Early Decay Time Calculator

Early Decay Time is a room acoustic parameter from ISO 3382 that measures the initial slope of the sound decay curve. It is the time for the first 10 dB of decay, scaled up by six to express it as the equivalent time for a full 60 dB drop. EDT correlates more closely with perceived reverberance than the later-slope reverberation time because listeners respond most to the early decay.

EDT = 6 * t10, where t10 is the time measured for the level to fall by 10 dB from the initial level. Multiplying by six extrapolates the 10 dB slope to a 60 dB decay. For example, if the level drops 10 dB in 0.1 seconds, the EDT is 0.6 seconds.

RT60 estimates such as T20 and T30 use later portions of the decay curve (from -5 to -25 dB or -5 to -35 dB) and extrapolate to 60 dB, describing the reverberant tail. EDT uses the first 10 dB and reflects what a listener perceives as reverberance. In a perfectly diffuse decay the two agree, but real rooms often show EDT differing from RT60, indicating early reflection structure.

Because hearing responds strongly to the onset of decay, EDT is a better predictor of subjective reverberance, clarity and intimacy than RT60 alone. Concert hall and auditorium designers use EDT alongside other ISO 3382 parameters to judge how live a space sounds at the listener's seat, where it can vary with position.

EDT is reported in seconds, the same as reverberation time. It is usually given per octave or one-third-octave band because decay slopes vary with frequency. Typical values range from a few tenths of a second in speech-oriented rooms to two seconds or more in large concert halls.