Phoneme Compression: processing of the speech signal and effects on speech intelligibility in hearing-Impaired listeners

Hearing-aid users often continue to have problems with poor speech understanding in difficult acoustical conditions. Another generally accounted problem is that certain sounds become too loud whereas other sounds are still not audible. Dynamic range compression is a signal processing technique that may be used in hearing aids to compensate for these remaining disabilities. Its main function is to provide sufficient amplification at low input levels without overloading the auditory system at high input levels. The time constants define the time needed by the compressor to realize a change in amplification. When using relatively large time constants, the compressor only reduces differences in overall level. This type of compression is known as Automatic Gain Control (AGC) or Automatic Volume Control (AVC).With short time constants the compressor also reduces the dynamic range of a fast-fluctuating signal like speech. This last type of system is therefore often called a syllabic or a phoneme compressor. The main goal of using phoneme compression is to optimize speech intelligibility by improving the detection of weak speech cues. We developed a phoneme compression system to improve the perception of high-frequency speech cues in hearing impaired listeners. The basic mechanism is a continuously changing balancing between low- and high-frequency amplification, steered by the input level of each speech part. As a consequence, the system should provide a relatively high amount of amplification to weak high-frequency speech cues. A specific configuration was developed to additionally reduce the negative effect of low-frequency amplification on the detection of high-frequency cues. This type of configuration is called anti-USOM processing as it is meant to compensate for “Upward-Spread-Of-Masking” (USOM) of high-frequency information by low-frequency signal parts. The main goal of the present thesis was to evaluate the effect of the different compression configurations on speech intelligibility in a group of hearing-impaired listeners with moderate-to-severe perceptive high-frequency losses (chapters 4 to 7). Additionally, we have investigated the effect of various types of compression on amplitude-modulated signals and speech(-like) signals (chapters 2 and 3). The acoustical measurements in chapters 2 and 3 provided a good insight in the effect of compression on modulating signals like speech. Speech can be considered as a stream of sounds with a continuously varying spectrum. These spectral differences lead to fluctuations of the envelope of the signal within individual frequency bands. The modulation depth is a measure for the amount of fluctuations. Phoneme compression will normally reduce the amount of fluctuations, resulting in a smaller modulation depth. By comparing the modulation depth in a signal before and after compression, the effective amount of compression can be obtained. This method was applied using an amplitude-modulated signal (chapter 2) and using speech(-like) signals (chapter 3). Another method compared the average level distributions of speech with and without compression (chapter 2). The results show that relatively short time constants were needed to affect the range of modulations that are relevant to speech intelligibility. Furthermore, an effective reduction of intensity differences within separate frequency channels was only possible if the compression was applied within independent frequency channels as well. Interestingly, the results were not only influenced by the compressor settings but also by the acoustical properties of the test signal. Intensity differences were reduced more effectively for speech in a stationary background noise compared to speech only. Chapters 4 and 6 describe the effects of different types of phoneme compression on speech intelligibility in hearing-impaired listeners. Phoneme scores were obtained in conditions with and without background noise. We evaluated the difference in performance between phoneme compression and a linear reference condition near comfortable presentation levels. This implies that the results could not be influenced by differences in overall level between the various conditions. The results described in chapter 4 show that hearing-impaired listeners may benefit from our type of phoneme compression in conditions without background noise. Consonant perception was improved by phoneme compression whereas the anti-USOM processing had an additional positive effect on vowel perception. Unfortunately, no such positive effects were found in conditions with background noise. Even substantially negative effects were found with the anti-USOM configuration that gave the best performance in quiet. The use of a more moderate type of anti-USOM in chapter 6 also resulted in a negative effect on phoneme recognition in background noise. No benefit was found for other types of phoneme compression in background noise. The use of a compression ratio of 4 resulted even in negative effects (chapter 