Preferred Strength of Noise Reduction for Normally Hearing and Hearing-Impaired Listeners.

Preference for noise reduction (NR) strength differs between individuals. The purpose of this study was (1) to investigate whether hearing loss influences this preference, (2) to find the number of distinct settings required to classify participants in similar groups based on their preference for NR strength, and (3) to estimate the number of paired comparisons needed to predict to which preference group a participant belongs. A paired comparison paradigm was used in which participants listened to pairs of speech-in-noise stimuli processed by NR with 10 different strength settings. Participants indicated their preferred sound sample. The 30 participants were divided into three groups according to hearing status (normal hearing, mild hearing loss, and moderate hearing loss). The results showed that (1) participants with moderate hearing loss preferred stronger NR than participants with normal hearing; (2) cluster analysis based solely on the preference for NR strength showed that the data could be described well by dividing the participants into three preference clusters; (3) the appropriate cluster membership could be found with 15 paired comparisons. We conclude that on average, a higher hearing loss is related to a preference for stronger NR, at least for our NR algorithm and our participants. The results show that it might be possible to use a limited set of pre-set NR strengths that can be chosen clinically. For our NR one might use three settings: no NR, intermediate NR, and strong NR. Paired comparisons might be used to find the optimal one of the three settings.

Individual Listener Preference for Strength of Single-Microphone Noise-Reduction; Trade-off Between Noise Tolerance and Signal Distortion Tolerance.

There are large interindividual variations in preference for strength of noise-reduction (NR). It is hypothesized that each individual's tolerance for background noise on one hand and signal distortions on the other hand form this preference. We aim to visualize and analyze this so-called trade-off between noise attenuation and signal quality. Dutch sentences in stationary background noise were processed with different NR strengths. We used an NR algorithm that allows us to separate the positive effects of NR (noise attenuation) from the negative effects (signal distortion). Stimuli consisted of speech in noise with different degrees of (1) background noise, (2) signal distortions, or (3) both (i.e., realistic NR at different NR strengths). With pairwise comparisons, participants chose which stimulus they preferred for prolonged listening. Twelve listeners with mild to moderate hearing loss participated in the study. For all participants, a trade-off between noise attenuation and signal quality was found and visualized. The strength of preference was estimated with the Bradley-Terry-Luce choice model and was different for all individuals but in the same order of magnitude for distortion effects and noise effects. Strength of preference of realistic NR was smaller by a factor of ten. This study used a unique setup to capture the individual trade-off between noise attenuation and signal quality in NR. Disturbance from signal distortions is as important as disturbance from background noise for determining preference for NR strength. Individual listeners differ in their sensitivity to both factors and as a consequence in their preferred NR strength.

Measuring the Influence of Noise Reduction on Listening Effort in Hearing-Impaired Listeners Using Response Times to an Arithmetic Task in Noise.

Single microphone noise reduction (NR) in hearing aids can provide a subjective benefit even when there is no objective improvement in speech intelligibility. A possible explanation lies in a reduction of listening effort. Previously, we showed that response times (a proxy for listening effort) to an auditory-only dual-task were reduced by NR in normal-hearing (NH) listeners. In this study, we investigate if the results from NH listeners extend to the hearing-impaired (HI), the target group for hearing aids. In addition, we assess the relevance of the outcome measure for studying and understanding listening effort. Twelve HI subjects were asked to sum two digits of a digit triplet in noise. We measured response times to this task, as well as subjective listening effort and speech intelligibility. Stimuli were presented at three signal-to-noise ratios (SNR; -5, 0, +5 dB) and in quiet. Stimuli were processed with ideal or nonideal NR, or unprocessed. The effect of NR on response times in HI listeners was significant only in conditions where speech intelligibility was also affected (-5 dB SNR). This is in contrast to the previous results with NH listeners. There was a significant effect of SNR on response times for HI listeners. The response time measure was reasonably correlated (R142 = 0.54) to subjective listening effort and showed a sufficient test-retest reliability. This study thus presents an objective, valid, and reliable measure for evaluating an aspect of listening effort of HI listeners.

Measuring Auditory Fitness in Locomotive Engineers: Development and Validation of a Signal Detection Test.

