Control of feedback for assistive listening devices

Acoustic feedback refers to the undesired acoustic coupling between the loudspeaker and microphone in hearing aids. This feedback channel poses limitations to the normal operation of hearing aids under varying acoustic scenarios. This work makes contributions to improve the performance of adaptive feedback cancellation techniques and speech quality in hearing aids. For this purpose a two microphone approach is proposed and analysed; and probe signal injection methods are also investigated and improved upon

Engineering data compendium. Human perception and performance. User's guide

The concept underlying the Engineering Data Compendium was the product of a research and development program (Integrated Perceptual Information for Designers project) aimed at facilitating the application of basic research findings in human performance to the design and military crew systems. The principal objective was to develop a workable strategy for: (1) identifying and distilling information of potential value to system design from the existing research literature, and (2) presenting this technical information in a way that would aid its accessibility, interpretability, and applicability by systems designers. The present four volumes of the Engineering Data Compendium represent the first implementation of this strategy. This is the first volume, the User's Guide, containing a description of the program and instructions for its use

Psychoacoustic measurements of bone conducted sound

Bone-conduction hearing aids (BCHAs) are a widely used method of treating conductive hearing loss, but the benefit of bilateral implantation is severely limited due to interaural cross-talk. In theory two BCHAs could deliver improved stereo separation using cross-talk cancellation. Sound vibrations from each BCHA would be cancelled at the contralateral cochlea by an out-of-phase signal of the same level from the psilateral BCHA. In order to achieve this the phase and level of sound at each cochlea needs to be known. A method to measure the level and phase required for these cancellation signals was developed and cross-validated with a second technique that combines air- and bone-conducted sound in normal hearing subjects. Levels measured with each method differed by <1 dB between 3-5 kHz. The phase results also corresponded well for the cancelled ear (11° mean difference). The newly developed method using only bone transducers is potentially transferable to a clinical population. To demonstrate cross-talk cancellation tone and speech reception thresholds (TRT and SRT) were investigated with and without unilateral cross-talk cancellation. Band limited noise was emitted from one BT whilst signal +/- cancellation signal was produced by the other. Benefits of cross-talk cancellation under this atypical listening situation were found to be 12.08 and 13.7 dB for TRT and SRT thresholds. In order to estimate the potential benefits of cross-talk cancellation in spatially realistic environments, phase and level elements of impulse responses from a BAHA 4 were convolved with speech. This found that cross-talk cancellation had the potential to lower SRTs in a clinical population by approximately 4.4 dB. Future work will focus on real-time processing and examine using a clinical population

Binaural hearing with bone conduction stimulation

It has been argued that apparent masking-level differences (MLDs) in users of bilateral bone-anchored hearing aids (BAHAs) provide evidence of binaural hearing. However, there is considerably less acoustical isolation between the two ears with bone conduction (BC) compared to air conduction (AC). The apparent MLDs may have arisen, at least in part, from inter-cranial interference between signals arising from the two BAHAs (i.e. monaural effect). That might also explain some of the inter-individual variation in both the magnitude and the direction of the MLDs reported in BAHA users. The present study was composed of three experimental stages with the main aim to investigate the influence of interference in normal hearing participants by measuring masking level difference in AC and BC to explore the conditions contributing to the reported variation. An additional aim was to investigate the performance of a newly designed BC transducer; the balanced electromagnetic separation transducer (BEST), for bone conduction research as well as more general clinical use.Stage 1 evaluated the performance of the BEST in comparison to the clinically used RadioEar B71 in a series of acoustical (sensitivity and harmonic distortion) and psychoacoustical (hearing thresholds and vibrotactile thresholds) measurements. The results from these studies led to the use of the BEST in the second and third stages because they produced significantly lower harmonic distortion at low frequencies (mainly 250 Hz). The psychoacoustic measurements alluded to the need to use different calibration values with the BESTs.Stage 2 was a preliminary investigation comparing the MLDs with standard bilateral configurations between the AC and BC in nine normal-hearing participants. Signals were pure tones at one of three frequencies (250, 500, 1000 Hz), presented via AC or BC. Broadband noise (100- 5000 Hz) was always presented via AC at 70 dB SPL. Thresholds were estimated using a three-alternative forced choice procedure combined with an adaptive staircase. Transducers used were insert earphones and the BESTs for BC testing. The results from this stage showed a statistical significant difference between AC and BC MLDs at 250, 500 and 1000 Hz (mean difference is 9.4, 6.6 and 3.5 dB respectively). Evidence of the change in the MLDs direction is observed at 250 Hz in three participants.Stage 3 consisted of the investigation of inter-cranial interference in eighteen normal hearing participants. This stage was composed of three main measurements. The first measurement compared the AC and BC MLDs at three test frequencies. The second measurement evaluated the transcranial attenuation (TA). The third measurement was the novel feature of the study it evaluated the monaural interference effect through the measurement of the diotic and dichotic conditions in one test ear. A significant discrepancy was found between the AC and BC MLDs of approximately 6, 1.5 and 2.5 dB at 500, 1000 and 2000 Hz, respectively. The TA was found to be lower than 10 dB at the three test frequencies. Measurable MTLDs were reported in some of the participants, high inter-subject variability was observed in the direction of the MTLDs.The BEST can reliably replace the B71 in clinical setup. Formal adjustment of the reference equivalent threshold force levels is advised. Binaural hearing was achieved through bilateral BC stimulation to a lesser magnitude compared to AC MLDs in normal hearing participants. The discrepancy between the AC and BC MLDs was reduced with the increase in the frequency. The discrepancy can partially be explained by the cross-talk of the signal in one ear. The results showed that in some participants the magnitude of the monaural tone level difference was similar to the magnitude of the BC MLD. Further investigation is recommended to investigate the association of the transcranial delay with the discrepancy between the AC and BC MLDs. This investigation also recommends the investigation of the AC and BC MLDs in patients fitted with bilateral BAHAs

