C50 (Clarity)

C50 is the Clarity Index for speech, defined as the logarithmic ratio of early sound energy (arriving within the first 50 milliseconds) to late sound energy (arriving after 50 ms), expressed in decibels. Specified in ISO 3382-1:2009 §4.3, C50 is directly related to speech intelligibility: higher values indicate clearer, more intelligible speech. The 50 ms limit corresponds to the temporal integration window for speech perception, within which reflections reinforce rather than degrade the direct sound. C50 values above 0 dB generally indicate acceptable speech clarity, with values above +2 dB preferred for classrooms and lecture halls. C50 is mathematically related to D50 (Definition): C50 = 10×log(D50/(1−D50)). It correlates strongly with STI and is often reported alongside it in room acoustic assessments. C50 is extracted from measured impulse responses and should be spatially averaged across multiple receiver positions per ISO 3382-1.