Extending the Hearing-Aid Speech Perception Index (HASPI): Keywords, sentences, and context.

The Hearing-Aid Speech Perception Index version 2 (HASPI v2) is a speech intelligibility metric derived by fitting subject responses scored as the proportion of complete sentences correct. This paper presents an extension of HASPI v2, denoted by HASPI w2, which predicts proportion keywords correct for the same datasets used to derive HASPI v2. The results show that the accuracy of HASPI w2 is nearly identical to that of HASPI v2. The values produced by HASPI w2 and HASPI v2 also allow the comparison of proportion words correct and sentences correct for the same stimuli. Using simulation values for speech in additive noise, a model of context effects for words combined into sentences is developed and accounts for the loss of intelligibility inherent in the impaired auditory periphery. In addition, HASPI w2 and HASPI v2 have a small bias term at poor signal-to-noise ratios; the model for context effects shows that the residual bias is reduced in converting from proportion keywords to sentences correct but is greatly magnified when considering the reverse transformation.

An overview of the HASPI and HASQI metrics for predicting speech intelligibility and speech quality for normal hearing, hearing loss, and hearing aids.

Alterations of the speech signal, including additive noise and nonlinear distortion, can reduce speech intelligibility and quality. Hearing aids present an especially complicated situation since these devices may implement nonlinear processing designed to compensate for the hearing loss. Hearing-aid processing is often realized as time-varying multichannel gain adjustments, and may also include frequency reassignment. The challenge in designing metrics for hearing aids and hearing-impaired listeners is to accurately model the perceptual trade-offs between speech audibility and the nonlinear distortion introduced by hearing-aid processing. This paper focuses on the Hearing Aid Speech Perception Index (HASPI) and the Hearing Aid Speech Quality Index (HASQI) as representative metrics for predicting intelligibility and quality. These indices start with a model of the auditory periphery that can be adjusted to represent hearing loss. The peripheral model, the speech features computed from the model outputs, and the procedures used to fit the features to subject data are described. Examples are then presented for using the metrics to measure the effects of additive noise, evaluate noise-suppression processing, and to measure the differences among commercial hearing aids. Open questions and considerations in using these and related metrics are then discussed.

A comparison of speech intelligibility and subjective quality with hearing-aid processing in older adults with hearing loss.

OBJECTIVE: This study characterised the relationship between speech intelligibility and quality in listeners with hearing loss for a range of hearing-aid processing settings and acoustic conditions. DESIGN: Binaural speech intelligibility scores and quality ratings were measured for sentences presented in babble noise and processed through a hearing-aid simulation. The intelligibility-quality relationship was investigated by (1) assessing the effects of experimental conditions on each task; (2) directly comparing intelligibility scores and quality ratings for each participant across the range of conditions; and (3) comparing the association between signal envelope fidelity (represented by a cepstral correlation metric) and intelligibility and quality. STUDY SAMPLE: Participants were 15 adults (7 females; age range 59-81 years) with mild to moderately severe sensorineural hearing loss. RESULTS: Intelligibility and quality showed a positive association both with each other and with changes to signal fidelity introduced by the entire acoustic and signal-processing system including the additive noise and the hearing-aid output. As signal fidelity decreased, quality ratings changed at a slower rate than intelligibility scores. Individual psychometric functions were more variable for quality compared to intelligibility. CONCLUSIONS: Variability in the intelligibility-quality relationship reinforces the importance of measuring both intelligibility and quality in clinical hearing-aid fittings.

Adding air absorption to simulated room acoustic models.

Room acoustics models are often used in auditory research. The simplest implementations of these models ignore air absorption, and this lack can affect the room response, especially at high frequencies and for large simulated spaces. This letter proposes a method for adding air absorption to a simulated room impulse response calculated without air absorption. The procedure creates a time-varying lowpass filter that approximates air absorption as a function of distance. The existing absorption-free room response is passed through the filter to create a new response that incorporates air absorption effects.

The Type of Noise Influences Quality Ratings for Noisy Speech in Hearing Aid Users.

Purpose The overall goal of the current study was to determine whether noise type plays a role in perceptual quality ratings. We compared quality ratings using various noise types and signal-to-noise ratio (SNR) ranges using hearing aid simulations to consider the effects of hearing aid processing features. Method Ten older adults with bilateral mild to moderately severe sensorineural hearing loss rated the sound quality of sentences processed through a hearing aid simulation and presented in the presence of five different noise types (six-talker babble, three-talker conversation, street traffic, kitchen, and fast-food restaurant) at four SNRs (3, 8, 12, and 20 dB). Results Everyday noise types differentially affected sound quality ratings even when presented at the same SNR: Kitchen and three-talker noises were rated significantly higher than restaurant, traffic, and multitalker babble, which were not different from each other. The effects of noise type were most pronounced at poorer SNRs. Conclusions The findings of this study showed that noise types differentially affected sound quality ratings. The differences we observed were consistent with the acoustic characteristics of the noise types. Noise types having lower envelope fluctuations yielded lower quality ratings than noise types characterized by sporadic high-intensity events at the same SNR.

