# 30 hz and 100 Hz standing wave



## Anthony (Oct 5, 2006)

In the copious measurements I made for the NeptuneEQ review I'm writing (should be up by the end of this week), I re-noticed an acoustics problem that I never did quite fix.

I have standing waves at 30 Hz (axial) and 100 hz (horizontal). Now my listening position is not in the dead center of either of these, but it is close enough for the "suckout" to be noticeable.

The nulls are modeled in CARA, so I know they are room related. I have no treatments, other than window coverings and heavy, thick, soft cloth couches. I am pretty well "absorbed" according to the CARA modeling, so I don't want to overdo it on any treatments and make the room dead.

Any ideas? Thanks.


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## bpape (Sep 14, 2006)

A couple of thick panels on the rear wall behind the listening position will help some. To do 30Hz, they'll need to be pretty beefy. Using a facing on them to avoid additional upper mid/high frequency absorption will minimize that issue. 

Another option is to play with subwoofer placement to try to generate a peak at 30Hz via SBIR to compensate a little.


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## Anthony (Oct 5, 2006)

How big is "beefy"? 

I have a blank wall in the back of the room. But it couldn't interfere with people's heads when they sat. So 4-6" would be the largest I could do. I could probably also get away with putting something behind the screen for some absorption.

Sadly, I don't have a lot of room to work with on the sides (100Hz problem).

Thanks for the help.


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## bpape (Sep 14, 2006)

On the sides, with 100Hz, a 2" panel with a 1" gap will work pretty well if it's purely an axial width mode.

4" isn't going to get 30Hz. 6" will dent it a little but not a ton. This is a situation where you might consider building a sealed membrane absorber from wood tuned to around that frequency. 

Bryan


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## Anthony (Oct 5, 2006)

I just found an old thread where I asked that exact same question (DOH!) Looks like panel absorber it is.

Thankfully I have a ton of scrap wood


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## bpape (Sep 14, 2006)

Just understand that there are 2 ways to tune low with that type of absorber. You can make the cavity deeper - or - you can make the front panel more massive. You don't have a lot of space so it's likely going to be mass. The more massive it is, the less efficient it is per square foot but it'll at least be in the right area.


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## Anthony (Oct 5, 2006)

yeah, I was looking at an online calculator. You need a lot of depth to get that low and a lot of mass. So much that you're probably looking at non-linear structural distortions to the "resonance" in the panel. Hmm. Easy enough to build and I have been meaning to get rid of all that 3/4" oak plywood. 

Thanks for the help. Looks like I have a project for this weekend.


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## bpape (Sep 14, 2006)

A couple of things. Make absolutely sure the back is VERY VERY rigid, solidly attached to the wall, and that it's all sealed air tight. Probably use 1" or 2" OC703 or equivalent for the inner absorption and have the face of the insulation about 3/8-1/2" from the front panel.

Also, you can increase the efficiency a little (and change the tuning center very slightly) by making something like a single 4'x6' instead of 3 2'x4' pieces.


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## Anthony (Oct 5, 2006)

What about using the riser as a helmholtz resonator? It's 12" tall, has two layers of 3/4" ply on top, and is stuffed with fiberglass bats. As of now, it's carpeted and mostly sealed.

Tuning would probably be a bear, though.

Just throwing out ideas. It looks like I'd have to have 13" cavity capped with 3/4" plywood at that frequency.


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## bpape (Sep 14, 2006)

To do a Helmholz, your tuning is very narrow. To do the membrane is a no-go because the top plywood is being damped by the seating.

Doing a tuned helmholz is difficult with the carpet over the top. Now, if you cut BIG holes in the front and used the 13" depth filled with insulation, that'd be thick enough to help at least a little.


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## Anthony (Oct 5, 2006)

No, I was thinking along the lines of "cut ports in the front or along the back edge". I could use subwoofer ports to get what I needed. It's a huge volume, though, so it might look comical to have all those holes.

Since the front of the riser is out into the room (i.e. in the null region) would that be the best place for the ports? Or would it be better to have the ports at the high pressure region, i.e. along the back wall?

Hmm. First idea should probably be testing new subwoofer locations. The ideal velocity source location (dipole sub) would be smack in the middle of the room. The wife may not like it, but at worst it's a coffee table  If that doesn't work, only then will I consider cutting holes in the riser.

If you don't mind, please check my math here, though, in case that's the path forward for me:

2*pi*f_rez = v*Sqrt(A/(V*L)) Where:
f_rez is tuning frequency
v is the speed of sound
A is the overall port area ( can that also be N*a for N smaller ports of area a?)
V is the volume of the cavity (how does stuffing affect? 30% more perceived volume?)
L is the port length

If all that's right, and I look for a tuning of 35 hz or so, I would need 4, 4" holes or 15 2" holes of just the thickness of the joist beam (1.5"). With increased perceived volume, I would need a 5th hole, if the 30% number is correct.

thanks again for the help.


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## bpape (Sep 14, 2006)

Here is the fomula to calculate slatted Helmholz resonators


fo = 2160*sqrt(r/((d*1.2*D)*(r+w)))

r = slot width
d = slat thickness
1.2 = mouth correction
D = cavity depth,
w = slat width
2160 = c/(2*PI) but rounded
c = speed of sound in inch/sec.

If you want to play what if games with different types, check here for a download

http://www.whealy.com/acoustics/Porous.html


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