Recording and classification of the acoustic environment of hearing aid users.

This article investigates the different acoustic signals that hearing aid users are exposed to in their everyday environment. Binaural microphone signals from recording positions close to the microphone locations of behind-the-ear hearing aids were recorded by 20 hearing aid users during daily life. The recorded signals were acoustically analyzed with regard to narrowband short-term level distributions. The subjects also performed subjective assessments of their own recordings in the laboratory using several questions from the Glasgow Hearing Aid Benefit Profile (GHABP) questionnaire. Both the questionnaire and the acoustic analysis data show that the importance, problems, and hearing aid benefit as well as the acoustic characteristics of the individual situations vary a lot across subjects. Therefore, in addition to a nonlinear hearing aid fitting, further signal classification and signal/situation-adaptive features are highly desirable inside modern hearing aids. These should be compatible with the variability of the individual sound environments of hearing-impaired listeners.

Performance of a fully adaptive directional microphone to signals presented from various azimuths.

The signal-to-noise ratio advantage of a directional microphone is achieved by reducing the sensitivity of the microphone to sounds from the sides and back. A fully adaptive directional microphone (one that automatically switches between an omnidirectional mode and various directional polar patterns) may allow the achievement of signal-to-noise (SNR) improvement with minimal loss on audibility to sounds that originate from the sides and back. To demonstrate such possibilities, this study compared the soundfield aided thresholds, speech in quiet at different input levels, and speech in noise performance of 17 hearing-impaired participants under three microphone modes (omnidirectional, fixed hypercardioid, and fully [or automatic] adaptive) as the stimuli were presented from 0 degrees to 180 degrees in 45 degrees intervals. The results showed a significant azimuth effect only with the fixed directional microphone. In quiet, the fully adaptive microphone performed similarly as the omnidirectional microphone at all frequencies, input levels, and azimuths. In noise, the fully adaptive microphone achieved similar SNR improvement as the fixed directional microphone. Clinical implications of the results of this study were discussed.

Reconsidering the concept of the aided threshold for nonlinear hearing AIDS.

The aided threshold (and functional gain) has been discussed in the context of linear hearing aids since the early 1960s. The use of nonlinear hearing aids, however, could change the meaningfulness of this verification tool because of their unique characteristics. The interpretation of the aided threshold (and functional gain) as it pertains to linear and nonlinear hearing aids is reviewed. Also discussed are the ideas of an optimal aided threshold, factors that may affect its magnitude, and a comparison between functional gain and insertion gain measures. Finally, how to improve the accuracy of the aided thresholds (and functional gain) through the use of in-situ unaided threshold measurements is discussed.

Prediction of Intelligibility of Non-linearly Processed Speech.

Speech in various noise backgrounds was processed through four different non-linear devices and the intelligibility of the processed signals was predicted from the Speech Transmission Index (STI). A novel calculation method was applied in order to avoid artifacts. Running speech was used as input signal and STI was calculated from the envelopes of the squared, noise-free speech signal and of the processed, squared, noisy signal in 23 critical bands. In situations with linearly processed speech and a stationary background noise, this calculation method gives results identical with the procedure described by Steeneken & Houtgast (6). However, in a number of situations with non-linearly processed speech, or a time varying background noise level, the calculation method used here is preferable. The predictions were evaluated in a psycho-acoustic listening test and the predictions agreed well with the listening test results.