Quantifying the Range of Signal Modification in Clinically Fit Hearing Aids.

OBJECTIVES: Hearing aids provide various signal processing techniques with a range of parameters to improve the listening experience for a hearing-impaired individual. In previous studies, we reported significant differences in signal modification for mild versus strong signal processing in commercially available hearing aids. In this study, the authors extend this work to clinically prescribed hearing aid fittings based on best-practice guidelines. The goals of this project are to determine the range of cumulative signal modification in clinically fit hearing aids across manufacturers and technology levels and the effects of listening conditions including signal to noise ratio (SNR) and presentation level on these signal modifications. DESIGN: We identified a subset of hearing aids that were representative of a typical clinical setting. Deidentified hearing aid fitting data were obtained from three audiology clinics for adult hearing aid users with sensorineural hearing loss for a range of hearing sensitivities. Matching laboratory hearing aids were programmed with the deidentified fitting data. Output from these hearing aids was recorded at four SNRs and three presentation levels. The resulting signal modification was quantified using the cepstral correlation component of the Hearing Aid Speech Quality Index which measures the speech envelope changes in the context of a model of the listener's hearing loss. These metric values represent the hearing aid processed signal as it is heard by the hearing aid user. Audiometric information was used to determine the nature of any possible association with the distribution of signal modification in these clinically fit hearing aids. RESULTS: In general, signal modification increased as SNR decreased and presentation level increased. Differences across manufacturers were significant such that the effect of presentation level varied differently at each SNR, for each manufacturer. This result suggests that there may be variations across manufacturers in processing various listening conditions. There was no significant effect of technology level. There was a small effect of pure-tone average on signal modification for one manufacturer, but no effect of audiogram slope. Finally, there was a broad range of measured signal modification for a given hearing loss, for the same manufacturer and listening condition. CONCLUSIONS: The signal modification values in this study are representative of commonly fit hearing aids in clinics today. The results of this study provide insights into how the range of signal modifications obtained in real clinical fittings compares with a previous study. Future studies will focus on the behavioral implications of signal modifications in clinically fit hearing aids.

Limitations of the Envelope Difference Index as a Metric for Nonlinear Distortion in Hearing Aids.

OBJECTIVES: The envelope difference index (EDI) compares the envelopes of two signals. It has been used to measure nonlinear distortion in hearing aids, but it also responds to linear processing. This article compares linear and nonlinear processing effects on the EDI. DESIGN: The EDI for spectral tilt and peak clipping distortion is computed to illustrate the effects of linear and nonlinear signal modifications. The EDI for wide dynamic-range compression is then compared with that obtained for linear amplification for a set of standard audiograms to show the expected range of EDI values for linear and nonlinear hearing aid processing. The EDI for hearing aid amplification and compression is also compared with a measure of time-frequency envelope modulation distortion for the same conditions. RESULTS: The EDI is shown to be as sensitive to linear amplification as it is to nonlinear processing. The EDI values for spectral tilt can exceed those for peak clipping, and the EDI values for linear amplification exceed those for wide dynamic-range compression for four of the nine audiograms considered. The agreement of the EDI with a nonlinear envelope distortion measure is shown to depend on the long-term spectra of the signals being compared when computing the EDI. CONCLUSIONS: The accuracy of the EDI as an indicator of nonlinear distortion for sentence materials can be improved by equalizing the long-term spectrum of the processed signal to match that of the unprocessed input. However, the EDI does not have a clear interpretation because of the confound between linear and nonlinear processing effects and the lack of an auditory model in calculating the signal differences.

Integrating a remote microphone with hearing-aid processing.

