D50 (Definition)

D50 (Deutlichkeit or Definition) is the ratio of early sound energy (first 50 ms) to total sound energy, expressed as a decimal or percentage. Defined in ISO 3382-1:2009 §4.4, D50 is a key speech intelligibility metric. Values above 0.50 (50%) indicate that more than half the sound energy arrives within the early integration window, correlating with good speech clarity. The metric was introduced by Thiele in 1953 and has been validated through extensive subjective correlation studies. D50 is mathematically equivalent to C50: D50 = 1/(1 + 10^(−C50/10)). Typical design targets are D50 > 0.50 for general speech rooms and D50 > 0.65 for classrooms and critical listening environments. D50 is calculated from the impulse response per octave band and should be spatially averaged. It is particularly useful in room acoustic design because it can be estimated from RT60 using simplified formulas, enabling early design-stage assessment of speech clarity.