Measuring the reverberation time and early decay time EDT
To determine the reverberation time T, the decay of the sound field, as shown in the figure, is evaluated between -5 and -35 dB below the sound pressure level of the continuous noise. It is extrapolated to -60 dB.
To measure the early decay time EDT, the decay process is to be registered between 0 and -10 dB by definition and likewise extrapolated to -60 dB.
In modern measurement procedures, continuous noise is replaced by an exponential sweep. Using the sweep, the impulse response is measured and the reverberation time is calculated.