RT60 at low frequencies ?

[Thomas1970]

I'm trying to measure reverberation time in my listening room. For some reason I do not get any shown on the graphs below 63Hz. I have tried to set the Buffer size to 128k and setting the measuring length to 1M, but this do not change getting data below 63Hz in RT60. Adjusting Analysis settings also do not change anything. What shall I do to get results below 63Hz?

Thanks, Thomas

 

[John Mulcahy]

RT60 is not meaningful at very low frequencies as behaviour there is dominated by modal resonances. Use the waterfall or decay graphs to observe low frequency behaviour.

 

[Falconer72]

I'm trying to measure RT-60 in loopback mode with virtual soundcard. Why not zero?

 

[John Mulcahy]

Because the octave/one-third octave filters have their own decay time. The time reversed filtering option can reduce that, see the help for more.

 

[Falconer72]

Because the octave/one-third octave filters have their own decay time. The time revered filtering option can reduce that, see the help for more.

Thanks. I understand.

 

[Falconer72]

Then is it practical to use Time reversed filtering option?

 

[John Mulcahy]

I'm not sure what you mean by practical, but it reduces the filter's contribution which can be useful if measuring an environment with low decay times.

 

[Falconer72]

Excuse me, I was careless. Now, I see that you wrote the correct answer in the first answer.

 

[Matthew J Poes]

RT60 is not meaningful at very low frequencies as behaviour there is dominated by modal resonances. Use the waterfall or decay graphs to observe low frequency behaviour.

What John said is right on. You will see huge swings in RT60 below a room's FS just by moving the mic around. This is due to modal changes, and not meaningful changes in decay. This is all minimum phase so EQ alone would change the decay time, but we don't typically think of RT60 as varying just based on level.

I also like to use filtered impulse response as a means of directly comparing the effects of treatments. Not sure if @John Mulcahy has thoughts on this being a bad practice. I found when looking at all the methods, it was the one that made differences most obvious.