Understanding hearing aid sound quality for music-listening

To improve speech intelligibility for individuals with hearing loss, hearing aids amplify speech using gains derived from evidence-based prescriptive methods, in addition to other advanced signal processing mechanisms. While the evidence supports the use of hearing aid signal processing for speech intelligibility, these signal processing adjustments can also be detrimental to hearing aid sound quality, with poor hearing aid sound quality cited as a barrier to device adoption. Poor sound quality is also of concern for music-listening, in which intelligibility is likely not a consideration. A series of electroacoustic and behavioural studies were conducted to study sound quality issues in hearing aids, with a focus on music. An objective sound quality metric was validated for real hearing aid fittings, enabling researchers to predict sound quality impacts of signal processing adjustments. Qualitative interviews with hearing aid user musicians revealed that users’ primary concern was understanding the conductor’s speech during rehearsals, with hearing aid music sound quality issues a secondary concern. However, reported sound quality issues were consistent with music-listening sound quality complaints in the literature. Therefore, follow-up experiments focused on sound quality issues. An examination of different manufacturers’ hearing aids revealed significant music sound quality preferences for some devices over others. Electroacoustic measurements on these devices revealed that bass content varied more between devices than levels in other spectral ranges or nonlinearity, and increased bass levels were most associated with improved sound quality ratings. In a sound quality optimization study, listeners increased the bass and reduced the treble relative to typically-prescribed gains, for both speech and music. However, adjustments were smaller in magnitude for speech compared to music because they were also associated with a decline in speech intelligibility. These findings encourage the increase of bass and reduction of treble to improve hearing aid music sound quality, but only to the degree that speech intelligibility is not compromised. Future research is needed on the prediction of hearing aid music quality, the provision of low-frequency gain in open-fit hearing aids, genre-specific adjustments, hearing aid compression and music, and direct-to-consumer technology

