Harmonics Problem.

[bwack]

Finished my attic studio this year and i'm having one issue.

I killed all the first reflections and did extensive bass trapping (grey area is 703 & 705 FSK bass traping and panels, the wall with the window behind the desk has 8" behind its 703).

the ceiling is at 8'

anyway, when listening to sweeping sine wave, around 240hz on up past 400hz i get a good bit of harmonic "build up" (i'm guessing modal stuff)(i'm new to acoustics but have been reading).

moving myself or speakers around the room helps a little but i'm guessing i'm gonna have to treat more.

the wall with the door is all built in bookcases (full). i was hoping for a little diffusion. the door has a panel on it.

so do i need to treat more of the end i'm sitting in, or more of the end behind me.

i'd love to not deaden the room too much...

thanks for any help in advance.

[jwl]

You need more bass trapping. The large bass traps are good, but bass trapping is as much about coverage area. Put some more panels across the corners of the room, you have a bunch of corners.

If you don't want to over-deaden the room, then cover the panels with a thin but stiff layer, ie, FRK or even thin cardboard. That'll help absorb more bass, and restore some reflectivity to the high end.

[Weasel9992]

I agree with jwl...more bass trapping is required. I'm also seeing the possibility of some real issues n the back part of the room with the untreated walls aimed right into the floor. Maybe not such a huge problem at the listening position, but it would be a big problem for tracking.

Book cases don't work very well from a diffusion standpoint. If they're filled with books they'll partially absorb high end stuff over much of the surface, but reflect it from part. Even if the whole surface of the bookcase did act reflectively, it's not really randomized at all. The result in both cases is not diffusion...it's closer to comb filtering. If it's not filled with books, then each nook acts like it's only little resonance chamber at particular frequencies depending on the specific material and density.

I think you could add a good bit more treatment without killing the room.

Frank

[Glenn Kuras]

Not to take away one bit of your post Frank as it is spot on. Here is what Jason Jones, the diffusor guru said on the matter.

The chances that a bookshelf will be an acoustic problem are really 100%. Designing a diffuser is SO much more complicated than throwing in some random depths.

Would you buy an absorber that had specs like this:

4 Khz 0.2
2 Khz 0.4
1 Khz 1.0
500 Khz 0.2
250 Khz 0.7
125 hz 0.2

This absorber will decrease your reverberation time across the board, but very unevenly. Compared to having nothing on your walls it will sound more tolerable. It would be a fine acoustical addition to a living space especially when you have a couple of these plus couches and curtains that have different absorption qualities.

So, in a pinch if you really have no alternatives a bookshelf is better than a completely bare wall. But, it will skew what you hear.

I certainly can be an impossible perfectionist sometimes.....but couldn't you find some mineral wool or rigid fiberglass over there and wrap it in fabric. That is still pretty cheap and I would happily bless that idea (Just in case, No I don't take myself THAT seriously ).

As to the Chinese wood screen: I am not usually much into Asian art, but that really is beautiful. I can see why you would want it on the wall. Do keep it on the wall, but don't kid yourself about it's acoustic properties. Put over an absorber it might help some around 3 Khz or 4 Khz IF you already have a room that is too dead; but, that's not really your problem.

Summary:

1. Pick up some mineral wool or rigid fiberglass. If you can find it local and cover it yourself it will be cheap and acoustically very good.
2. The bookshelf is better than nothing. You can use it if you can't do #1 but it would be a delusion to think that it was more than a D grade (A-F scale) diffuser.

Jason
__________________
GIK Acoustics
GIK Acoustics.

Glenn

[bwack]

more treatment it is...

i'll probably make the two smaller panels on the 45 degree walls bigger and make more. i guess i should treat the window extrusion area as well. are you guys seeing the 135 degree corners at the ceiling as the "many corners that still need trapping."

if you are refering to the wall behind the desk, it is a fake wall of 3" 703 with eight inches of air before the drywall.

so there really arent drywall corners exposed fully (does this make sense) but if i still need to treat those corners more let me know. also, the fake wall stops at the 4' high bass traps, it does not continue down to the far edge.


i'll probably leave the bookcases as a last ditch effort if everything else i do is still not enough. maybe i could make two rolling vertical panels (gobo?) to roll in front of the shelves when tracking and mixing.

do you think i'm facing a good way... i'm at the end of the room with my back to it. part of me wants to turn around with myself and the speakers more near the 38% space. this would also put my spekers out in the room.

thanks for the quick responses. much appreciate all the time you guys spend helping those of us newbies.

[jwl]

Hi bwack,

Glad to help.

Yes, you have many ceiling corners begging for bass traps. :-)

I'm not sure I'm understanding what you are saying about the drywall. Do you mean that parts of the walls are unfinished, and that the room is not sealed up with drywall?

Pulling your desk out from the front wall would be good. Find the 38% point back from the front wall, move your chair to that spot, and then move the desk forward until the speakers are an equilateral triangle. Most likely that will sound much better. You'll also need to tweak your room treatment placement for your RFZ.

Do you do tracking in that room? If so, I'd most likely stay in the section where you are, that way bands/performers can come in and set up behind you in a slightly larger space. Plus it's more convenient to the door.

[bwack]

Originally Posted by jwl

I'm not sure I'm understanding what you are saying about the drywall. Do you mean that parts of the walls are unfinished, and that the room is not sealed up with drywall?

sorry,

yes we sealed the whole room with drywall on resilient channel. then after painting it all, we framed fake walls (like the 4" high traps on my left and right)(also one 8" out from the drywall around the window) and filled them with 703 leaving the air brehind.

so that entire wall is like a big custom fit panal 8" out from the drywall.

but i guess i need more still. also, this is mostly a listening/editing room, but i do help people out with projects from time to time (tracking/mixing).

i'll post some pics next week