Reverb Clarity Calculator: C50, C80 and modulation depth from decay time, reverb level and pre-delay

How much outline a reverb leaves a vocal or a groove depends mainly on three settings: the decay time, the level of the reverb against the dry signal and the pre-delay. For these, the calculator shows the room-acoustic clarity indices C50 and C80 and the modulation depth, meaning how much of the fluctuation of syllables and notes remains after the reverb.

1 means the level fluctuation is fully kept; 0 means the reverb flattens it. Quarter, eighth and sixteenth notes follow the tempo, syllables lie at about 4 Hz.

Model: direct sound with energy 1 and a reverb tail with energy g = 10^(reverb level/10) that decays exponentially from the pre-delay on and falls by 60 dB in T.

C50 and C80: 10·log(early energy / late energy) with the boundary at 50 or 80 ms after the direct sound, per ISO 3382-1.

Modulation depth: m(F) = |1 + g·e^(−j2πF·pre-delay) / (1 + j2πF·T/13.82)| / (1 + g).

Limits: the model knows no early reflections, no high-frequency damping and no stereo width. In the measurement for the reverb article, the calculated values differed from convolution with ffmpeg by at most 0.31 dB for C50 and C80 and by at most 0.020 for the modulation depth.

How decay time, pre-delay, reverb level and a high-pass in the send change clarity is measured in the article “Reverb Measured”.