Approaching speech intelligibility enhancement with inspiration from Lombard and Clear speaking styles

Abstract

Lombard and Clear speech represent two acoustically and perceptually distinct speaking styles that humans employ to increase intelligibility. For Lombard speech, increased spectral energy in a band spanning the range of formants is consistent, effectively augmenting loudness, while vowel space expansion is exhibited in Clear speech, indicating greater articulation. On the other hand, analyses in the first part of this work illustrate that Clear speech does not exhibit significant spectral energy boosting, nor does the Lombard effect invoke an expansion of vowel space. Accordingly, though these two acoustic phenomena are largely attributed with the respective intelligibility gains of the styles, present analyses would suggest that they are mutually exclusive in human speech production. However, these phenomena can be used to inspire signal processing algorithms that seek to exploit and ultimately compound their respective intelligibility gains, as is explored in the second part of this work. While Lombard-inspired spectral shaping has been shown to successfully increase intelligibility, Clear speech-inspired modifications to expand vowel space are rarely explored. With this in mind, the latter part of this work focuses mainly on a novel frequency warping technique that is shown to achieve vowel space expansion. The frequency warping is then incorporated into an established Lombard-inspired Spectral Shaping method that pairs with dynamic range compression to maximize speech audibility (SSDRC). Finally, objective and subjective evaluations are presented in order to assess and compare the intelligibility gains of the different styles and their inspired modifications.