A remote microphone (RM) links a talker's microphone to a listener's hearing aids (HAs). The RM improves intelligibility in noise and reverberation, but the binaural cues necessary for externalization are lost. Augmenting the RM signal with synthesized binaural cues and early reflections enhances externalization, but interactions of the RM signal with the HA processing could reduce its effectiveness. These potential interactions were evaluated using RM plus HA processing in a realistic listening simulation. The HA input was the RM alone, the augmented RM signal, the acoustic inputs at the HA microphones, including reverberation measured using a dummy head, or a mixture of the augmented RM and acoustic input signals. The HA simulation implemented linear amplification or independent dynamic-range compression at the two ears and incorporated the acoustic effects of vented earmolds. Hearing-impaired listeners scored sentence stimuli for intelligibility and rated clarity, overall quality, externalization, and apparent source width. Using the RM improved intelligibility but reduced the spatial impression. Increasing the vent diameter reduced clarity and increased the spatial impression. Listener ratings reflect a trade-off between the attributes of clarity and overall quality and the attributes of externalization and source width that can be explained using the interaural cross correlation.

Externalization of remote microphone signals using a structural binaural model of the head and pinna.

In a remote microphone (RM) system, a talker speaks into a microphone and the signal is transmitted to the hearing aids worn by the hearing-impaired listener. A difficulty with remote microphones, however, is that the signal received at the hearing aid bypasses the head and pinna, so the acoustic cues needed to externalize the sound source are missing. The objective of this paper is to process the RM signal to improve externalization when listening through earphones. The processing is based on a structural binaural model, which uses a cascade of processing modules to simulate the interaural level difference, interaural time difference, pinna reflections, ear-canal resonance, and early room reflections. The externalization results for the structural binaural model are compared to a left-right signal blend, the listener's own anechoic head-related impulse response (HRIR), and the listener's own HRIR with room reverberation. The azimuth is varied from straight ahead to 90° to one side. The results show that the structural binaural model is as effective as the listener's own HRIR plus reverberation in producing an externalized acoustic image, and that there is no significant difference in externalization between hearing-impaired and normal-hearing listeners.

The dynamic gammawarp auditory filterbank.

Auditory filterbanks are an integral part of many metrics designed to predict speech intelligibility and speech quality. Considerations in these applications include accurate reproduction of auditory filter shapes, the ability to reproduce the impact of hearing loss as well as normal hearing, and computational efficiency. This paper presents an alternative method for implementing a dynamic compressive gammachirp (dcGC) auditory filterbank [Irino and Patterson (2006). IEEE Trans. Audio Speech Lang. Proc. 14, 2222-2232]. Instead of using a cascade of second-order sections, this approach uses digital frequency warping to give the gammawarp filterbank. The set of warped finite impulse response filter coefficients is constrained to be symmetrical, which results in the same phase response for all filters in the filterbank. The identical phase responses allow the dynamic variation in the gammachirp filter magnitude response to be realized as a sum, using time-varying weights, of three filters that provide the responses for high-, mid-, and low-intensity input signals, respectively. The gammawarp filterbank offers a substantial improvement in execution speed compared to previous dcGC implementations; for a dcGC filterbank, the gammawarp implementation is 24 to 38 times faster than the dcGC Matlab code of Irino.

Using Objective Metrics to Measure Hearing Aid Performance.

OBJECTIVES: The performance of hearing aids is generally characterized by a small set of standardized measurements. The primary goals of these procedures are to measure basic aspects of the hearing aid performance and to ascertain that the device is operating properly. A more general need exists for objective metrics that can predict hearing aid outcomes. Such metrics must consider the interaction of all the signal processing operating in the hearing aid and must do so while also accounting for the hearing loss for which the hearing aid has been prescribed. This article represents a first step in determining the clinical applicability of the hearing aid speech perception index (HASPI) intelligibility and hearing aid speech quality index (HASQI) speech quality metrics. The goals of this article are to demonstrate the feasibility of applying these metrics to commercial hearing aids and to illustrate the anticipated range of measured values and identify implementation concerns that may not be present for conventional measurements. DESIGN: This article uses the HASPI intelligibility and HASQI speech quality metrics to measure the performance of commercial hearing aids. These metrics measure several aspects of the processed signal, including envelope fidelity, modifications of the temporal fine structure, and changes in the long-term frequency response, all in the context of an auditory model that reproduces the salient aspects of the peripheral hearing loss. The metrics are used to measure the performance of basic and premium hearing aids from three different manufacturers. Test conditions include the environmental factors of signal to noise ratio and presentation level, and the fitting configurations were varied to provide different degrees of processing from linear to aggressive nonlinear processing for two different audiograms. RESULTS: The results show that the metrics are capable of measuring statistically significant differences across devices and processing settings. HASPI and HASQI measure both audibility and nonlinear distortion in the devices, and conditions are identified where predicted intelligibility is high but predicted speech quality is substantially reduced. The external signal properties of signal to noise ratio and presentation level are both statistically significant. Hearing loss is significant for HASPI but not for HASQI, and degree of processing is significant for both metrics. A quadratic model for manufacturer showed large effect sizes for HASPI and HASQI, but basic versus premium hearing aid model is not significant. CONCLUSIONS: The results presented in this article represent a first step in applying the HASPI and HASQI metrics to commercial hearing aids. Modern hearing aids often use several different processing strategies operating simultaneously. The proposed metrics provide a way to predict the total effect of this processing, including algorithm interactions that may be missed by conventional measurement procedures. The measurements in this article show significant differences between manufacturers, processing settings, and adjustment for different hearing losses. No significant differences were found between basic and premium hearing aid models. Further research will be needed to determine the clinical relevance of these measurements and to provide target values appropriate for successful fittings.

