Historically the most important acoustical parameter is the Reverberation Time (T or RT) defined as the time, in seconds, that would be required for the sound pressure to decrease by 60dB after the sound source has ceased; this is why the most used indication you find in literature for the reverberation time is RT60. As it is normally practically difficult to obtain a sufficient dynamic range to measure the reverberation time directly,the norm provides for its evaluation based on a smaller decay of 30dB; in this case the reverberation time, indicated as RT30, should be the time evaluated considering a linear decay given by the least-square regression of the measured curve from -5dB to -35dB. Also provided is the possibility of evaluating RT20 and RTUser(based on user defined limits).