C80 (Clarity)

C80 is the Clarity Index for music, defined as the logarithmic ratio of early sound energy (first 80 milliseconds) to late sound energy (after 80 ms), expressed in decibels. Specified in ISO 3382-1:2009 §4.3, C80 quantifies the balance between clarity and reverberance in music performance spaces. The 80 ms integration limit reflects the longer temporal processing window for music compared to speech. Positive C80 values favor clarity and note definition, while negative values favor a blended, reverberant quality. Optimal values for symphonic music are typically −2 to +2 dB at mid-frequencies (averaged across 500, 1000, and 2000 Hz), balancing definition against warmth. Chamber music and opera may prefer slightly higher C80 values (+1 to +3 dB). C80 is position-dependent: seats close to the stage have higher C80 (more direct sound), while rear seats have lower C80 (more reverberant energy). Concert hall design optimizes C80 uniformity across the audience area.