[CAK]

I did some more room measurments today as I have a -12db null from 54hz to 68hz.  Walking around with my SPL meter I came across something interesting that I never picked up on before. The null is completely height dependent (from floor to ceiling) and not so much distance (front to back wall).  Of course null starts at about 15 inches off the floor through my listening height of 38 inches and ends around 55 inches from the floor. Room size is 12x14x8 with speakers on the short wall 44 inches out, 28 from side walls and my ears are 30 inches from back wall.  I have a 2x4 x2" panel on the front wall behind each speaker, two panels on each side wall at 1st reflection points and two panels on rear wall directly behind listening position.

I have a feeling (guess) this has something to do with room height but not sure how to test and then how to treat it...any ideas guys?  Thanks!

[bpape]

If it's purely height related (doubtful since it's so wide height wise - but treating the height may still help), treating the ceiling with something THICK will likely help you.  The other option is to build a sealed wood panel type resonator that's tuned to the middle of the null and has a width of approximately 1 octave.

Bryan

[CAK]

It's interesting...walking the meter front to back yields little difference in SPL. Side to side there is a +3db difference in front of each speaker as opposed to the center of the room...but the up and down is where it really just dissapears.  When I run the freq sweep as the hz goes up so does the height of the null off the floor.  When I sit down it's as if someone just turned the stereo off.  For kicks I moved the speakers back towards the rear wall five inches which actually helped +5db but I think all the low frequency's got a boost respectivly.  What kind of phenomenon I'm I dealing with here? A room null, comb filtering? confused....

[rockadanny]

CAK - Are you running a sub? What settings? Where is it located?

[pecker]

I have several rooms in my house, not much bigger than yours. If I sit about 2 feet out from the wall behind me, there is a pretty deep null in the frequency range you are experiencing...60hz +/-.

If I move back towards the wall behind me, I get a big improvement...at least at that frequency.

I'm pretty sure it's due to room dimensions, and  minor treatments are not going to fix it.

There used to be an online room mode/standing wave calculator, but my link for it is dead now.

[bpape]

Moving the speakers closer to a boundary is changing the SBIR (speaker boundary interface response).  This is a useful tuning aid if needed. 

So you say that the null does change in intensity as you go up and down vertically then? Not the same for the middle 50% of the height?  If that's the case, it very well may be primarily a height related issue and my previous recommendations would be appropriate.

Bryan

[MaxCast]

  The other option is to build a sealed wood panel type resonator that's tuned to the middle of the null and has a width of approximately 1 octave.

Bryan

How does one build one of these resonators?  Does it work on peeks?   I have a big peak at the same frequencies.

[CAK]

Moving the speakers closer to a boundary is changing the SBIR (speaker boundary interface response).  This is a useful tuning aid if needed. 

So you say that the null does change in intensity as you go up and down vertically then? Not the same for the middle 50% of the height?  If that's the case, it very well may be primarily a height related issue and my previous recommendations would be appropriate.

Bryan

Bryan yes. Where the speakers were originaly, if I moved the spl meter forward or back (towards the speakers from my listening position or away from the speakers towards the back wall) the db change was minimal.  When I stand up and sit down is where that particular range just completly dissapears and it doesn't seem to matter much where in the room I do this as long as I am in front of the speaker plane towrds the back wall. Moving the speakers back five inches did bump that range a bit but it is still dropping off on a verticle plane. the drop of starts just above the floor, about 15 inches, and reaches it's maximum null just around my listening height. As I raise the meter above my ears it come's back and at around 55 inches from the floor it comes back altogether.  The height of the null increases with the frequency.

Rockadanny, no sub.

[bpape]

CAK

OK.  So you can deal somewhat with it by introducing a peak based on speaker baffle to wall distance.  The rest sounds like it's definitely the height and an absorber overhead is the solution.

Max

You have to build a rigid, sealed box and have insulation in it but not touching the front panel.  The depth of the box cavity and the density of the front wood (the membrane) will determine the center frequency. 

I have to take off to get on the road for the weekend pretty quick. Remind me and I'll do the calcs and post what's required.

Bryan

[bpape]

Here is the formula

F =170/SQRT(M*D)

F = the frequency you're aiming to absorb

M = the mass of the panel in lbs per square foot

D = the depth of the air space in inches.

Keep in mind that these will function most effectively over about 1 octave or a bit more total.    There's more to it than that realistically as the efficiency and center frequency will also change with the size.  For example, if you tried to make one of these 6"x6" it would be different than one that's 2'x4'.  From past experience, if you make them 2'x4', F is pretty close to the calculated value.

What you're effectively doing is building a drum in reverse.  Mount the absorption close to the front membrane but not touching it.  Maybe leave 1/2". 

Here is a picture from Ethan's site with some plans for a couple of different implementations.  You can also use a solid back on it instead of toggle bolting the frame into the ceiling directly.  This makes it easier to seal IMO as you can do pretty much everything on a workbench and then just put it up.



Bryan

[MaxCast]

Bryan, thanks for the quick work.  Have a safe trip.

Do these have to go on the ceiling?  If so, where?  If they do not, where should they go?
Does anyone know the weight of a 2'x4' sheet of 1/4 plywood? 

[Ethan Winer]

Here's the complete article:

Build a Better Bass Trap

--Ethan