Tim Van den Bogaert

Horizontal localization with bilateral hearing aids: Without is better than with

This paper studies the effect of bilateral hearing aids on directional hearing in the frontal horizontal plane. Localization tests evaluated bilateral hearing aid users using different stimuli and different noise scenarios. Normal hearing subjects were used as a reference. The main research questions raised in this paper are: (i) How do bilateral hearing aid users perform on a localization task, relative to normal hearing subjects? (ii) Do bilateral hearing aids preserve localization cues, and (iii) Is there an influence of state of the art noise reduction algorithms, more in particular an adaptive directional microphone configuration, on localization performance? The hearing aid users were tested without and with their hearing aids, using both a standard omnidirectional microphone configuration and an adaptive directional microphone configuration. The following main conclusions are drawn. (i) Bilateral hearing aid users perform worse than normal hearing subjects in a localization task, although more than one-half of the subjects reach normal hearing performance when tested unaided. For both groups, localization performance drops significantly when acoustical scenarios become more complex. (ii) Bilateral, i.e., independently operating hearing aids do not preserve localization cues. (iii) Overall, adaptive directional noise reduction can have an additional and significant negative impact on localization performance.

Speech enhancement with multichannel Wiener filter techniques in multimicrophone binaural hearing aids

This paper evaluates speech enhancement in binaural multimicrophone hearing aids by noise reduction algorithms based on the multichannel Wiener filter (MWF) and the MWF with partial noise estimate (MWF-N). Both algorithms are specifically developed to combine noise reduction with the preservation of binaural cues. Objective and perceptual evaluations were performed with different speech-in-multitalker-babble configurations in two different acoustic environments. The main conclusions are as follows: (a) A bilateral MWF with perfect voice activity detection equals or outperforms a bilateral adaptive directional microphone in terms of speech enhancement while preserving the binaural cues of the speech component. (b) A significant gain in speech enhancement is found when transmitting one contralateral microphone signal to the MWF active at the ipsilateral hearing aid. Adding a second contralateral microphone showed a significant improvement during the objective evaluations but not in the subset of scenarios tested during the perceptual evaluations. (c) Adding the partial noise estimate to the MWF, done to improve the spatial awareness of the hearing aid user, reduces the amount of speech enhancement in a limited way. In some conditions the MWF-N even outperformed the MWF possibly due to an improved spatial release from masking.

Sound localization, sound lateralization, and binaural masking level differences in young children with normal hearing.

Objectives: In this study, procedures for measuring sound localization, sound lateralization, and binaural masking level differences (BMLDs) in young children were developed. Sensitivity for these tasks was assessed in large groups of children between 4 and 9 yr of age to investigate potential developmental trends. Design: Sound localization was measured in the sound field, with a broadband bell-ring presented from one of nine loudspeakers positioned in the frontal horizontal field. A group of 33 children between 4 and 6 yr of age and 5 adults took part in this experiment. Sound lateralization based on interaural time differences was measured with headphones in 49 children between 4 and 9 yr of age and 10 adults. A low-frequency stimulus containing harmonics 2 to 5 from a click train with a rate of 160 Hz was used. In the BMLD test, the same filtered click train was presented diotically or dichotically (phase reversed or time delayed) in a broadband (200 to 1000 Hz) frozen noise to 23 children between 4 and 6 yr of age and 10 adults. For comparison with literature, additional measurements with a 500-Hz sinusoid were administered to adults. All tasks were adapted to the interest and attention span of young children. Results: Children of 5 yr of age did not perform significantly different from adults on the sound localization task, but mean absolute errors were larger for the 4-yr-olds. Also on the BMLD task, 5-yr-old children performed at the adult level, whereas the 4-yr-old children obtained significantly less binaural unmasking compared with the adults. Concerning sound lateralization, a small but significant difference between adults and children existed, but no age effects were apparent in the 4- to 9-yr-old group. Overall, the variation was relatively large in the 4-yr-old group, with some of the children performing at adult level, in all three tasks. Conclusions: The results of this study show that the modified procedures are suitable for testing children from the age of 4 to 5 yr. Furthermore, it seems that binaural hearing capacities of the 5-yr-olds are similar to those of adults. Several observations led to the hypothesis that the observed age differences between 4-yr-olds and older subjects on localization and BMLD or between those 4- to 9-yr old and adults on lateralization, were attributable to both a development in binaural hearing and to nonauditory factors, such as task comprehension, attention, and testing conditions. It is possible that the developmental process is more obvious and prolonged in other aspects of binaural hearing, which require more dynamic or more central processing.