OBJECTIVES: To perform their job safely and effectively, locomotive engineers are required to detect auditory warning signals in the noisy work setting of a train cabin. Based on audio recordings of Dutch train cabins, we have developed a task and job-specific test for assessing the engineer's ability to detect the two acoustic warning signals for the Dutch situation. The aim of this study was to evaluate the reliability, agreement, and construct validity of this test. DESIGN: Two experiments were performed. In the first experiment, reliability and agreement of the signal detection test were assessed. Normally hearing individuals (N = 12) completed a signal detection test twice in 12 driving conditions. In the second experiment, construct validity was assessed. We retrospectively identified locomotive engineers, suspected of being hearing impaired, who were referred to the Amsterdam UMC for an auditory fitness for job assessment. All included engineers (N = 83) performed the signal detection test in four driving conditions, underwent tone audiometry and two speech perception in noise tests, and rated the effort and concentration it takes to detect the auditory signals. Seven a priori formulated hypotheses were tested. RESULTS: In the first experiment, sufficient reliability and agreement were found in nine driving conditions (ICC = 0.54-0.81; standard error of measurement = 1.15-1.92), poor reliability in two driving conditions (ICC < 0.50), and poor agreement in one driving condition (standard error of measurement = 2.67 dBA). In the second experiment, the results of the signal detection test correlated moderately with the pure-tone thresholds, speech reception threshold in fluctuating noise, and engineer's subjective rating of effort and concentration, but not with the speech reception threshold in continuous noise. According to the hypotheses, poorer test scores were obtained by hearing aid users compared with nonhearing aid users. CONCLUSIONS: The signal detection test has sufficient reliability and agreement in all but three driving conditions. This study provides evidence supporting the construct validity of the signal detection test in locomotive engineers. The moderate associations with conventional hearing tests show that the conventional hearing tests did not cover the whole construct measured with the signal detection test. The results, therefore, underpin the importance of evaluating the ability to detect auditory warning signals separately from other hearing-critical job tasks.

Detectability of auditory warning signals in the ambient noise of Dutch train cabins.

Locomotive engineers need to detect auditory warning signals for safe and effective job performance. We measured the levels and spectra of the warning signals and noises present in Dutch train cabins to evaluate the effectiveness of these signals. Audio-recordings were made in six train types during normal operation. Signal detectability was estimated using the Detectsound software and compared against ISO 7731. Signal detectability was also measured in six normally-hearing individuals in a laboratory setting. Signal levels ranged between 68 and 84 dBA. Noise levels ranged between 53 and 77 dBA. The acoustical requirements for signal detectability were not met in multiple driving conditions, especially at higher speed. Sufficient signal-to-noise ratio's were achieved in the laboratory measurements, but difficulties can be expected in unfavourable driving situations or when the engineer suffers from hearing loss. Acoustical, environmental, or work modifications might be required to prevent situations with insufficient audibility in hearing-impaired engineers. Practitioner summary: The audibility of the warning signals in Dutch trains was evaluated by comparing signal and noise spectra. The results showed that sufficient audibility is not always guaranteed. Under laboratory conditions, normally-hearing individuals could compensate for the suboptimal acoustic circumstances, but acoustical, environmental, or work modifications might be required to prevent situations with insufficient audibility in hearing-impaired engineers.

Subjective evaluation of single microphone noise reduction with different time constants.

Objective: Previous studies on single microphone noise reduction (NR) in hearing aids (HAs) have shown that some NR algorithms provide beneficial effects in terms of listener preference. To improve HA user satisfaction, we are interested in characteristics that determine preferences for NR, and in the inter-individual variability. The aim of this study was to test if dynamic properties of NR influence listener preference. Design: The gain reduction at speech offsets of a NR algorithm was slowed down by applying temporal exponential smoothing. At speech onsets the gain recovery was left unchanged. Test signals consisted of speech in continuous and modulated speech-shaped background noise, processed with three time constants: 0, 100, and 200 ms. Study sample: 16 Normal hearing (NH) and 16 hearing impaired (HI) subjects participated in a paired-comparison listening test. Results: NH subjects as a group had a significant preference for NR with time constants of 100, or 200 ms (slower acting NR). HI listeners as a group preferred NR over no NR, but had no clear preference for fast or slow NR. Patterns of preference differed between individual listeners. Conclusions: NR dynamics had an impact on individual listener preference and should be considered when optimising HAs.

The Influence of Noise Reduction on Speech Intelligibility, Response Times to Speech, and Perceived Listening Effort in Normal-Hearing Listeners.

