9. Adaptive Dynamic Range Compression for Improving Envelope-Based Speech Perception: Implications for Cochlear Implants

The temporal envelope is the primary acoustic cue used in most cochlear implant (CI) devices for eliciting speech perception in implanted patients. Due to biological constraints, a compression scheme is required to adjust the wide dynamic range (DR) of input signals to a desirable level. Static envelope compression (SEC) is a well-known strategy used in CI speech processing, where a fixed compression ratio is adopted to narrow the envelope DR. More recently, a novel adaptive envelope compression (AEC) strategy has been proposed. In contrast to the SEC strategy, the AEC strategy more effectively enhances the modulation depth of the envelope waveforms to make the best use of the DR, in order to achieve higher intelligibility of envelope-based speech. In this chapter, we first introduce the theory of and implementation procedures for the AEC strategy. Then, we present four sets of experiments that were designed to evaluate the performance of the AEC strategy. In the first and second experiments, we investigated AEC performance under two types of challenging listening conditions: noisy and reverberant. In the third experiment, we explore the correlation between the adaptation rate using the AEC strategy and the intelligibility of envelope-compressed speech. In the fourth experiment, we investigated the compatibility of the AEC strategy with a noise reduction (NR) method, which is another important facet of a CI device. The AEC-processed sentences could provide higher intelligibility scores under challenging listening conditions than the SEC-processed sentences. Moreover, the adaptation rate was an important factor in the AEC strategy for producing envelope-compressed speech with optimal intelligibility. Finally, the AEC strategy could be integrated with NR methods to enhance speech intelligibility scores under noisy conditions further. The results from the four experiments imply that the AEC strategy has great potential to provide better speech perception performance than the SEC strategy, and can thus be suitably adopted in CI speech processors.