The reverberation time is usually defined as the time it takes for sound to decrease by 60 dB. To determine the length of this time, different parts of the reverberation curve are used.
When measuring Early Decay Time (EDT), an interval of 10 dB is used. At T20, an interval of 20 dB is used. When determining T20, the evaluation does not start until after the sound level has already fallen by 5 dB. At T30, an interval of 30 dB is used and here too, the evaluation starts after the sound level has fallen by 5 dB. If the reverberation curve is straight, the EDT, T20, T30, will all produce the same value. In practice, the reverberation curve is not straight (dashed line), which means that the descriptors will differ.
The descriptors T20 and T30 are usually called “late reverberation times” as they measure at the later part of the curve. EDT is called “early reverberation” and is considered to better reflect how we perceive the reverberance in the room.