Lyndal Carter

The effect of multi-channel wide dynamic range compression, noise reduction, and the directional microphone on horizontal localization performance in hearing aid wearers

This study examined the effect that signal processing strategies used in modern hearing aids, such as multi-channel WDRC, noise reduction, and directional microphones have on interaural difference cues and horizontal localization performance relative to linear, time-invariant amplification. Twelve participants were bilaterally fitted with BTE devices. Horizontal localization testing using a 360° loudspeaker array and broadband pulsed pink noise was performed two weeks, and two months, post-fitting. The effect of noise reduction was measured with a constant noise present at 80° azimuth. Data were analysed independently in the left/right and front/back dimension and showed that of the three signal processing strategies, directional microphones had the most significant effect on horizontal localization performance and over time. Specifically, a cardioid microphone could decrease front/back errors over time, whereas left/right errors increased when different microphones were fitted to left and right ears. Front/back confusions were generally prominent. Objective measurements of interaural differences on KEMAR explained significant shifts in left/right errors. In conclusion, there is scope for improving the sense of localization in hearing aid users.

NAL-NL2 Empirical Adjustments

NAL-NL1, the first procedure from the National Acoustic Laboratories (NAL) for prescribing nonlinear gain, was a purely theoretically derived formula aimed at maximizing speech intelligibility for any input level of speech while keeping the overall loudness of speech at or below normal loudness. The formula was obtained through an optimization process in which speech intelligibility and loudness were predicted from selected models. Using updated models and applying some revisions to the derivation process, a theoretically derived NAL-NL2 formula was obtained in a similar way. Further adjustments, directed by empirical data collected in studies using NAL-NL1 as the baseline response, have been made to the theoretically derived formula. Specifically, empirical data have demonstrated that (a) female hearing aid users prefer lower overall gain than male users; (b) new hearing aid users with more than a mild hearing loss prefer increasingly less gain with increasing degree of hearing loss than experienced hearing aid users, and require up to 2 years to adapt to gain levels selected by experienced hearing aid users; (c) unilaterally and bilaterally fitted hearing aid users prefer overall gain levels that vary less than estimated by the bilateral correction factor; (d) adults prefer lower overall gain than children; and (e) people with severe/profound hearing loss prefer lower compression ratios than predicted when fitted with fast-acting compression. The literature and data leading to these conclusions are summarized and discussed in this article, and the procedure for implementing the adjustments to the theoretically derived NAL-NL2 formula is described.

The relationship between cortical auditory evoked potential (CAEP) detection and estimated audibility in infants with sensorineural hearing loss

Abstract Objective: To determine the effectiveness of objective statistical detection in CAEP testing to evaluate audibility in young infants with sensorineural hearing loss. Design: CAEP recordings to speech-based stimuli were made at three presentation levels (55, 65, or 75 dB SPL) when a group of hearing-impaired infants were either aided or unaided. Later-obtained behavioral audiograms were used as the gold standard against which to evaluate the accuracy of the automatic detection of the presence/absence of CAEP responses. Study sample: Participants were 18 infants with confirmed sensorineural hearing loss. Results: Higher sensation levels led to a greater number of present CAEP responses being detected. More CAEP waveforms were detected in the aided condition than in the unaided condition. Conclusion: Our results suggest that the presence/absence of CAEP responses defined by the automatic statistical criterion was effective in showing whether increased sensation levels provided by amplification were sufficient to reach the cortex. This was clearly apparent from the significant increase in cortical detections when comparing unaided with aided testing.

The detection of adult cortical auditory evoked potentials (CAEPs) using an automated statistic and visual detection

