# Room treatment is creating a null?!



## NoDestiny (Oct 28, 2009)

Hey gang,

I've done more reading that I can even recall, prepping a listening room. The room itself is 14' 2.5" deep, 10' 1" wide, and 100" high (4.33M, 3.07M, and 2.54M). It is new/old construction mixed, so I had some studded walls to play with. On the old construction, the walls were studded on both sides, so I took full advantage and decided to use that for making a dead front wall area. The front 6 1/2 feet of the left/right front portion of the walls is 3.5" studs, while the front wall itself is 7" deep studs. The rest is covered in drywall. I setup with the 38% and put the speakers at ear level and triangulated distance.

So, here is a completely empty room, no treatment:









I put 3.5" of pink fluff fiberglass in the 7" deep cavities, up against the the wall:









The, slapped Roxul Safe n sound in front of that, filling the 7" cavities on the front wall.









The frontal side walls filled with SnS...









I filled the rear "false wall" corners with 9" of pink fluff...









*And this is where things get weird... *I faced the pink fluff in the rear with a layer of the same 3" SnS I've been using...









Here is a quick representation of what the room looks like at this point...









The center of the rear wall was still bare, so I put 3x sheets just laying up against the wall to cover the rear reflection points...









The null is very noticeable. I take 3 bags of SnS in their compressed state and put a bag in 3 of 4 corners (2 front, 1 right rear)...









What the?! The null is making it's presence known!

Maybe my mic is having an issue? Move the mic over in front of the rear right bag of SnS...









Guess not.

So, I've still got a lot of plans for treatment in this area, including large superchunk fronts, soffit trapping all around, and an acoustic cloud (not to mention some diffusion on the rear), but I'm a bit baffled as to why this null in my bass response has dramatically increased? Any ideas?


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## bpape (Sep 14, 2006)

You've already got a ton of mid/high absorption in the room. Maybe too much. Before you do much of anything else, I'd play with head position. Move the mic forward a foot and measure, then back to where you are and move it left a foot. Look at the differences in response and see what's changing - might go away, might just change center frequency but it's a clue as to where the problem is coming from.


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## NoDestiny (Oct 28, 2009)

I did some testing this morning with moving the mic about a foot forward and back from my nominal position. The null didn't move much frequency wise, but did tame down a bit. I plan on moving the desk today (with the speakers) and seeing what mic position combo works with that.

What really has me wondering is... why did the null get worse?

Oh, and as you mentioned, the high end is starting to suffer. The plans for that is to add wood slatting to the walls to liven things back up once I get the low end satisfactory. I'll also be doing some diffusion in the future on the back wall and mildy on the ceiling. I'll make sure this room isn't an acoustic vacuum where it counts.


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## bpape (Sep 14, 2006)

Hard to say since you moved it in 2 directions at the same time.


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## sdurani (Oct 28, 2010)

NoDestiny said:


> What really has me wondering is... why did the null get worse?


Just a guess but it is possible that reflected energy was partially filling in the null, making it look less severe. Adding absorption to the room reduced the reflected energy and unmasked the true depth of the null. Rare, but I've seen it happen before, hence the guess.


NoDestiny said:


> I did some testing this morning with moving the mic about a foot forward and back from my nominal position. The null didn't move much frequency wise, but did tame down a bit.


In that case, the null seems to be one of the length modes of your room. The mode will stay at the same frequency but it will change from null to peak as you move the mic along the length of the room. 

To get a rough idea of the frequency, divide the speed of sound by the length of your room. 

1130 ÷ 14 = 81 Hz

Pretty close to what you measured. Try placing the mic at 1/3rd room length and re-measure.


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## NoDestiny (Oct 28, 2009)

OK, I did some testing...

33%, 38%, 50%, 66% front front wall with mic center of room...









38% center, left half of room, and up 50% height (from 45" to 77.5")...









And finally, I moved the speakers back about 13" on some stands (too hard to move desk)(stands are 4" ABS/MDF stands full of sand). Just curious, I stood to the left of the mic, held over 2 layers of 3" SnS about 2' over the tip of the mic...









Thoughts?


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## bpape (Sep 14, 2006)

Looks like it might be a combination of some SBIR off the front wall and cancellation off the back wall. If it were a length mode, it wouldn't be worse at 33 and 38% than it is at 50. The 2nd harmonic would be a peak, the 3rd harmonic would be a null again.


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## NoDestiny (Oct 28, 2009)

bpape,

Thanks for the reply. I wish I had some of your guys' brains on this stuff. I'm grateful for internet forums and the helpful communities that power them, that's for sure!

I did some more testing within the room. I put the speakers 1" / 8.5" from the front wall (rear corners distance, after triangulation. Triangulation = 4' 3") with both tweets inward and flipped them and did tweets outward...









Here is the on stands near wall (above, tweets in) VS on the desk. Note: on desk, was not triangulated to seating position!









I'll be doing some ceiling treatment in the near future and see what that does. That includes soffits and a cloud over the listening position.


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## bpape (Sep 14, 2006)

Not sure what you mean about triangulated with seating position? Mic wasn't in the right place or not in an equilateral triangle? Regardless the change in height and additional distance from the desk did change some things. Ceiling in terms of modes would be a peak as it would be a 2nd harmonic, but could be some SBIR there too as well as off the desktop.


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