4). This means that the performance in background noise gets poorer with an increasing amount of phoneme compression. The temporal behaviour of the background noise did not influence the results (chapter 6). We hoped to find positive effects from phoneme compression in a fluctuating background noise, but no such improvement was found. The results of chapter 5 can be used to understand the measured effects of compression on speech intelligibility. Two methods were used to analyse the perceptual confusions of chapter 4. INDSCAL was used to identify and visualise the most relevant differences in phoneme perception. However the interpretation of these differences was not always easy because the perceptual dimensions could be related to several perceptual features. Therefore, SINFA was used as a second method. The advantage of using this method was that the various effects could be separated for the different predefined articulatory features. In quiet, positive effects were found on the perception of features containing mainly high-frequency information. This is according to our original goal to improve the identification of high-frequency cues by phoneme compression. However, the perception of high-frequency cues appeared to be highly deteriorated at critical background noise conditions. As a consequence, the features containing low-frequency information had become of major importance. The use of anti-USOM processing removed low-frequency information that appeared to be relevant for the perception of low-frequency cues. Additionally, we evaluated three phoneme compression conditions in a small field study using an experimental body-worn hearing aid (chapter 7). The phoneme compression configurations were embedded in a slow-acting non-linear system to compensate for differences in overall level. The listeners used the system for a period of six weeks next to the own hearing aids. The performance with the various compression programs was measured every week. The main question was if the performance could be influenced by a frequent use of the system. In general, the results were similar to that in previous experiments. The performance in background noise tended to be poorer than the performance in quiet. Interestingly, the overall recognition score with phoneme compression improved over time. However, a large part of this improvement was also found for the reference condition. The tendency for a small additional improvement with phoneme compression may be attributed to acclimatization to the speech processing. The experiences of the hearing-impaired listeners with the phoneme compression programs differed between listeners and depended of the difference in performance with the own hearing aids. Generally they had no problems with the sound of the new programs.Hoortoesteldragers houden vaak problemen met het verstaan van spraak in lastige akoestische omstandigheden. Een ander bekend probleem is dat door de versterking van het hoortoestel bepaalde geluiden als te hard ervaren worden, terwijl andere belangrijke geluiden nog steeds niet worden gehoord. Ter compensatie van deze beperkingen kan dynamiekcompressie worden toegepast in het hoortoestel. Dit is een vorm van signaalbewerking die voor voldoende versterking zorgt bij lage niveaus, zonder het gehoor te overbelasten bij hoge signaalniveaus. Een compressiesysteem heeft tijd nodig om een verandering in versterking te realiseren. Deze is instelbaar via de zogenaamde tijdsconstanten. Bij hoge waarden van de tijdsconstanten reduceert de compressor alleen globale verschillen in signaalniveau. Deze vorm van compressie wordt Automatische Volume Controle (AVC) of Automatic Gain Control (AGC) genoemd. Indien de tijdsconstanten kort gehouden worden reduceert de compressor ook intensiteitsverschillen tussen opeenvolgende spraakklanken. Deze vorm van dynamiekcompressie wordt daarom vaak aangeduid met syllabische compressie of foneemcompressie. Het doel hiervan is het optimaliseren van spraakverstaan door een verbeterde detectie van zwakkere spraakklanken. In dit proefschrift wordt een vorm van foneemcompressie onderzocht die bedoeld is om de perceptie van hoogfrequente spraakklanken te verbeteren bij slechthorenden. Het systeem verandert continu de verhouding tussen laag- en hoogfrequente versterking, afhankelijk van de niveaus van de afzonderlijke spraakklanken. Dit leidt tot extra versterking van de zwakke klanken. Verder is een aparte configuratie ontwikkeld om de negatieve invloed van laagfrequente versterking te verminderen op de perceptie van hoogfrequente spraak (“anti-upward-spread-of-masking” of “anti-USOM” genoemd). Ons hoofddoel is om de effecten van deze vormen van foneemcompressie te evalueren op spraakverstaan bij slechthorenden (hoofdstukken 4 t/m 7). Tevens is onderzocht hoe het spraaksignaal beïnvloed wordt door verschillende vormen van compressie (hoofdstukken 2 en 3). De akoestische metingen in hoofdstukken 2 en 3 geven een goed inzicht in hoe spraak en andere snel fluctuerende signalen beïnvloed worden door compressie. Spraak kan beschouwd worden als een continue stroom geluiden met een variërend spectrum. Deze spectrale