Speaking in noise: How does the Lombard effect improve acoustic contrasts between speech and ambient noise?

What makes speech produced in the presence of noise (Lombard speech) more intelligible than conversational speech produced in quiet conditions? This study investigates the hypothesis that speakers modify their speech in the presence of noise in such a way that acoustic contrasts between their speech and the background noise are enhanced, which would improve speech audibility. Ten French speakers were recorded while playing an interactive game first in quiet condition, then in two types of noisy conditions with different spectral characteristics: a broadband noise (BB) and a cocktail-party noise (CKTL), both played over loudspeakers at 86dB SPL. Similarly to (Lu and Cooke, 2009b), our results suggest no systematic ''active'' adaptation of the whole speech spectrum or vocal intensity to the spectral characteristics of the ambient noise. Regardless of the type of noise, the gender or the type of speech segment, the primary strategy was to speak louder in noise, with a greater adaptation in BB noise and an emphasis on vowels rather than any type of consonants. Active strategies were evidenced, but were subtle and of second order to the primary strategy of speaking louder: for each gender, fundamental frequency (f"0) and first formant frequency (F1) were modified in cocktail-party noise in a way that optimized the release in energetic masking induced by this type of noise. Furthermore, speakers showed two additional modifications as compared to shouted speech, which therefore cannot be interpreted in terms of vocal effort only: they enhanced the modulation of their speech in f"0 and vocal intensity and they boosted their speech spectrum specifically around 3kHz, in the region of maximum ear sensitivity associated with the actor's or singer's formant.