Author's Rebuttal to Smits et al. (2018), "Comment on 'Sensitivity of the Speech Intelligibility Index to the Assumed Dynamic Range' by Jin et al. (2017)".

Purpose: The purpose of this letter is to refute the comments written by Smits, Goverts, and Versfeld (2018). Method: Refutations to each issue including the fixed mathematical relationship between dynamic range (DR) and a fitting constant (Q value), deviating results for small DRs, and determination of Speech Intelligibility Index (SII) model parameters are described. Results: Although Smits et al. (2018) correctly identified several issues, those comments do not diminish the results of the original article (Jin, Kates, & Arehart, 2017) in providing new insights for the SII. Conclusions: Jin et al. (2017) clearly provided the impact of languages and DR on the SII, which was the main result of the study.

Speech quality and stable gain trade-offs in adaptive feedback cancellation for hearing aids.

This paper addresses trade-offs in adaptive feedback cancellation (AFC) for hearing aids. Aggressive AFC for improved added stable gain (ASG) reduces speech quality. In this paper, the hearing-aid speech quality index (HASQI) is used to investigate AFC performance before the system becomes unstable. It is demonstrated that for a desired speech quality, multiple AFC algorithms can be evaluated for their ASG and computational efficiency. An example is presented with HASQI = 0.8, baseline AFC, and two advanced approaches. For the advanced AFCs, ASG gains of 4 and 7 dB were obtained at additional computational complexity of 8% and 11%, respectively.

Sensitivity of the Speech Intelligibility Index to the Assumed Dynamic Range.

Purpose: This study aims to evaluate the sensitivity of the speech intelligibility index (SII) to the assumed speech dynamic range (DR) in different languages and with different types of stimuli. Method: Intelligibility prediction uses the absolute transfer function (ATF) to map the SII value to the predicted intelligibility for a given stimuli. To evaluate the sensitivity of the predicted intelligibility to the assumed DR, ATF-transformed SII scores for English (words), Korean (sentences), and Mandarin (sentences) were derived for DRs ranging from 10 dB to 60 dB. Results: Increasing the assumed DR caused steeper ATFs for all languages. However, high correlation coefficients between predicted and measured intelligibility scores were observed for DRs from 20 dB to 60 dB for ATFs in English, Korean, and Mandarin. Conclusions: Results of the present study indicate that the intelligibility computed from the SII is not sensitive to the assumed DR. The 30-dB DR commonly used in computing the SII is thus a reasonable assumption that produces accurate predictions for different languages and different types of stimuli.

Using envelope modulation to explain speech intelligibility in the presence of a single reflection.

A single reflection is the simplest simulation of reverberation and provides insights into more complex scenarios of listening in rooms. This paper presents an analysis of the effects of a single reflection as its delay and intensity are systematically varied. The changes to the envelope modulations are analyzed using not only the traditional within-auditory-band analysis approach but also an across-band spectro-temporal analysis using cepstral correlation coefficients. The use of an auditory model allowed the extension of the simulations to include sensorineural hearing loss. Short delays did not interfere with the envelope modulations at low modulation rates (<16 Hz) and impact predicted intelligibility, while longer delays caused substantial distortion at these rates. The patterns of envelope modulation distortions caused by a single reflection were shown to be similar in models of normal hearing and hearing impairment.

Does Language Matter When Using a Graphical Method for Calculating the Speech Intelligibility Index?