Uma proposta para a redução do efeito de oclusão em aparelhos auditivos

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Elétrica, Florianópolis, 2017.A deficiência auditiva, também conhecida como surdez, consiste na perda parcial ou total da capacidade de audição de um indivíduo. De forma geral, essa limitação pode ser compensada pela utilização de aparelhos auditivos. Esses dispositivos são projetados não apenas para amplificar o campo acústico nas adjacências do ouvido, mas também para aumentar a inteligibilidade e o conforto acústico do usuário. Apesar do avanço da tecnologia digital, estudos recentes relatam a insatisfação dos usuários com relação a diferentes aspectos, entre eles: a amplificação do ruído ambiente, a realimentação acústica e o efeito de oclusão. Essas características tendem a reduzir o tempo de uso diário do aparelho pelos usuários.A realimentação acústica decorre do acoplamento acústico entre o alto-falante e o microfone do aparelho auditivo e se manifesta na forma de um apito estridente que causa desconforto físico ao usuário, podendo ser ouvido por pessoas no seu entorno. A prevenção da realimentação acústica usualmente é realizada através da limitação do ganho máximo do dispositivo, da utilização de canceladores ativos de realimentação ou do estreitamento ou fechamento do canal auditivo por um molde (de forma a aumentar a impedância acústica entre alto-falante e microfone). Embora o estreitamento ou fechamento do canal auditivo seja o método mais usual e efetivo, sua utilização resulta no chamado efeito de oclusão. O efeito de oclusão ocorre quando o aparelho auditivo, posicionado na porção cartilaginosa do canal, possui uma abertura de ventilação com tamanho insuficiente para promover a necessária dissipação de energia sonora conduzida ao canal auditivo através do crânio e da mandíbula, causando aumento significativo de potência em baixas frequências (predominantemente na faixa de 200 a 500 Hz) e fazendo com que o usuário ouça sua própria voz de forma abafada. Este trabalho apresenta, inicialmente, a análise de um sistema de cancelamento de realimentação na presença do efeito de oclusão. Simulações para validação do modelo mostraram que o efeito de oclusão não altera o desempenho do sistema de cancelamento. Como contribuição principal, um novo sistema adaptativo de controle ativo de ruído para reduzir o efeito de oclusão em aparelhos auditivos sem duto de ventilação é proposto. Em contraste com os canceladores de efeito de oclusão previamente desenvolvidos, esse sistema oferece uma estrutura de cancelamento não realimentada, que permite a análise de seu comportamento como um problema de identificação de um filtro linear com resposta ao impulso finita. Equações recursivas determinísticas foram derivadas para predição do erro quadrático médio e comportamento médio dos coeficientes, tanto para transitório como em regime permanente. Tais modelos são de particular interesse para os projetistas de aparelhos auditivos como ferramentas de guia para definir parâmetros ótimos, de forma a obter o desempenho desejado. As simulações computacionais concordam com as predições teóricas obtidas pelas equações derivadas, indicando uma redução média de 5,4 dB do efeito de oclusão na faixa de 200-500 Hz. Experimentos subjetivos corroboram a funcionalidade da arquitetura proposta.Abstract : Hearing loss is the partial or total loss of hearing ability of an individual. In general, this limitation can be compensated by the use of hearing aids. These devices are designed not only to amplify the sound field in the vicinity of the ear, but also to increase the intelligibility and acoustic comfort of the user. Even with the advancement of digital technology, recent studies have reported the dissatisfaction of users with respect to several aspects, including: environmental noise amplification, acoustic feedback and occlusion effect. Such characteristics tend to reduce the daily use of the device by users.The acoustic feedback results from the acoustic coupling between the loudspeaker and the microphone of the hearing aid, and is manifested as a shrill whistle that causes physical discomfort to the user and it is perceived by the user and people around as an unpleasant sound. Prevention of acoustic feedback is usually accomplished by limiting the maximum gain of the device, the use of active feedback cancellers or by narrowing or closing of the auditory canal by a mould (in order to increase the acoustic impedance between loudspeaker and microphone). The narrowing or closure of the auditory canal is the most common and effective method, but its use reinforces the occlusion perception. The occlusion effect occurs when the earmould, placed in the cartilaginous part of the ear canal, has a ventilation oppening insufficient to dissipate the necessary acoustical energy conducted to the ear canal through the skull and jaw, resulting in a power increase at low frequencies (predominantly in the range of 200 to 500 Hz), leading the user to perceive a muffled version of his own voice.This work presents, initially, an analysis of a feedback system cancellation in the presence of the occlusion effect. The predicted results of the mean weight behaviour are compared with simulations to show that the occlusion effect does not affect the feedback system performance. Following, a new adaptive active-noise-control system to reduce the occlusion effect in unvented hearing aids is proposed. In contrast to previously developed occlusion-effect cancellers, this system is based on a feedforward cancelling structure that permits the analysis of its behaviour as a finite-impulse-response linear-filter identification problem. Deterministic recursive equations were derived with the aim to theoretically predict its mean square error and mean coefficient behaviour, both in transient and steady state conditions. Such models are of particular interest to hearing aid designers as guide tools for setting parameters to obtain a desired performance. Computational simulations accurately agree with theoretical predictions obtained by the derived equations, indicating a mean reduction of 5.4 dB of the occlusion effect in the range of 200?500 Hz. Subjective experiments corroborate the functionality of the proposed architecture