Clarity, C80 compares the amount of sound energy arriving within the first 80 ms to the total amount of energy until it ceases. In simple words, C80 compares the early with the late energy.
This parameter is based on the fact that later reflections in the room acoustics have a greater “sound-blurring effect than early reflections”. The sound will be experienced as less clear, when later reflections are more dominant in the space than early reflections. Therefore, Clarity typically grows in opposite direction than Reverberation time (RT). Lower C80 is a result of longer RT, while higher C80 means shorter RT. It should be noted that much acoustical research has been made for classical music, but not for rhythmical music (pop, rock, etc.). Therefore these findings might not apply the same way.
Typical range for C80 in concert halls is -1 to 3 dB.
Definition (D50) is very similar to Clarity, but focuses on an even earlier part of the response, up to 50 ms.