Single-microphone noise reduction leads to subjective benefit, but not to objective improvements in speech intelligibility. We investigated whether response times (RTs) provide an objective measure of the benefit of noise reduction and whether the effect of noise reduction is reflected in rated listening effort. Twelve normal-hearing participants listened to digit triplets that were either unprocessed or processed with one of two noise-reduction algorithms: an ideal binary mask (IBM) and a more realistic minimum mean square error estimator (MMSE). For each of these three processing conditions, we measured (a) speech intelligibility, (b) RTs on two different tasks (identification of the last digit and arithmetic summation of the first and last digit), and (c) subjective listening effort ratings. All measurements were performed at four signal-to-noise ratios (SNRs): -5, 0, +5, and +∞ dB. Speech intelligibility was high (>97% correct) for all conditions. A significant decrease in response time, relative to the unprocessed condition, was found for both IBM and MMSE for the arithmetic but not the identification task. Listening effort ratings were significantly lower for IBM than for MMSE and unprocessed speech in noise. We conclude that RT for an arithmetic task can provide an objective measure of the benefit of noise reduction. For young normal-hearing listeners, both ideal and realistic noise reduction can reduce RTs at SNRs where speech intelligibility is close to 100%. Ideal noise reduction can also reduce perceived listening effort.

Acoustical and Perceptual Comparison of Noise Reduction and Compression in Hearing Aids.

PURPOSE: Noise reduction and dynamic-range compression are generally applied together in hearing aids but may have opposite effects on amplification. This study evaluated the acoustical and perceptual effects of separate and combined processing of noise reduction and compression. DESIGN: Recordings of the output of 4 hearing aids for speech in babble noise at +4 dB signal-to-noise ratio were used in 3 experiments: (a) acoustical measurements to determine the influence of processing on speech and noise levels; (b) perceptual measurements to determine the detectability of processing differences for 16 listeners with hearing impairment; and (c) perceptual measurements to determine the effect of processing on speech intelligibility, noise annoyance, speech naturalness, and overall preference. RESULTS: Noise reduction and compression processing differed between hearing aids. The combined processing (noise reduction with compression) most strongly reduced noise and speech levels. The combined processing was detectably different between hearing aids, but compression processing alone was not. The combined processing did not influence speech intelligibility. Preference for combined processing was lower than previously observed for noise reduction without compression. CONCLUSIONS: Differences in processing between hearing aids are perceptually salient. The effect of compression should be taken into account during the development and evaluation of hearing aid noise reduction.

Optimization of the Dutch matrix test by random selection of sentences from a preselected subset.

Matrix tests are available for speech recognition testing in many languages. For an accurate measurement, a steep psychometric function of the speech materials is required. For existing tests, it would be beneficial if it were possible to further optimize the available materials by increasing the function's steepness. The objective is to show if the steepness of the psychometric function of an existing matrix test can be increased by selecting a homogeneous subset of recordings with the steepest sentence-based psychometric functions. We took data from a previous multicenter evaluation of the Dutch matrix test (45 normal-hearing listeners). Based on half of the data set, first the sentences (140 out of 311) with a similar speech reception threshold and with the steepest psychometric function (≥9.7%/dB) were selected. Subsequently, the steepness of the psychometric function for this selection was calculated from the remaining (unused) second half of the data set. The calculation showed that the slope increased from 10.2%/dB to 13.7%/dB. The resulting subset did not allow the construction of enough balanced test lists. Therefore, the measurement procedure was changed to randomly select the sentences during testing. Random selection may interfere with a representative occurrence of phonemes. However, in our material, the median phonemic occurrence remained close to that of the original test. This finding indicates that phonemic occurrence is not a critical factor. The work highlights the possibility that existing speech tests might be improved by selecting sentences with a steep psychometric function.

Effects of noise reduction on speech intelligibility, perceived listening effort, and personal preference in hearing-impaired listeners.

This study evaluates the perceptual effects of single-microphone noise reduction in hearing aids. Twenty subjects with moderate sensorineural hearing loss listened to speech in babble noise processed via noise reduction from three different linearly fitted hearing aids. Subjects performed (a) speech-intelligibility tests, (b) listening-effort ratings, and (c) paired-comparison ratings on noise annoyance, speech naturalness, and overall preference. The perceptual effects of noise reduction differ between hearing aids. The results agree well with those of normal-hearing listeners in a previous study. None of the noise-reduction algorithms improved speech intelligibility, but all reduced the annoyance of noise. The noise reduction that scored best with respect to noise annoyance and preference had the worst intelligibility scores. The trade-off between intelligibility and listening comfort shows that preference measurements might be useful in addition to intelligibility measurements in the selection of noise reduction. Additionally, this trade-off should be taken into consideration to create realistic expectations in hearing-aid users.

Detection threshold for sound distortion resulting from noise reduction in normal-hearing and hearing-impaired listeners.

