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Discussion Starter #1
In examining the sound decay in my listening room of an open baffle loudspeaker system with the REW software I have observed that some resonances appear to increase in frequency with decay whereas others appear to decrease in frequency. The attached views show these phenomena.

The first shows how a resonance at 78Hz (the origin of which is only clearly seen at the 60mS trace) reduces in frequency to 68Hz. The second shows how a resonance apparently starting at 195Hz appears to drift up on decaying to 256Hz.

Am I observing real physical phenomena in the frequency drift here or are they artefacts of the measuring procedure?
 

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There are multiple resonances with different rates of decay, the window width has a smoothing effect causing the overall peaks of groups of resonances to be in different positions in different slices. Using a longer window would increase frequency resolution and change the picture.
 

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Discussion Starter #3
Thank you. Following your explanation I experimented with the window values. Then I realised that I have become accustomed to using 1/3 octave filtering which smooths the curves into an orderly pattern that the eye can follow. However it also blurred the distinction between different resonances, so there were actually two distinct resonances with 1/6 octave filtering: at 195Hz and 256Hz. No doubt with less filtering I would find even more.

Can you explain why it is that the RT60 spectral display only starts its 1/3 octave analysis at (presumably) 63Hz? The SPL and Decay displays indicate that I have a room resonance (in some mike positions) at 53Hz, which then doesn't show up on the RT60.
 

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The lowest centre frequency for the RT60 plot is 50 Hz at 1/3 octave and 63 Hz at 1 octave, per ISO3382. RT60 measurements are not valid at low frequencies in (acoustically) small rooms since the underlying assumption of random incidence is not met. Use the waterfall, spectrogram and decay plots for low frequencies.
 

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Discussion Starter #5
Thanks. Now I understand the limitations better. Would some of this not be useful information for the HELP file? Can I assist the writer of that?
 
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