Originally Posted by BasementBob
OK, so you have a ringing mode where you have that microphone, at 52.6hz (2,0,0 axial), and another at 85.3hz (0,2,0 axial). There might be one at 26.3hz and/or 42.6hz that your microphone isn't showing, perhaps your microphone is in a null node (remeasure to test hypothesis).
In any event your big issue is the length mode -- so, what can you do to increase absorption of that?
I did a few quick new tests this morning, with the microphone in a new location. This time it's roughly in the middle of the riser. Running the same measurement test as before showed markedly different results, as we'd expect:
The 52.6Hz spike is much more muted in this location and most of the frequencies are a bit flatter. But what I'm really interested to see is how the different vents affect different modal frequencies. As in, do the back vents work on the length frequencies and the side vents work on width ones? In that case, what would work on the height modes?
You can see why I was hiding results below 30Hz before. It looks like garbage results.
There's something interesting about all of the peaks. For instance, here are my room modes up to 200Hz
Length Modes: 26.3 Hz 52.6 Hz 78.8 Hz 105.1 Hz 131.4 Hz 157.7 Hz 184.0 Hz
Width Modes: 42.6 Hz 85.3 Hz 127.9 Hz 170.6 Hz
Height Modes: 69.9 Hz 139.9 Hz
Now look at the frequencies where the ringing is. I can tell you from playing with REW that they are nearly ALL the length modes ringing! There's a cute little ringing at the 2nd order width mode of 85.3Hz and maybe the 3rd order 127.9Hz combines with the 5th order length mode of 131.4Hz. But otherwise, they aren't visible at all in the waterfall and neither of the height modes show up even a little bit.
So for test one, I loosely covered the back vents with the cut-off pieces of the riser (two layers of 3/4" OSB glued together). It's not a perfect cover, since there are some air gap slivers roughly the width of the jig-saw blade kerf, but they are mostly covered. The results:
Ringing increased in pretty much all of the length modes, ranging from maybe 10ms on the low end to around 40ms on the upper end. The one known axial width mode didn't budge at all.
I then uncovered the back vents and loosely covered the side vents:
There aren't any strong width modes in play, so we can't expect much change there. I do note that the lower frequency length modes don't budge at all, while the upper ones do go a little. In fact, although the back vents seem to affect all of the modes, the side vents only really target the 4th, 6th, and 7th length modes. Whatever is at 200Hz seems to also be massively affected, but that doesn't correspond with any axial modes.
More info that's not in these screen captures -- if I increase my window to 300Hz, then I see that the side vents affect the 200-300Hz band quite a bit more than the lower frequencies. That implies its use as a broadband absorber in addition to one targeting modal frequencies.
My tentative conclusions at this point:
1. My length modes DOMINATE
2. The back vents have more impact than the side vents
3. The side vents do still help, but only a little on the notable modes and more on the higher frequencies
- According to this image, you could extend the 3 vents along the top to cover the entire wall, and you could make those three vents twice as wide. Either, or both, of those would increase the absorption along that 52.6 (2,0,0) axial mode. Similarly, your seats riser at the other end of the room, same thing. Obviously leave enough deck for structural integrity, or remove the deck plywood and add 2x4 blocking and replace the deck plywood.
Yeah, I'll be messing with these later. I don't want to touch them now since my columns are going to be around there and I'm not yet sure what their size will be. I think I'll revisit the back vent size after my soffit and columns are done.
I'm not entirely certain what you mean by "seats riser on the other end of the room"? Only the stage is on the other end of the room.