The effect of multimicrophone noise reduction systems on sound source localization by users of binaural hearing aids

This paper evaluates the influence of three multimicrophone noise reduction algorithms on the ability to localize sound sources. Two recently developed noise reduction techniques for binaural hearing aids were evaluated, namely, the binaural multichannel Wiener filter (MWF) and the binaural multichannel Wiener filter with partial noise estimate (MWF-N), together with a dual-monaural adaptive directional microphone (ADM), which is a widely used noise reduction approach in commercial hearing aids. The influence of the different algorithms on perceived sound source localization and their noise reduction performance was evaluated. It is shown that noise reduction algorithms can have a large influence on localization and that (a) the ADM only preserves localization in the forward direction over azimuths where limited or no noise reduction is obtained; (b) the MWF preserves localization of the target speech component but may distort localization of the noise component. The latter is dependent on signal-to-noise ratio and masking effects; (c) the MWF-N enables correct localization of both the speech and the noise components; (d) the statistical Wiener filter approach introduces a better combination of sound source localization and noise reduction performance than the ADM approach.

Sound source localization using hearing aids with microphones placed behind-the-ear, in-the-canal, and in-the-pinna.

Abstract Objective: The effect of different commercial hearing aids on the ability to resolve front-back confusions and on sound localization in the frontal horizontal and vertical plane was studied. Design: Commercial hearing aids with a microphone placed in-the-ear-canal (ITC), behind-the-ear (BTE), and in-the-pinna (ITP) were evaluated in the frontal and full horizontal plane, and in the frontal vertical plane. Study Sample: A group of 13 hearing-impaired subjects evaluated the hearing aids. Nine normal-hearing listeners were used as a reference group. Results and Conclusions: Differences in sound localization in the front-back dimension were found for different hearing aids. A large inter-subject variability was found during the front-back and elevation experiments. With ITP or ITC microphones, almost all natural spectral information was preserved. One of the BTE hearing aids, which is equipped with a directional microphone configuration, generated a sufficient amount of spectral cues to allow front-back discrimination. No significant effect of hearing aids on elevation performance in the frontal vertical plane was observed. Hearing-impaired subjects reached the same performance with and without the different hearing aids. In the unaided condition, a frequency-specific audibility correction was applied. Some of the hearing-impaired listeners reached normal hearing performance with this correction. Sumario Objetivo: Se estudio el efecto de diferentes auxiliares auditivos sobre la habilidad para resolver confusiones en dirección antero-posterior y en la localización del sonido en los planos fronto-horizontal y vertical. Diseño: Se analizaron auxiliares auditivos comerciales que tienen un micrófono localizado dentro del canal auditivo (ITC) y en la oreja (ITP) en el plano frontal, el horizontal y en el plano fronto-vertical. Muestra: Un grupo de 13 sujetos hipoacúsicos evaluaron los auxiliares auditivos. Nueve sujetos normoacúsicos sirvieron como grupo de referencia. Resultados y conclusiones: Se encontraron diferencias en la localización del sonido en la dimensión fronto-posterior con varios auxiliares auditivos. Se encontró una gran variabilidad inter-sujeto en los experimentos de dirección antero-posterior y de elevación. Uno de los auxiliares auditivos tipo curveta que está equipado con un micrófono direccional generó una cantidad suficiente de claves espectrales para poder discriminar en el plano antero-posterior. No se observó un efecto significativo de los auxiliares auditivos en el desempeño por elevación en el plano vertical. Los sujetos hipoacúsicos alcanzaron el mismo desempeño con y sin los diferentes auxiliares auditivos. Cuando los usaban se aplicó una corrección de audibilidad específica en frecuencia. Algunos de lo sujetos hipoacúsicos alcanzaron un desempeño de audición normal con esta corrección.

