To examine the temporal behaviour of the sound field in a room, you consider the sequentially arriving pulses being created by a single Dirac-Pulse that is sent out from the loudspeaker and is many times reflected at the room walls.

Displaying these pulses in a time diagram results in the Transient Room Response. Squaring the 'Transient Room Response' yields a measure for the Sound Energy Density (sound energy per unit volume at the listening place) as a function of time. This diagram is called Reverberation Diagram (red curve).

Summing up the energy densities of all the time intervals (time integration from T=0 to T=end) results in the Total Energy Density at the listening place. This 'Total Energy Density' is identical to the energy density at the listening place of the steady state sound field being permanently excited by a white noise signal.

The time dependence of the 'Total Energy Density' is calculated by integrating the 'Reverberation Diagram' from time T=t to T=end. This curve (green) is in CARA called Integral of Reverberation Diagram.

From the temporal decay of that curve the Reverberation Time or the so-called Early Decay Time (e.g. T10) may be derived.