The potential binaural consequences of two envelope-based speech enhancement strategies (broadband compression and expansion) were examined. Sensitivity to interaural time differences imposed on four single-word stimuli was measured in listeners with normal hearing and sensorineural hearing loss. While there were no consistent effects of compression or expansion across all words, some potentially interesting word-specific effects were observed.
Fluctuations in the amplitude envelope play a critical role in the spatial perception of sounds. For example, listeners place increased perceptual weight on binaural cues occurring at onsets, and the steepness of onset slopes can influence binaural sensitivity. For speech stimuli, we recently showed that the temporal weighting of binaural cues is non-uniform and depends in part on the detailed behavior or the envelope. Here, we examined the potential binaural consequences of two monaural speech enhancement strategies that operate by altering the speech envelope: consonant enhancement (CE) and envelope expansion (EE). While CE and EE improve speech intelligibility under certain conditions, their effect on binaural perception has not been considered. We measured sensitivity to interaural time differences (ITDs) carried by single-word stimuli that were either unprocessed or processed with CE/EE. Thresholds were measured in quiet, in the presence of multi-talker babble, or for a vocoded condition designed to limit the availability of fine-structure ITDs. For ten listeners with normal hearing, while sensitivity depended on the specific word token, there were no systematic effects of speech enhancement on ITD sensitivity. Ongoing work is considering supra-threshold binaural tasks and listeners with hearing loss who have reduced spatial sensitivity. [Work supported by NIDCD.]
A novel speech enhancement scheme is being developed in our lab with the goal of improving speech intelligibility in “cocktail party” listening environments. By manipulating the temporal envelope to increase the salience of acoustic onsets, the algorithm improves access to binaural cues sampled at these onsets. The hope is that this will lead to a more robust spatial perception and improved streaming of competing talkers that could impact a variety of applications including hearing aids and cochlear implants. The algorithm is designed to run in real time, and in order to test its efficacy in real-world environments, it must be implemented on a portable device. For this project, the algorithm is being written in the Teensyduino software and uploaded to a Tympan Rev E DSP unit fitted with a Tympan AIC CODEC daughterboard. External speech is both captured by and played back through a pair of Tympan BTE earpieces, and all processing is performed independently for each ear by the Tympan’s Teensy 4.1 microcontroller. Assuming that the algorithm is successfully implemented, speech signals delivered to the listener will have perceptibly higher crest factors (i.e., higher transient energy) than at input. The portable, bilateral, real-time implementation developed here will allow us to evaluate any associated binaural effects and their consequences in complex real-world listening tasks.
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