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Speech produced in the context of real or imagined communicative difficulties is characterized by hyperarticulation. Phonological neighborhood density (ND) conditions similar patterns in production: Words with many neighbors are hyperarticulated relative to words with fewer; Hi ND words also show greater coarticulation than Lo ND words e.g., Scarborough, R. (2012). "Lexical similarity and speech production: Neighborhoods for nonwords," Lingua 122(2), 164-176. Coarticulatory properties of "clear speech" are more variable across studies. This study examined hyperarticulation and nasal coarticulation across five real and simulated clear speech contexts and two neighborhood conditions, and investigated consequences of these details for word perception. The data revealed a continuum of (attempted) clarity, though real listener-directed speech (Real) differed from all of the simulated styles. Like the clearest simulated-context speech (spoken "as if to someone hard-of-hearing"-HOH), Real had greater hyperarticulation than other conditions. However, Real had the greatest coarticulatory nasality while HOH had the least. Lexical decisions were faster for words from Real than from HOH, indicating that speech produced in real communicative contexts (with hyperarticulation and increased coarticulation) was perceptually better than simulated clear speech. Hi ND words patterned with Real in production, and Real Hi ND words were clear enough to overcome the dense neighborhood disadvantage.
Scarborough et al. (Fri,) studied this question.