How does foreigner-directed speech differ from other forms of listener-directed clear speaking styles?

Forty talkers participated in problem-solving tasks with another talker in conditions differing in communication difficulty for the interlocutor. A linguistic barrier condition (L2 interlocutor) was compared to acoustic barrier conditions (native interlocutors hearing vocoded or noisy speech). Talkers made acoustic-phonetic enhancements in all barrier conditions compared to the no-barrier condition, but talkers reduced their articulation rate less and showed less increase in vowel hyperarticulation in foreigner-directed speech than in the acoustic barrier condition, even though communicative difficulty was greater in the L2 condition. Foreigner-directed speech was also perceived as less clear. This suggests that acoustic enhancements in clear speech are not simply a function of the level of communication difficulty.