Comparison of Reduced-Bandwidth MWF-Based Noise Reduction Algorithms for Binaural Hearing Aids

In a binaural hearing aid noise reduction system, binaural output signals are generated by sharing information between the two hearing aids. When each hearing aid has multiple microphones and all microphone signals are transmitted between the hearing aids, a significant noise reduction can be achieved using the binaural multi-channel Wiener filter (MWF). To limit the number of signals being transmitted between the hearing aids, in order to comply with bandwidth constraints of the binaural link, this paper presents reduced-bandwidth MWF-based algorithms, where each hearing aid uses only a filtered combination of the contralateral microphone signals. One algorithm uses the output of a monaural MWF on the contralateral microphone signals, whereas a second algorithm involves a distributed binaural MWF scheme. Experimental results compare the performance of the presented algorithms.

Amplification of interaural level differences improves sound localization in acoustic simulations of bimodal hearing

Users of a cochlear implant and contralateral hearing aid are sensitive to interaural level differences (ILDs). However, when using their clinical devices, most of these subjects cannot use ILD cues for localization in the horizontal plane. This is partly due to a lack of high-frequency residual hearing in the acoustically stimulated ear. Using acoustic simulations of a cochlear implant and hearing loss, it is shown that localization performance can be improved by up to 14 degrees rms error relative to 48 degrees rms error for broadband noise by artificially introducing ILD cues in the low frequencies. The algorithm that was used for ILD introduction is described.

The use of virtual acoustics in the evaluation and development of binaural hearing aid algorithms

The development of noise reduction algorithms for hearing aids (HA) is not longer only related to the improvement of signal to noise ratio, but also to the quality of hearing, e.g. binaural aspects of hearing. This is very important for the recognition of the localization of sound sources but also for an improved speech intelligibility in noisy situations due to spatial release from masking eects. New design and signal processing algorithms for binaural HA’s need to be tested and validated in dierent acoustical scenarios. As it is too laborious and time consuming to perform sucient numbers of perceptual evaluations in dierent rooms with dierent acoustical parameters, advanced acoustic modeling of dierent virtual acoustical environments might be needed. Virtual acoustics in our research relates to the convolution of the measured or simulated binaural signals (head related transfer functions - HRTF’s) with the impulse response generated from a computer model of a room (using ODEON R ∞ software) to simulate binaural sounds. This study investigates the usage of virtual acoustics in the framework of developing algorithms for binaural hearing aids. It evaluates and quantifies the fidelity of binaural signals generated by commercially available virtual acoustics software with respect to the localization of sound and speech intelligibility in dierent acoustical scenarios.

Speech‐in‐noise enhancement and sound localization with improved binaural hearing instruments

Multi-microphone noise reduction schemes have become standard in commercial hearing aids and cochlear implants. Recent studies with bilateral hearing aids have shown that common adaptive directional microphone systems tend to distort localization cues, leading to inappropriate and reduced spatial awareness for bilateral hearing aid users. Here we show that binaural multi-microphone signal processing based on multi-channel Wiener filter (MWF) are capable of combining noise reduction with the preservation of directional hearing. Physical simulations and perceptual results from 10 listeners have been studied for dierent noise source scenarios, in dierent reverberant conditions, and for a number of signal processing schemes using up to 4 microphone inputs (2 each side). An overview is given of the localization performance and the speech reception benefits in these dierent listening conditions for the dierent noise reduction strategies. An adaptive directional microphone system (ADM) is used as a reference system. Signal processing based on MWF does, unlike ADM, provides a combination of noise reduction and preservation of spatial awareness. Moreover, in some conditions it even oers an improved spatial release from masking. The MWF outperforms the ADM in terms of localization and noise reduction if signals are not arriving from the most forward field of view.