verschillen leiden tot fluctuaties in de omhullende van het spraaksignaal binnen verschillende frequentiebanden, ook wel modulaties genoemd. De sterkte van de modulaties wordt uitgedrukt door de modulatiediepte. Foneemcompressie zal, indien effectief toegepast, de modulatiediepte binnen het spraaksignaal verkleinen. De effectieve mate van compressie kan geschat worden door de modulatiediepte voor en na compressie met elkaar te vergelijken. Deze methode is toegepast voor “eenvoudige” amplitude-gemoduleerde signalen (hoofdstuk 2) en spraak(-achtige) signalen (hoofdstuk 3). Een andere methode bestaat uit het vergelijken van de niveauverdelingen binnen spraak voor en na compressie (hoofdstuk 2). De resultaten laten zien dat relatief korte tijdsconstanten nodig zijn om de voor het spraakverstaan relevante modulaties te beïnvloeden. Verder blijkt dat alleen compressie uitgevoerd in verschillende frequentiekanalen in staat is intensiteitsverschillen te reduceren in de afzonderlijke frequentiebanden. Opmerkelijk genoeg blijken niet alleen de instellingen van de compressor invloed te hebben op de mate van effectieve compressie, maar ook het gebruikte testsignaal. Voor spraak in stationaire ruis wordt een hogere mate van effectieve compressie gemeten dan voor spraak afzonderlijk. In hoofdstukken 4 en 6 worden de resultaten beschreven van spraaktesten bij slechthorenden met verschillende vormen van foneemcompressie. Er zijn foneemscores bepaald in condities met en zonder achtergrondruis. Hierbij is het verschil bekeken tussen de resultaten met foneemcompressie en een geoptimaliseerde referentieconditie aangeboden op het zelfde signaalniveau. De resultaten in hoofdstuk 4 laten zien dat slechthorenden in staat zijn te profiteren van foneemcompressie zolang er geen achtergrondgeluid wordt toegevoegd aan de spraak. Compressie blijkt vooral een positieve invloed te hebben op de herkenning van de beginmedeklinker. Daarnaast geeft anti-USOM een positief effect op de klinkerherkenning. De beste resultaten worden behaald met een combinatie van beide technieken. Helaas wordt er voor de condities met achtergrondgeluid juist negatieve effecten gevonden met deze configuratie. Ook met een meer gematigde vorm van anti-USOM blijft het resultaat negatief (hoofdstuk 6). Andere configuraties met foneemcompressie geven eveneens geen verbetering van spraakverstaan in achtergrondlawaai. Voor een grote mate van compressie wordt een verslechtering gevonden. Ook bij een fluctuerende achtergrondruis worden geen duidelijke voordelen gevonden van foneemcompressie. De resultaten in hoofdstuk 5 geven meer inzicht in de effecten van foneemcompressie op de perceptie van spraakklanken. De verwisselingen van spraakklanken in de spraaktesten met slechthorenden zijn onderzocht door middel van twee methoden. INDSCAL identificeert en visualiseert de meest relevante verschillen in foneemperceptie door het definiëren van onafhankelijke perceptuele dimensies. De interpretatie van deze dimensies in termen van fonetische kenmerken blijkt echter niet altijd even gemakkelijk. Als aanvulling is daarom SINFA gebruikt, met als voordeel dat de verschillende fonetische kenmerken hierbij vooraf gedefinieerd kunnen worden. Zowel foneemcompressie als anti-USOM blijken een positief effect te hebben op de perceptie van hoogfrequente spraakklanken. Met name de kenmerken fricatief en de hogere klinkerformanten worden beter doorgegeven. Dit klopt met de originele doelstelling. In de condities met achtergrondgeluid is de perceptie van hoogfrequente spraakklanken echter zodanig verstoord dat het gebruik van foneemcompressie geen uitkomst meer biedt. De laagfrequente spraakkenmerken worden nu belangrijker voor het spraakverstaan. Juist deze blijken te worden aangetast worden door het toepassen van anti-USOM, waarmee de in hoofdstukken 4 en 6 gevonden negatieve effecten verklaard kunnen worden. In een kleine veldstudie met experimentele digitale hoortoestellen zijn een drietal van de eerder geteste compressieconfiguraties geëvalueerd door slechthorenden (hoofdstuk 7). De foneemcompressor is opgenomen in een langzaam niet-lineair systeem waarmee de globale niveauverschillen gereduceerd kunnen worden. De programma’s zijn paarsgewijs getest over een periode van 6 weken. Vanwege praktische redenen was het toegestaan om ook de eigen hoortoestellen te blijven gebruiken. Wekelijks zijn er spraaktesten uitgevoerd, met als hoofdvraag of de prestaties beïnvloed worden door een regelmatig gebruik van de programma’s. Er worden opnieuw slechtere resultaten met compressie gevonden in achtergrondlawaai. Opmerkelijk is echter dat de spraakscores toenemen over de periode van 6 weken. Een groot deel van deze toename wordt ook gevonden voor de referentieconditie met alleen langzame compressie. Het gebruik van foneemcompressie lijkt tot een lichte extra stijging te leiden. Deze kan alleen verklaard worden door gewenning aan het luisteren met foneemcompressie. De ervaringen van de slechthorenden met de compressieprogramma’s zijn wisselend, afhankelijk van de prestaties ten opzichte van het eigen toestel. Het geluid wordt over het algemeen als positief ervaren

