John S Bradley, Rebecca D Reich, and S.G. Norcross (1999)
A just noticeable difference in C50 for speech
Applied Acoustics, 58(2):99–108.
C50 is an early-to-late arriving sound ratio used to assess the influence of room acoustics on the clarity and intelligibility of speech. A just noticeable difference in C50 values was determined for speech sounds in simulated sound fields. Over a range of C50 values from −3 to +9 dB, representing most situations in rooms for speech, a just noticeable difference was estimated to be 1.1 dB. The corresponding just noticeable difference in Speech Transmission Index (STI) values was 0.03. This is similar to previous related estimates for speech and musical signals. To improve the acoustical characteristics of a room for speech, it is probably necessary to increase C50 by approximately 3 dB to create a readily detectable improvement in everyday situations.