Hearing-aid noise reduction should reduce background noise, but not disturb the target speech. This objective is difficult because noise reduction suffers from a trade-off between the amount of noise removed and signal distortion. It is unknown if this important trade-off differs between normal-hearing (NH) and hearing-impaired (HI) listeners. This study separated the negative effect of noise reduction (distortion) from the positive effect (reduction of noise) to allow the measurement of the detection threshold for noise-reduction (NR) distortion. Twelve NH subjects and 12 subjects with mild to moderate sensorineural hearing loss participated in this study. The detection thresholds for distortion were determined using an adaptive procedure with a three-interval, two-alternative forced-choice paradigm. Different levels of distortion were obtained by changing the maximum amount of noise reduction. Participants were also asked to indicate their preferred NR strength. The detection threshold for overall distortion was higher for HI subjects than for NH subjects, suggesting that stronger noise reduction can be applied for HI listeners without affecting the perceived sound quality. However, the preferred NR strength of HI listeners was closer to their individual detection threshold for distortion than in NH listeners. This implies that HI listeners tolerate fewer audible distortions than NH listeners.

Investigation into the applicability and optimization of the Dutch matrix sentence test for use with cochlear implant users.

OBJECTIVE: Matrix sentence tests use words from a fixed word matrix to compose syntactically equivalent, but semantically unpredictable sentences. These tests are suitable for monitoring performance of cochlear implant (CI) users by repeated speech intelligibility testing. This study evaluates the Dutch matrix sentence test in CI users in quiet and in noise. It then investigates the possibility to improve the test-retest reliability for CI users by selecting subsets of sentences. DESIGN: Repeated speech intelligibility testing was performed in quiet and in noise. The effect of sentence selection on the test-retest reliability was predicted by computer simulations and experimentally evaluated using a cross-over design. STUDY SAMPLE: Fifteen post-lingually deafened CI users, of which eleven participated in the cross-over study. RESULTS: The test-retest reliability equaled 2.3 dB in quiet and 1.3 dB in noise. The simulations predicted an improvement in test-retest reliability, especially in quiet. The cross-over study did not confirm the predictions. CONCLUSIONS: The results of the study suggest that the homogeneity of the sentences is not the prime component underlying the test-retest reliability. The Dutch matrix speech material and the selected subsets of sentences were equally suitable for speech intelligibility testing in CI users.

Development of a Dutch matrix sentence test to assess speech intelligibility in noise.

OBJECTIVE: A Dutch matrix sentence test was developed and evaluated. A matrix test is a speech-in-noise test based on a closed speech corpus of sentences derived from words from fixed categories. An example is "Mark gives five large flowers." DESIGN: This report consists of the development of the speech test and a multi-center evaluation. STUDY SAMPLE: Forty-five normal-hearing participants. RESULTS: The developed matrix test has a speech reception threshold in stationary noise of - 8.4 dB with an inter-list standard deviation of 0.2 dB. The slope of the intelligibility function is 10.2 %/dB and this is slightly lower than that of similar tests in other languages (12.6 to 17.1 %/dB). CONCLUSIONS: The matrix test is now also available in Dutch and can be used in both Flanders and the Netherlands.

Using response time to speech as a measure for listening effort.

OBJECTIVE: Speech signals that do not differ in intelligibility might differ in listening effort. This study aimed to investigate the effect of background noise on response time to intelligible speech. DESIGN: We added various amounts of stationary noise to spoken digit triplets and measured the influence of noise on the response time for both an identification and an arithmetic task: Task 1 'identify the final digit in a triplet', and Task 2 'calculate the sum of the initial and the final digits in a triplet.' STUDY SAMPLE: Twelve normal-hearing participants with a mean age of 30.6 years (range: 28-44 years). RESULTS: Response time increased with lower (i.e. worse) signal to noise ratios for both tasks, even for signal to noise ratios with almost maximum intelligibility (close to 100%). The response time during the arithmetic task was more affected by the noise than during the identification task, but the arithmetic task demonstrated higher variance. CONCLUSIONS: The response time to digit triplets reduces significantly for increasing signal to noise ratios, even where speech intelligibility is optimal. These differences in response time might be related to listening effort and as such might be used to evaluate hearing-aid signal processing at positive SNRs.

Perceptual effects of noise reduction with respect to personal preference, speech intelligibility, and listening effort.