The relation of hearing-specific patient-reported outcome measures with speech perception measures and acceptable noise levels in cochlear implant users

Objective: To investigate the relation of a hearing-specific patient-reported outcome measure (PROM) with speech perception and noise tolerance measurements. It was hypothesised that speech intelligibility in noise and noise tolerance may explain a larger part of the variance in PROM scores than speech intelligibility in quiet.Design: This cross-sectional study used the Speech, Spatial, Qualities (SSQ) questionnaire as a PROM. Speech recognition in quiet, the Speech Reception Threshold in noise and noise tolerance as measured with the acceptable noise level (ANL) were measured with sentences.Study sample: A group of 48 unilateral post-lingual deafened cochlear implant (CI) users.Results: SSQ scores were moderately correlated with speech scores in quiet and noise, and also with ANLs. Speech scores in quiet and noise were strongly correlated. The combination of speech scores and ANL explained 10-30% of the variances in SSQ scores, with ANLs adding only 0-9%.Conclusions: The variance in the SSQ as hearing-specific PROM in CI users was not better explained by speech intelligibility in noise than by speech intelligibility in quiet, because of the remarkably strong correlation between both measures. ANLs made only a small contribution to explain the variance of the SSQ. ANLs seem to measure other aspects than the SSQ

Efficient Adaptive Speech Reception Threshold Measurements Using Stochastic Approximation Algorithms

This study examines whether speech-in-noise tests that use adaptive procedures to assess a speech reception threshold in noise (SRT50n) can be optimized using stochastic approximation (SA) methods, especially in cochlear-implant (CI) users. A simulation model was developed that simulates intelligibility scores for words from sentences in noise for both CI users and normal-hearing (NH) listeners. The model was used in Monte Carlo simulations. Four different SA algorithms were optimized for use in both groups and compared with clinically used adaptive procedures. The simulation model proved to be valid, as its results agreed very well with existing experimental data. The four optimized SA algorithms all provided an efficient estimation of the SRT50n. They were equally accurate and produced smaller standard deviations (SDs) than the clinical procedures. In CI users, SRT50n estimates had a small bias and larger SDs than in NH listeners. At least 20 sentences per condition and an initial signal-to-noise ratio below the real SRT50n were required to ensure sufficient reliability. In CI users, bias and SD became unacceptably large for a maximum speech intelligibility score in quiet below 70%. In conclusion, SA algorithms with word scoring in adaptive speech-in-noise tests are applicable to various listeners, from CI users to NH listeners. In CI users, they lead to efficient estimation of the SRT50n as long as speech intelligibility in quiet is greater than 70%. SA procedures can be considered as a valid, more efficient, and alternative to clinical adaptive procedures currently used in CI users

Optimising the effect of noise reduction algorithm ClearVoice in cochlear implant users by increasing the maximum comfort levels

