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Originally Posted by DanDan  I am disappointed that REW only goes down to 80Hz |
Ah, we have been talking at cross purposes. The business of making acoustic measurements is set out in an ISO standard, ISO 3382. That recommends analysis in the six 1-octave bands from 125Hz to 4kHz. Most measurement software, REW included, extend this to include an extra octave above and below so 62.5Hz to 8kHz. To extend the range to include the 31.25Hz octave would really go beyond the point where an RT60 figure tells you anything meaningful at all, as the lowest filter would then cover such a narrow range of frequencies that even in a large room the behaviour of relatively few isolated modes would dominate the result, giving Schroeder curves that bear little relation to straight lines. Many software packages, again REW included, offer analysis in 1/3rd octave bands, but it would then be typical to restrict the lowest frequency to the band centred on 80Hz to have some hope of spanning a sufficient frequency range to allow a meaningful result. I would personally use waterfall and spectral decay plots for everything below 200Hz.