Quote:
Originally Posted by DanDan  I wonder is REW as good as it looks here, or is it actually applying fairly similar small numbers when uncertain or out of range. Do the Waterfalls tell the same story? Sorry, but it looks far far too good to be true! I would love to hear from REW's creator on this. Over what range of RT is REW accurate or even defined? |
REW doesn't guess, approximate, invent or otherwise fiddle the RT60 figures
They are derived in the usual manner by applying linear fits to the Schroeder integral, REW uses an iterative procedure to estimate the best starting point for the integration, often called "Lundeby's Method" (from the paper by A. Lundeby, T. E. Vigran, H. Bietz, and M. Vorländer, “Uncertainties of Measurements in Room Acoustics,” Acustica, vol. 81, pp. 344–355 (1995)). The correlation coefficients for the linear fits can be viewed to get a feel for how reliable any particular octave or 1/3 octave band figure is. On the specific RT measure displayed, you can choose from EDT (line fit from 0 to -10dB), T20 (fit from -5 to -25dB), T30 (fit from -5 to -35dB)and what I have chosen to call "Topt", which is an "optimal" decay time estimate 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 (often the case in small rooms) the Topt measure uses a start point based on the intersection of the EDT and T30 lines, otherwise it uses -5dB. REW then tests every end point in 1dB steps to the end of the Schroeder curve and chooses the one which gives the best linear fit. I find that to give more reliable and repeatable results, particularly in small rooms where the early decay can go through a couple of slope changes before settling to something more linear.