# Improved tactile speech perception and noise robustness using audio-to-tactile sensory substitution with amplitude envelope expansion

## TL;DR

This study shows that expanding the amplitude envelope in audio-to-tactile conversion improves speech perception, especially in noisy environments, for haptic hearing aids.

## Contribution

The study introduces multi-band amplitude envelope expansion to enhance vowel features and improve noise robustness in tactile speech perception.

## Key findings

- Envelope expansion improved vowel discrimination by 10.3% in quiet environments.
- Overall phoneme discrimination improved by 9.6% in noisy conditions with envelope expansion.
- The improvement was consistent for both vowels and consonants in noise.

## Abstract

Recent advances in haptic technology could allow haptic hearing aids, which convert audio to tactile stimulation, to become viable for supporting people with hearing loss. A tactile vocoder strategy for audio-to-tactile conversion, which exploits these advances, has recently shown significant promise. In this strategy, the amplitude envelope is extracted from several audio frequency bands and used to modulate the amplitude of a set of vibro-tactile tones. The vocoder strategy allows good consonant discrimination, but vowel discrimination is poor and the strategy is susceptible to background noise. In the current study, we assessed whether multi-band amplitude envelope expansion can effectively enhance critical vowel features, such as formants, and improve speech extraction from noise. In 32 participants with normal touch perception, tactile-only phoneme discrimination with and without envelope expansion was assessed both in quiet and in background noise. Envelope expansion improved performance in quiet by 10.3% for vowels and by 5.9% for consonants. In noise, envelope expansion improved overall phoneme discrimination by 9.6%, with no difference in benefit between consonants and vowels. The tactile vocoder with envelope expansion can be deployed in real-time on a compact device and could substantially improve clinical outcomes for a new generation of haptic hearing aids.