Objective: ClearVoice is a single-microphone noise reduction algorithm in Advanced Bionics cochlear implant(CI) systems with the aim to improve performance in background noise. The present study investigated a hypothesised increased effect of ClearVoice if combined with a structural increase of maximum comfort stimulation levels (M-levels) in the CI fitting. Design: We tested performance with ClearVoice (Medium) in four conditions, defined by combined settings of ClearVoice off/on and with/without 5% increase of M-levels. The main outcome measures were the Acceptable Noise Level (ANL) and the speech reception threshold in continuous background noise (SRTn). Study sample: Participants were 16 experienced cochlear implant recipients with Advanced Bionics implants and a Naida Q70 processor. Results: The ANL significantly improved by using either ClearVoice or an increase of M-levels. Combining both settings gave the largest improvement in ANL. For the SRTn, we found a small, but significant interaction between ClearVoice and an increase of M-levels, implying that ClearVoice improved speech understanding slightly, but only if combined with a 5% increase of M-levels. Conclusions: Optimal profit from ClearVoice is obtained if combined with a structural 5% increase of M-levels

The Important Role of Contextual Information in Speech Perception in Cochlear Implant Users and Its Consequences in Speech Tests

This study investigated the role of contextual information in speech intelligibility, the influence of verbal working memory on the use of contextual information, and the suitability of an ecologically valid sentence test containing contextual information, compared with a CNC (Consonant-Nucleus-Consonant) word test, in cochlear implant (CI) users. Speech intelligibility performance was assessed in 50 postlingual adult CI users on sentence lists and on CNC word lists. Results were compared with a normal-hearing (NH) group. The influence of contextual information was calculated from three different context models. Working memory capacity was measured with a Reading Span Test. CI recipients made significantly more use of contextual information in recognition of CNC words and sentences than NH listeners. Their use of contextual information in sentences was related to verbal working memory capacity but not to age, indicating that the ability to use context is dependent on cognitive abilities, regardless of age. The presence of context in sentences enhanced the sensitivity to differences in sensory bottom-up information but also increased the risk of a ceiling effect. A sentence test appeared to be suitable in CI users if word scoring is used and noise is added for the best performers

Evaluation of a wireless remote microphone in bimodal cochlear implant recipients

Objective: To evaluate the benefit of a wireless remote microphone (MM) for speech recognition in noise in bimodal adult cochlear implant (CI) users both in a test setting and in daily life. Design: This prospective study measured speech reception thresholds in noise in a repeated measures design with factors including bimodal hearing and MM use. The participants also had a 3-week trial period at home with the MM. Study sample: Thirteen post-lingually deafened adult bimodal CI users. Results: A significant improvement in SRT of 5.4 dB was found between the use of the CI with the MM and the use of the CI without the MM. By also pairing the MM to the hearing aid (HA) another improvement in SRT of 2.2 dB was found compared to the situation with the MM paired to the CI alone. In daily life, participants reported better speech perception for various challenging listening situations, when using the MM in the bimodal condition. Conclusion: There is a clear advantage of bimodal listening (CI and HA) compared to CI alone when applying advanced wireless remote microphone techniques to improve speech understanding in adult bimodal CI users

Contributing Determinants to Hearing Loss in Elderly Men and Women: Results from the Population-Based Rotterdam Study

To contribute to a better understanding of the etiology in age-related hearing loss, we carried out a cross-sectional study of 3,315 participants (aged 52-99 years) in the Rotterdam Study, to analyze both low- and high-frequency hearing loss in men and women. Hearing thresholds with pure-tone audiometry were obtained, and other detailed information on a large number of possible determinants was collected. Hearing loss was associated with age, education, systolic blood pressure, diabetes mellitus, body mass index, smoking and alcohol consumption (inverse correlation). Remarkably, different associations were found for low- and high-frequency loss, as well as between men and women, suggesting that different mechanisms are involved in the etiology of age-related hearing loss

Prevalence of permanent neonatal hearing impairment: systematic review and Bayesian meta-analysis

