How to look for room modes in REW measurements

[No. 5]

Consider these two waterfalls. For purpose of illustration, I increased the gain so that the peak is at 130dB. The noise floor is unknown because I did not measure it.

Thank you for explaining your position, I see what you are talking about in regard to providing a reference point. I'm going to risk being called pedantic now, and propose that you did actually measure the noise floor.

Consider this waterfall:

This is a measurement sweep (sort of), level calibrated to 94dB and taken with no speakers connected, and the microphone wrapped up in a plastic bag and burred in a large bucket of sand in a quiet basement on a quiet evening. I'm sure that there is still some ambient noises being picked up (especially at low frequency), but for the most part, it looks to me to be in line with an independent lab's assessment that this mic has a broadband noise floor of 33dB. Calculated with the mic's sensitivity, that's -96.9dBu of broadband noise in every measurement made with this microphone. Which also means that I can't measure ambient noise levels that are less than 40dB with this microphone because of the constant presence of 33dB of noise. Now look at the waterfall from my second example again (I think it was taken with the same mic... I can't remember now):

This time there is ambient room noise in there in addition to the inherent noise of the mic and electronics, but the main thing is that this accumulated noise floor continues on after the signal dies away. Another thing of note: I used a longer window in this waterfall than I did in the previous post because favoring time resolution in this case will give the illusion of a lower noise floor than is actually present.

 

[No. 5]

It's relative to room length. Length to speaker orientation so always use longer axe for orientation/placement. In first example it's just above 2 m, in second about 3.7 m if it whose let's say about 5 m first peak would be about 34 Hz and could be used controled for the boost.

I'm sorry, but I'm not following how that's connected to knowing the absolute SPL of a measurement, could you explain? I mean, a virtual bass array is still cool and all...

 

[ZolaIII]

I'm sorry, but I'm not following how that's connected to knowing the absolute SPL of a measurement, could you explain? I mean, a virtual bass array is still cool and all...

Only corelation where the room mode will be is room length. It will be present on background noise and pretty much any SPL level. Of course it will get worse on very loud level and mix with other refractions so going pretty wiled. You want it clearly present but not going wiled so you do measurements on about 76 dB or - 12 from calibration point which is considered subjective as twice as loud (+12 dB).
Table is to pin them down accurate and not be able to miss them, REW room simulator show you the same thing more graphical.

 

[JohnPM]

To see the noise floor tick the "Capture noise floor" box on the Measure dialog and look at the result on the Distortion graph. REW can tell you the frequencies and decay times of resonances in a measurement.

 

[CleanSound]

To see the noise floor tick the "Capture noise floor" box on the Measure dialog and look at the result on the Distortion graph. REW can tell you the frequencies and decay times of resonances in a measurement.

Thanks John. I will play around with that. And keep up the great work!