BACKGROUND: Graphical methods for calculating the speech intelligibility index (SII), such as the count-the-dot audiogram, are useful tools in quantifying how much weighted audibility is restored when amplification is used for individuals with hearing loss. The band-importance function (BIF), which is an important component of the SII, depends on the language. Thus, language may affect the prediction of weighted audibility using the graphical SII. PURPOSE: The purpose of this study was to apply language-specific BIFs to develop and compare graphical SIIs for English, Korean, and Mandarin. RESEARCH DESIGN: The graphical SIIs were developed and compared using a research design that applied and analyzed existing datasets. DATA COLLECTION AND ANALYSIS: Language-specific BIFs and dynamic ranges were used to derive graphical SIIs for English, Korean, and Mandarin. SII predictions were compared by calculating the language-specific predictions for the same audiometric configurations. RESULTS: The graphical SIIs for English, Korean, and Mandarin yielded different unaided and aided predictions for the same audiogram configurations. CONCLUSIONS: A graphical SII helps patients easily understand their weighted audibility for unaided and aided conditions; thus, it is a useful counseling tool in the clinic. The most accurate graphical SII's will, however, be based on a patient's spoken language.

The Hearing-Aid Audio Quality Index (HAAQI).

This paper presents an index designed to predict music quality for individuals listening through hearing aids. The index is "intrusive", that is, it compares the degraded signal being evaluated to a reference signal. The index is based on a model of the auditory periphery that includes the effects of hearing loss. Outputs from the auditory model are used to measure changes in the signal time-frequency envelope modulation, temporal fine structure, and long-term spectrum caused by the hearing aid processing. The index is constructed by combining a term sensitive to noise and nonlinear distortion with a second term sensitive to changes in the long-term spectrum. The index is fitted to an existing database of music quality judgments made by listeners having normal or impaired hearing. The data comprise ratings for three music excerpts (classical orchestra, jazz trio, and jazz singer), each processed through 100 conditions representative of hearing-aid processing and listening situations. The overall accuracy of the index is high, with a correlation coefficient of 0.970 when computed over all of the processing conditions and averaged over the combined groups of listeners having normal and impaired hearing.

Comparing the information conveyed by envelope modulation for speech intelligibility, speech quality, and music quality.

This paper uses mutual information to quantify the relationship between envelope modulation fidelity and perceptual responses. Data from several previous experiments that measured speech intelligibility, speech quality, and music quality are evaluated for normal-hearing and hearing-impaired listeners. A model of the auditory periphery is used to generate envelope signals, and envelope modulation fidelity is calculated using the normalized cross-covariance of the degraded signal envelope with that of a reference signal. Two procedures are used to describe the envelope modulation: (1) modulation within each auditory frequency band and (2) spectro-temporal processing that analyzes the modulation of spectral ripple components fit to successive short-time spectra. The results indicate that low modulation rates provide the highest information for intelligibility, while high modulation rates provide the highest information for speech and music quality. The low-to-mid auditory frequencies are most important for intelligibility, while mid frequencies are most important for speech quality and high frequencies are most important for music quality. Differences between the spectral ripple components used for the spectro-temporal analysis were not significant in five of the six experimental conditions evaluated. The results indicate that different modulation-rate and auditory-frequency weights may be appropriate for indices designed to predict different types of perceptual relationships.

Derivations of the band-importance function: A cross-procedure comparison.

The purpose of this study was to compare band-importance functions (BIFs) derived by two different procedures. Intelligibility data from native speakers of Korean were used. BIFs were derived using an established procedure reported by Studebaker and Sherbecoe [J. Speech Lang. Hear. Res. 34, 427-438 (1991)] and by a recently published method reported by Kates [J. Acoust. Soc. Am. 134(5), EL459-EL464 (2013)]. The BIFs produced by the two different procedures were similar. These results indicate that the BIF calculation procedure by Kates is accurate for sentences as well as for the monosyllables used in its derivation.

Objective Quality and Intelligibility Prediction for Users of Assistive Listening Devices.

This article presents an overview of twelve existing objective speech quality and intelligibility prediction tools. Two classes of algorithms are presented, namely intrusive and non-intrusive, with the former requiring the use of a reference signal, while the latter does not. Investigated metrics include both those developed for normal hearing listeners, as well as those tailored particularly for hearing impaired (HI) listeners who are users of assistive listening devices (i.e., hearing aids, HAs, and cochlear implants, CIs). Representative examples of those optimized for HI listeners include the speech-to-reverberation modulation energy ratio, tailored to hearing aids (SRMR-HA) and to cochlear implants (SRMR-CI); the modulation spectrum area (ModA); the hearing aid speech quality (HASQI) and perception indices (HASPI); and the PErception MOdel - hearing impairment quality (PEMO-Q-HI). The objective metrics are tested on three subjectively-rated speech datasets covering reverberation-alone, noise-alone, and reverberation-plus-noise degradation conditions, as well as degradations resultant from nonlinear frequency compression and different speech enhancement strategies. The advantages and limitations of each measure are highlighted and recommendations are given for suggested uses of the different tools under specific environmental and processing conditions.