Objective: To investigate the variance in reported prevalence rates of permanent neonatal hearing impairment (HI) worldwide. Design: A systematic review and meta-analysis was performed on reported prevalence rates of sensorineural and permanent conductive or mixed HI worse than 40 dB in neonates, detected as a result of a screening programme or audiometric study. Study sample: For meta-analysis, 35 articles were selected, 25 from high-income countries and 10 from middle-income countries according to the world bank classification system. Results: The prevalence rate of permanent uni- and bilateral HI worse than 40 dB in neonates varied from 1 to 6 per 1000, the overall prevalence was 2.21 per 1000 [1.71, 2.8]. In NICU populations the prevalence rate was higher with a larger fraction of bilateral cases. Although not significant, prevalence rates were slightly higher in Asia compared to Europe and the number of infants lost to follow-up appeared higher in countries with lower gross national income. Conclusion: Substantial variations exist in prevalence rates of neonatal permanent HI across countries and regions. There is a strong need for more data from low-income countries to identify demographic factors that account for this variability in reported prevalence rates. Reporting these data in a uniform way is advocated

Assessing hearing loss in older adults with a single question and person characteristics; Comparison with pure tone audiometry in the Rotterdam Study

INTRODUCTION: Hearing loss (HL) is a frequent problem among the elderly and has been studied in many cohort studies. However, pure tone audiometry-the gold standard-is rather time-consuming and costly for large population-based studies. We have investigated if self-reported hearing loss, using a multiple choice question, can be used to assess HL in absence of pure tone audiometry. METHODS: This study was performed within 4,906 participants of the Rotterdam Study. The question (in Dutch) that was investigated was: 'Do you have any difficulty with your hearing (without hearing aids)?'. The answer options were: 'never', 'sometimes', 'often' and 'daily'. Mild hearing loss or worse was defined as PTA0.5-4(Pure Tone Average 0.5, 1, 2 & 4 kHz) ≥20dBHL and moderate HL or worse as ≥35dBHL. A univariable linear regression model was fitted with the PTA0.5-4 and the answer to the question. Subsequently, sex, age and education were added in a multivariable linear regression model. The ability of the question to classify HL, accounting for sex, age and education, was explored through logistic regression models creating prediction estimates, which were plotted in ROC curves. RESULTS: The variance explained (R2) by the univariable regression was 0.37, which increased substantially after adding age (R2 = 0.60). The addition of sex and educational level, however, did not alter the R2 (0.61). The ability of the question to classify hearing loss, reflec

Sensorineural hearing loss and language development following neonatal extracorporeal membrane oxygenation

OBJECTIVE: To determine the prevalence of hearing loss in school-age children who have undergone neonatal extracorporeal membrane oxygenation (ECMO) treatment and to identify any effects of hearing loss on speech- and language development. DESIGN: Prospective longitudinal follow-up study within the framework of a structured post-ECMO follow-up program. SETTING: Outpatient clinic of a level III university hospital. RESULTS: Tone audiometry was performed by standardized protocol in 136 children aged 5 to 12 years. Hearing loss was considered clinically significant when >20dB. Hearing was normal in 75.7% of children. Five children (3.7%) had bilateral sensorineural or combined hearing loss; 3 of them received special audiological care (2.2% of total sample). Of the 24 children with congenital diaphragmatic hernia, 19 (79.2%) had normal hearing; and only 2 (8.3%) had mild SNHL, unilateral in one of them. Follow-up at 24 months of age had shown normal verbal and non-verbal developmental scores. Language development and intelligence median (range) scores at 5 years of age were also normal: receptive language development 104 (55-133); syntactical development 104 (68-132); and lexical development 101 (50-141) for 89 children; intelligence quotient was 104 (68-132) in 106 children. Scores did not differ between those with normal hearing, and those with mild hearing loss, or those with moderate to severe hearing loss (p=0.800, p=0.639, p=0.876, and p=0.886, for the respective developmental tests). CONCLUSIONS: We found normal language development and intelligence in a cohort of neonatal ECMO survivors. The prevalence of bilateral sensorineural hearing loss was in accordance with that of larger series in the United States – which exceeds the prevalence in the normal population