C-50 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 C-50 values was determined for speech sounds in simulated sound fields. Over a range of C-50 values from -3 to +9dB, 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 C-50 by approximately 3 dB to create a readily detectable improvement in everyday situations.