OBJECTIVES: Most modern hearing aids use noise reduction to increase listening comfort in noisy environments. However, it is unclear whether perceptual effects (e.g., intelligibility, listening effort, and preference) of noise reduction differ among hearing aids and among listeners. The authors compared perceptual scores across different hearing aid noise-reduction systems to determine (1) whether noise-reduction systems differ perceptually and (2) which factors underlie the overall preference of individual listeners. DESIGN: The authors recorded hearing aid noise-reduction outputs and used these signals in a laboratory experiment. The recording method allowed the evaluation of noise reduction in an isolated form, without the dominant effects of hearing aid frequency response and interactions with dynamic-range compression. Ten normal-hearing subjects listened to speech in babble noise processed by noise reduction from four different hearing aids. The subjects performed (1) speech-intelligibility tests, (2) listening-effort ratings, and (3) paired-comparison ratings with respect to noise annoyance, speech naturalness, and overall preference. RESULTS: Noise-reduction systems from different hearing aids differed in the degree to which they influenced the noise annoyance and speech naturalness perceived by the normal-hearing listeners. Small differences in intelligibility and effort scores were found among different noise-reduction systems but not between having noise reduction on and off. Subjects differed in whether their overall preference was more strongly related to noise annoyance or to speech naturalness. CONCLUSIONS: The authors conclude that noise annoyance and speech naturalness are determining factors for the overall preference of normal-hearing listeners for a specific noise-reduction condition, and found individual differences in the preferred weighting of these factors even in a homogeneous group of normal-hearing listeners. Subsequent experiments should include hearing-impaired subjects to determine whether these conclusions also hold for a more heterogeneous group of listeners. If these results can be extrapolated to hearing-impaired listeners, the fitting and fine-tuning of noise reduction in hearing aids needs considerable revision.

Perceptual effects of noise reduction by time-frequency masking of noisy speech.

Time-frequency masking is a method for noise reduction that is based on the time-frequency representation of a speech in noise signal. Depending on the estimated signal-to-noise ratio (SNR), each time-frequency unit is either attenuated or not. A special type of a time-frequency mask is the ideal binary mask (IBM), which has access to the real SNR (ideal). The IBM either retains or removes each time-frequency unit (binary mask). The IBM provides large improvements in speech intelligibility and is a valuable tool for investigating how different factors influence intelligibility. This study extends the standard outcome measure (speech intelligibility) with additional perceptual measures relevant for noise reduction: listening effort, noise annoyance, speech naturalness, and overall preference. Four types of time-frequency masking were evaluated: the original IBM, a tempered version of the IBM (called ITM) which applies limited and non-binary attenuation, and non-ideal masking (also tempered) with two different types of noise-estimation algorithms. The results from ideal masking imply that there is a trade-off between intelligibility and sound quality, which depends on the attenuation strength. Additionally, the results for non-ideal masking suggest that subjective measures can show effects of noise reduction even if noise reduction does not lead to differences in intelligibility.

A method to remove differences in frequency response between commercial hearing aids to allow direct comparison of the sound quality of hearing-aid features.

GOAL: We want to remove differences in frequency response between different commercial hearing aids so that we can compare the sound quality of signal processing features from different hearing-aid in a future paired-comparison set-up. More specifically, we want to control for the confounding effects of the linear hearing aid response when evaluating nonlinear processing. This article presents a control procedure and evaluates its effectiveness. METHOD: We increased the similarity of hearing-aid recordings in three steps and used both an objective quality metric and listening tests to investigate if the recordings from different hearing aids were perceptually similar. RESULTS: Neither was it sufficient to manually adjust the hearing-aid insertion gain, nor was it sufficient to add an additional bandwidth limitation to the recordings. Only after the application of an inverse filter the perceptual differences between recordings were removed adequately. CONCLUSION: It was possible to level the ground between different hearing devices, so to speak. This will allow future research to evaluate the sound quality of nonlinear signal processing features.

Multicenter evaluation of signal enhancement algorithms for hearing aids.

In the framework of the European HearCom project, promising signal enhancement algorithms were developed and evaluated for future use in hearing instruments. To assess the algorithms' performance, five of the algorithms were selected and implemented on a common real-time hardware/software platform. Four test centers in Belgium, The Netherlands, Germany, and Switzerland perceptually evaluated the algorithms. Listening tests were performed with large numbers of normal-hearing and hearing-impaired subjects. Three perceptual measures were used: speech reception threshold (SRT), listening effort scaling, and preference rating. Tests were carried out in two types of rooms. Speech was presented in multitalker babble arriving from one or three loudspeakers. In a pseudo-diffuse noise scenario, only one algorithm, the spatially preprocessed speech-distortion-weighted multi-channel Wiener filtering, provided a SRT improvement relative to the unprocessed condition. Despite the general lack of improvement in SRT, some algorithms were preferred over the unprocessed condition at all tested signal-to-noise ratios (SNRs). These effects were found across different subject groups and test sites. The listening effort scores were less consistent over test sites. For the algorithms that did not affect speech intelligibility, a reduction in listening effort was observed at 0 dB SNR.