This seems to have remained a constant throughout all the testing and treatment I've done. I understand the concept of taking low frequencys out of the room, but how does one add them back in?
You already did it!
![Smile :)](./images/smilies/icon_smile.gif)
Believe it or not, you put the energy back in to the room by taking energy out of the room..
![Cool 8)](./images/smilies/icon_cool.gif)
Sounds weird, I know, but that's what you are doing in effect.
The basic issue is this: you were looking at a null in a standing wave, but you were only seeing it because your mic was at that specific location in the room. At any other location, it would not have been in that null, but rather a "neutral" zone, or even the peak... for that specific frequency. A standing wave is called that because it ... well, it "stands": even though the wave is still there, and still moving through the room, the pattern of pressure peaks and pressure nulls that it creates is stationary with respect to the room boundary, so the wave appears to be standing still. Hence the name: "standing wave". In reality, it isn't actually stationary: it's only the patterns of pressure peaks and nulls that remains fixed, but the energy of that wave is still traveling around the room. So what you hear depends where you are in the room. Move the mic a bit and you'll be out of that null (and probably into a null for a different frequency). That's why it is so important to get the mic back to the exact same location each time you do a measurement in REW.
So, the energy of the wave is still moving around the room. For every null there is a corresponding peak, and since this is a room mode, the peak is located... in the corner of the room! All modes terminate in corners. So by treating the corner with thick absorption, you have damped the
resonance of that mode: You brought down the peak, which also automatically brings up the null. Magic! The mode is still there, and still "standing", but now it is standing much more weakly, since it is damped. (And you also drove the frequency up a tad: it is now at about 131 Hz.) So you actually did, indeed, "fill in" the null by
taking away some energy from that wave. But you took it away at the location where it is peaking, by damping its resonance.
It sounds totally illogical that you can treat an empty hole by apparently taking something out of it, but it make more sense when you realize that the "hole" is only part of the entire wave, and there's a corresponding peak somewhere else in the room, sort of the "other end" of the wave. So you are not actually "taking stuff out" of the hole, but rather "taking it out" of the peak, and that has an effect on the hole too.
Ain't acoustics wonderful? So intuitive, so logical, so clear...
Not sure if I managed to explain that clearly at all: Reading it back, I think I just confused myself, actually!
- Stuart -