standing wave?

BoB_DoLe · Post by **BoB_DoLe** » Mon Jul 03, 2006 12:01 am

ok hello!

i recently finished my studio (ok its more a CR) and i think it sounds fine overall, BUT (there allways has to be a problem, huh?) i noticed i huge peak at about 140Hz at my listening position.
but first more infos on the room, the plan is here, my listening position is at about 1,2meters high and about 1,8meters from the front wall.
the problem is like i said a huge overemphasis at 140Hz, what is a real problem as you could imagine. its really just like the room itself oszillates, what makes it even worse. the good thing (well, i hope so) is that when i stand up on the same position, this peak is totally gone, so i have normal response. when i roll back in my chair, it gets less very slowly, so you can say its not getting better. when i move to side from my listening position, nothing changes. so that makes me believe its a standing wave between the ceiling and floor. the room height is about 2,35meters, so i dont really see a 140Hz standing wave there?
so am i wrong and its e totally different problem?
and, no matter what it is, how can i manage it, some clouds over the mix position or what?
its really frustrating, cause right now ive put so much work in this room (and money, of course), so i am a bit disappointed.. but i hope theres some solution for that!

thank you for any help!

David French · Post by **David French** » Mon Jul 03, 2006 2:11 am

Sounds to me like that's exactly what the problem is: the second mode between the ceiling and the floor. Your description of how that sound changes is spot on, the calculated mode is 147 Hz, and your head is just about exactly in on of the worst places for this mode in the height dimension (the center).

Luckily, there's an easy way to handle this. Place four to six 4" thick panels of rigid mineral wool across your wall-ceiling intersections. This will damp the resonance to the point where it will no longer be audible.

If the rest of your room isn't treated already, I would highly suggest you look into it.

BoB_DoLe · Post by **BoB_DoLe** » Mon Jul 03, 2006 4:00 am

that are good news, thank you!
so you mean the mineral wool across the corners, so i get a triangle?

David French wrote: If the rest of your room isn't treated already, I would highly suggest you look into it.

well, take a look at my other thread, i have corner traps in all four corners, they shouldve been working at 150Hz or so, seems they differ a bit from theory

and i have rear wall absorbers on front and rear wall!
ive measured my room for some reason now (equip was samson resolv 65a speakers and a behringer b2 mic), even though i know this wouldnt be very representative.. ive noticed that theres a dip in the frequencies below 100Hz, i believe that this comes from the corner traps, perhaps a little overdosed? or its just the response of the speakers, they are perhaps too little to go down so far..
then there is the ~140Hz peak, thats really audible without measuring, its a pain so far..
and then theres a small dip at about 400Hz, sont know where that comes from, perhaps this is the effect of the corner traps? but as far as i can say right now, this is not that worse and barely noticeable..

so i think i go with your solution and hope it works

thanks again!

David French · Post by **David French** » Mon Jul 03, 2006 5:13 am

Yes, so you have a triangle. The panels make an angle of 45 degrees with the wall and with the ceiling.

Your vertical traps won't be all that effective at damping resonances between the floor and the ceiling; The placement I described will.

Hmm... a drop off below 100 Hz... did you have your mic in cardioid perhaps? I think you already know that you can't stust anything that wou measure with an improper mic. If you want to do it right, get a cheap Nady CM-100 or Behringer ECM8000. Also, you certainly didn't over-absorb the low end; there's no such thing. Speakers may be at fault as well.

BoB_DoLe · Post by **BoB_DoLe** » Mon Jul 03, 2006 5:31 am

David French wrote:Yes, so you have a triangle. The panels make an angle of 45 degrees with the wall and with the ceiling.

Your vertical traps won't be all that effective at damping resonances between the floor and the ceiling; The placement I described will.

ok, i will try that!
i already imagined that the vertical traps wont help with that.
but i hoped so

Hmm... a drop off below 100 Hz... did you have your mic in cardioid perhaps? I think you already know that you can't stust anything that wou measure with an improper mic. If you want to do it right, get a cheap Nady CM-100 or Behringer ECM8000. Also, you certainly didn't over-absorb the low end; there's no such thing. Speakers may be at fault as well.

the mic was in omni, but yeah i shouldnt realy trust that.. the speakers have a roll off at about 65Hz, for the mic you cant get any useful information. perhaps when i have a little more money i buy such a ecm8000, but its not that important for me as long as the room sounds good to my ears

well at least im glad that i didnt over-absorb the room!