Abstract The detection of adult cortical auditory evoked potentials (CAEPs) can be challenging when the stimulus is just audible. The effectiveness of a statistic compared with expert examiners in (1) detecting the presence of CAEPs when stimuli were present, and (2) reporting the absence of CAEPs when no stimuli were present, was investigated. CAEPs recorded from ten adults, using two speech-based stimuli, five stimulus presentation levels, and non-stimulus conditions, were given to four experienced examiners who were asked to determine if responses to auditory stimulation could be observed, and their degree of certainty in making their decision. These recordings were also converted to multiple dependent variables and Hotellings T2 was applied to calculate the probability that the mean value of any linear combination of these variables was significantly different from zero. Results showed that Hotellings T2 was equally sensitive to the best of individual experienced examiners in differentiating a CAEP from random noise. It is reasonable to assume that the difference in response detection for a novice examiner and Hotellings T2 would be even greater. Sumario La detección de los potenciales evocados auditivos corticales (CAEP) del adulto puede ser un desafío cuando el estímulo es apenas audible. Se investigó la efectividad de una comparación estadística con examinadores expertos para (1) detectar la presencia del CAEP cuando el estimulo estuvo presente, y (2) reportar la ausencia del CAEP cuando ningún estímulo estaba presente. Los CAEP registrados para diez adultos, usando dos estímulos con base en lenguaje, cinco niveles de presentación del estímulo, y condiciones sin estímulo, fueron presentados a cuatro examinadores con experiencia, a quienes se solicitó que determinaran si las respuestas a estímulos auditivos podían ser observadas y su grado de certeza en la toma de sus decisiones. Estos registros fueron también convertidos a múltiples variables dependientes y se aplicó el T2 de Hotelling para calcular la probabilidad de que el valor medio de una combinación lineal de estas variables fuera significativamente diferente de cero. Los resultados mostraron que el T2 de Hotelling era igualmente sensible que lo mejor de los examinadores experimentados individuales para diferenciar un CAEP de un ruido aleatorio. Es razonable asumir que la diferencia en detección de respuestas para un examinador novato y el T2 de Hotelling serían aún mayor.

The leisure-noise dilemma: hearing loss or hearsay? What does the literature tell us?

The authors undertook a review of the literature, focussing on publications describing the following: (1) Pure tone threshold data for adolescents/young adults; (2) Measurements/estimates of noise exposure from leisure activities; and (3) The relationship between hearing threshold levels (HTLs) and leisure-noise exposure. There is a large volume of published materials relevant to these topics, and opinion among authors regarding the relationship between leisure-noise exposure and HTLs varies significantly. At one extreme is the view that the effects of leisure-noise are minimal. The opposing belief is that as a direct result of leisure-noise exposure, significant HTL shifts and possibly significant hearing disability are occurring in a large (and increasing) proportion of young people. It has been claimed that behaviors relating to leisure-noise are “as threatening to young people’s health as more traditional risk behaviors” (Bohlin & Erlandsson 2007, p. 55). This view has been reiterated by the popular media. This review revealed that while sufficient data confirm that some leisure pursuits provide potentially hazardous noise levels, the nature of the exposure–injury relationship for leisure-noise is yet to be determined. Specific information about the quality-of-life effects of threshold shift related to leisure-noise exposure is also lacking. The scope and limitations of a large sample of relevant publications and an overview of the methodological issues in this area of research are briefly presented. Considerations for future research are raised.

Sensitivity of cortical auditory evoked potential detection for hearing-impaired infants in response to short speech sounds

Cortical auditory evoked potentials (CAEPs) are an emerging tool for hearing aid fitting evaluation in young children who cannot provide reliable behavioral feedback. It is therefore useful to determine the relationship between the sensation level of speech sounds and the detection sensitivity of CAEPs, which is the ratio between the number of detections and the sum of detections and non-detections. Twenty-five sensorineurally hearing impaired infants with an age range of 8 to 30 months were tested once, 18 aided and 7 unaided. First, behavioral thresholds of speech stimuli /m/, /g/, and /t/ were determined using visual reinforcement orientation audiometry. Afterwards, the same speech stimuli were presented at 55, 65, and 75 dB sound pressure level, and CAEPs were recorded. An automatic statistical detection paradigm was used for CAEP detection. For sensation levels above 0, 10, and 20 dB respectively, detection sensitivities were equal to 72±10, 75±10, and 78±12%. In 79% of the cases, automatic detection P-values became smaller when the sensation level was increased by 10 dB. The results of this study suggest that the presence or absence of CAEPs can provide some indication of the audibility of a speech sound for infants with sensorineural hearing loss. The detection of a CAEP might provide confidence, to a degree commensurate with the detection probability, that the infant is detecting that sound at the level presented. When testing infants where the audibility of speech sounds has not been established behaviorally, the lack of a cortical response indicates the possibility, but by no means a certainty, that the sensation level is 10 dB or less.

