Impact of Different Speech Types on Listening Effort

Listeners are exposed to different types of speech in everyday life, from natural speech to speech that has undergone modifications or has been generated synthetically. While many studies have focused on measuring the intelligibility of these distinct speech types, their impact on listening effort is not known. The current study combined an objective measure of intelligibility, a physiological measure of listening effort (pupil size) and listeners’ subjective judgements, to examine the impact of four speech types: plain (natural) speech, speech produced in noise (Lombard speech), speech enhanced to promote intelligibility, and synthetic speech. For each speech type, listeners responded to sentences presented in one of three levels of speech-shaped noise. Subjective effort ratings and intelligibility scores showed an inverse ranking across speech types, with synthetic speech being the most demanding and enhanced speech the least. Pupil size measures indicated an increase in listening effort with decreasing signal-to-noise ratio for all speech types apart from synthetic speech, which required significantly more effort at the most favourable noise level. Naturally and artificially modified speech were less effortful than plain speech at the more adverse noise levels. These outcomes indicate a clear impact of speech type on the cognitive demands required for comprehension.