BoB_DoLe · Post by **BoB_DoLe** » Tue Jul 04, 2006 4:41 am

two more questions, please

first, do the panels have to be 45degree to the wall or could i go with, say, 60° to the wall?
and, what dimensions should they be, how wide/what depth from the wall?
i just dont want to make it to small (because of my little space) to work properly in the target frequency..
and (ok, now weve got three

), is the material i will use ok for that, its isover TP1, and they have a airflow-resistance of >5kPa*s/m², is that ok?

thanks for helping me so much!

David French · Post by **David French** » Tue Jul 04, 2006 5:45 am

45 degrees is optimal, but it will still work at other angles, albeit at ever-decreasing effectiveness.

The size depends on the size of your materials. Please describe the dimensions and density of the material you plan to use. I am not familiar with Isover TP1.

BoB_DoLe · Post by **BoB_DoLe** » Tue Jul 04, 2006 6:06 am

ok i will try

the panels are about 24" by 49" wide and about 4" thick (they are available in other thicknesses as well). density is such a thing, i dont really have informations on that one.. they only tell the airflow-resistance in the datasheets

the counterpart from isover switzerland is at about 25kg/m³.. sounds too less, huh?

David French · Post by **David French** » Tue Jul 04, 2006 6:22 am

25 kg/m^3 isn't dense enough to make panels out of, but you could sure make chunks. If you can't find a denser material, this is what I would recommend doing.

BoB_DoLe · Post by **BoB_DoLe** » Tue Jul 04, 2006 6:38 am

hmm, the chunks look good to me, do they work well compared to panels?
there is shure denser material but no shop i know sells it!
although it would be more expensive, and by now im really on a budget, all the other stuff was a heavy load..
which size should i go then, is the 24" pattern good for me?

BoB_DoLe · Post by **BoB_DoLe** » Tue Jul 04, 2006 7:13 am

i try to answer the size question, help me if im wrong:
wavelength of 140Hz is about 2,4meters, the highest peak is abt 1/4 wavelength, so at 60centimeters.
so i take a triangle and make the height 60cm(from the corner to the longest side).
so with tan i get 120cm for the longest side and therefore about 43cm for both other sides.
so id have to cut triangles that size, and with the dimensions of a panel, i would get two triangles out of one, with a lot of stuff remaining.. not that efficient i guess, id need about 25 panels for my room, thats a bit too much for my budget i think.

but, as far as i dont have that much experience in acoustics, would it still help if i use the smaller chunks or would this be totaly waste of money then?

sorry if i am a bit annoying

David French · Post by **David French** » Tue Jul 04, 2006 7:50 am

The chunks are somewhat better than panels. Check the graphs here for a comparison of 24" face panels vs. 34" face chunks (unfair comparison, of course. Even for the standard 24" size, chunks are better than panels, not becuase they extend the low frequency limit (becuase they don't really), but because you get a smoother absorption curve. Even absorption at all frequencies=good. Panels are by far the most cost effective solution, so if money is a major issue, I would hunt down something in the 50 kg/m^3 density range and use panels instead. If you can afford it, the 34" face 'superchunks' made from the less dense stuff will provide significantly more absorption.

David French · Post by **David French** » Tue Jul 04, 2006 7:56 am

For your last post, don't worry about these types of calculations. Panels and chunks are both extremely effective at 140 Hz.

Like I said, find denser stuff and make panels if cost is a major concern. 12 or so 2' x' 4' x 4" panels will straighten your room right out for 60 Hz and above.

BoB_DoLe · Post by **BoB_DoLe** » Tue Jul 04, 2006 11:53 pm

ok, i think i got it now, must have surveyed the graphs somehow..
as for the money, i was a bit wrong, cause my fibreglass is the same size as in the link you posted, so i can get 4 chunks out of one panel, what makes 24chunks (each 4inch thick) meaning 96inches of chunks per package for about 12euros, thats not too much i guess

so i can stuff the whole wall-ceiling-joint with two packages, chunk size 34".
i hope this isnt too much for my room, as its really a small one (this time not sound-wise, but space-wise), but i check for this..

i think im ready with this by sunday, then i make new tests (and listenings, of course!) and come back with results!
thank you very much so far!

David French · Post by **David French** » Wed Jul 05, 2006 3:01 am

The smaller your room, the more severe you must be with your bass absorption, so this is definitely not overkill. You will, however want to use chunks in other areas besides the floor to ceiling corners. You will also need to do something with the area where the ceiling joins the walls. This will help tame the resonances that exist between the floor and the ceiling. Chunks are always good, but you should be able to get away with 4" thick panels if you want to go that route.