combined, are incorporated in the maskers. This use of difference scores minimizes the effects of between-listener variation in factors such as linguistic skills and general cognitive ability on LISN-S performance. Design: An initial experiment was conducted to determine the relative intelligibility of the sentences used in the test. Up to 30 sentences were presented adaptively to 24 children ages 8 to 9 yr to estimate the SRT (eSRT). Fifty sentences each were then presented at each participants eSRT, eSRT +2 dB, and eSRT −2 dB. Psychometric functions were fitted and the sentences were adjusted in amplitude for equal intelligibility. After adjustment, intelligibility increased across sentences by approximately 17% for each 1 dB increase in signal-to-noise ratio (SNR). A second experiment was conducted to gather normative data on the LISN-S from 82 children with normal hearing, ages 5 to 11 yr. Results: For the 82 children in the normative data study, regression analysis showed that there was a strong trend of decreasing SRT and increasing advantage as age increased across all LISN-S performance measures. Analysis of variance revealed that significant differences in performance were most pronounced between the 5-yr-olds and the other age groups on the LISN-S measures that assess the ability to use spatial cues to understand speech in background noise, suggesting that binaural processing skills are still developing at age 5 yr. Inter-participant variation in performance on the various SRT and advantage measures was minimal for all groups, including the 5- and 6-yr-olds who exhibited standard deviations ranging from only 1.0 dB to 1.8 dB across measures. The intra-participant standard error ranged from 0.6 dB to 2.0 dB across age groups and conditions. Total time taken to administer all four LISN-S conditions was on average 12 minutes. Conclusions: The LISN-S provides a quick, objective method of measuring a childs ability to understand speech in background noise. The small degree of inter- and intra-participant variation in the 5- and 6-yr-old children suggests that the test is capable of assessing auditory processing in this age group. However, because there appears to be a strong developmental curve in binaural processing skills in the 5-yr-olds, it is suggested that the LISN-S be used clinically with children from 6 yr of age. Cut-off scores, calculated as 2 standard deviations below the mean adjusted for age, were calculated for each performance measure for children ages 6 to 11 yr. These scores, which represent the level below which performance on the LISN-S is considered to be outside normal limits, will be used to in future studies with children with suspected central auditory processing disorder.