## RT60 Graph

The RT60 Reverberation Time curves at each octave or one-third octave filter centre frequency are displayed on this graph, with separate traces for the Early Decay time (EDT) and the 60dB decay times T20, T30 and REW's Topt. See below for descriptions of each of these parameters.

### RT60 Controls

The control panel for the RT60 graph has these controls:

The Time reversed filtering control applies the octave band filters backwards in time, this reduces the filter's own contribution to the measured decay. When using 1/3 octave filters at low frequencies the filter decay time can be significant, over 200 ms for a 100Hz 1/3 filter, for example. Applying the filter in reverse reduces this decay to less than 50 ms, but it does affect the response somewhat, such that Early Decay Time (EDT) figures using Time-Reversed filters may not be valid.

Zero phase filtering applies 4th order bandpass filters in two passes through the data, one forwards and one reversed, to give a response with overall zero phase shift. This reduces the contribution of the filter's decay time similar to time revered filtering but without significantly affecting early decay time.

If the Show correlation factor box is checked the graph legend names shows the quality of the line fit for the various decay measures. The "r" value shown after each decay measure is the regression coefficient, which measures how well the data corresponds to a straight line. A value of -1 would indicate a perfect fit, values lower in magnitude than -0.98 indicate the corresponding decay figure may not be reliable. Unreliable values are italicised. The decay measures available are:

EDT
  Early Decay Time, based on the slope of the Schroeder curve between 0dB and -10dB.
T20
  Decay time based on the slope of the Schroeder curve between -5dB and -25dB.
T30
  Decay time based on the slope of the Schroeder curve between -5dB and -35dB.
Topt
  An "optimal" decay time based on the slope of the Schroeder curve over a variable range chosen to yield the best linear fit. If the early decay time is much shorter than T30 the Topt measure uses a start point based on the intersection of the EDT and T30 lines, otherwise it uses -5dB. REW tests every end point in 1dB steps to the end of the Schroeder curve and chooses the one which gives the best linear fit. The range over which the value has been calculated is shown in the trace legend.

The RT60 plot can show horizontal bars centred on each filter frequency and spanning the filter's bandwidth, or lines joining the filter centre frequencies, according to the Use bars on RT60 plot control setting.

The reverberation times for the current measurement can be written to a text file using the File -> Export -> RT60 data as text menu entry.

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