REW uses Topt: information about this Topt:
RT60 Calculation
The RT60 values are estimated by calculating the slope of the Schroeder curve, which is a plot of the energy (squared values) of the impulse response that is backwards integrated (summed starting from the end and moving backwards). The vertical axis of the plot is in dB. The different RT60 measures (e.g. T20, T30, REW's Topt) are derived by calculating the slope of a best fit line to the Schroeder curve over different ranges (detailed below). In a diffuse field the curve with the dB vertical scale is quite linear until it reaches the noise floor.
The start point for the classical T20 and T30 measures of RT60 is where the Schroeder curve has dropped 5 dB below its peak. That works well in the large spaces for which RT60 is most applicable, particularly if the source used for the measurement is omnidirectional. In domestically sized rooms using normal, directional loudspeakers as sources the initial drop of the Schroeder curve is quite sharp (the Early Decay Time is quite short) meaning the -5 dB point lies within the early decay region rather than the diffuse field region. That in turn means the T20 and T30 figures underestimate the RT60 time. Where the EDT is much shorter than the T30 RT60 figure REW's Topt RT60 calculation uses a start point based on the intersection of the EDT and T30 regression lines, to determine a point that lies within the diffuse field region. It then tests each possible end point in 1 dB steps and picks the one that gives a regression line with the best linear fit. That produces a more reliable RT60 figure.
You can find this Topt: your measurements results window under the tab RT60. Gooid value is around 0.500mS for livin rooms.