Variation in preferred gain with experience for hearing-aid users

This study aimed to determine whether gain adaptation occurs, and at which frequency bands, among new hearing aid (HA) users. Fifty new and 26 experienced HA users were fitted with three listening programs (NAL-NL1 and NAL-NL1 with low- and high-frequency cuts) in the same hearing instrument family. Real-life gain preferences and comfortable loudness levels were measured one, four, and 13 months post-fitting for the new HA users, and one month post-fitting for the experienced HA users. Relative to experienced HA users, new HA users preferred progressively less overall gain than prescribed as the hearing loss became more severe. Gain adaptation occurred in new HA users with greater hearing loss, but was not complete 13 months post-fitting, and was not explained by changes in loudness perception. Preferences for a high-frequency gain cut by half of all study participants could not be predicted from audiological data. Gain adaptation management is recommended for new HA users with more than a mild hearing loss.

Spatial release from masking in normal-hearing children and children who use hearing aids.

Listening to speech in competing sounds poses a major difficulty for children with impaired hearing. This study aimed to determine the ability of children (3-12 yr of age) to use spatial separation between target speech and competing babble to improve speech intelligibility. Fifty-eight children (31 with normal hearing and 27 with impaired hearing who use bilateral hearing aids) were assessed by word and sentence material. Speech reception thresholds (SRTs) were measured with speech presented from 0° azimuth, and competing babble from either 0° or ±90° azimuth. Spatial release from masking (SRM) was defined as the difference between SRTs measured with co-located speech and babble and SRTs measured with spatially separated speech and babble. On average, hearing-impaired children attained near-normal performance when speech and babble originated from the frontal source, but performed poorer than their normal-hearing peers when babble was spatially separated from target speech. On average, normal-hearing children obtained an SRM of 3 dB whereas children with hearing loss did not demonstrate SRM. Results suggest that hearing-impaired children may need enhancement in signal-to-noise ratio to hear speech in difficult listening conditions as well as normal-hearing children.

Threshold measurements by self-fitting hearing aids: feasibility and challenges.

A self-fitting, self-contained hearing aid is a device that can be managed entirely by the user, without assistance from a hearing health care professional or the need for special equipment. A key component of such a device is an automated audiometer that will enable the user to self-administer measurements of in situ thresholds, which the hearing aid will use to prescribe a baseline setting for the wearer. The success of the device therefore depends on the validity and reliability of in situ threshold measurements and automatically measured thresholds. To produce a complete and self-contained device, the self-fitting hearing aid will also enable identification of audiograms that are contraindicative of hearing aid usage. The feasibility and challenges of achieving these characteristics are explored and discussed. While the overall concept seems feasible, several challenges were identified that need thorough investigation and/or development. These include the use of instructions to self-manage hearing aid insertion and in situ threshold measurements, selection of an appropriate transducer and instant-fit tip that will allow measurements of a wide range of threshold levels, control of ambient noise during threshold measurements, and self-manageable procedures that enable identification of such audiogram characteristics as asymmetry and conductive hearing loss.

Hearing threshold levels for a population of 11 to 35 year old Australian females and males

Abstract Objective: Detailed information on the hearing threshold levels (HTLs) of young Australians was gathered as part of a large-scale study of the relationship between HTL and leisure-noise exposure in young Australians. Design: HTL data for the study population (18–35 year olds) was carefully collected, as well as otoscopy, tympanometry, contra-lateral acoustic reflexes, and otoacoustic emissions (transient and distortion product), together with a comprehensive hearing health history – both past and present. Study sample: The sample cohort consisted of 1407 individuals, females and males. Results: Prior to analysis, HTL data were filtered according to both a ‘Low’ and ‘High’ set of exclusion criteria. The results obtained for both high-screen and low-screen datasets were around +5 dB above the traditionally accepted values of audiometric zero. This is consistent with previous published reports. Conclusion: Comparison with ‘ISO 7029 Acoustics: Statistical distribution of hearing thresholds as a function of age’ indicated that threshold values for this dataset have a similar distribution to those of the Standard. This data provides a suitable reference HTL (‘normative’) database for young Australians.