Quick Slot Absorber Questions?

Hello. I have been working on our 20.4'L x 13'W x 10.75'H control room for a couple of weeks and I just have a quick question or three about slot resonators. I want to build a 4'H x 8'W x 9"D slot absorber on the middle-bottom portion of the back wall (behind the couch) to get in the low 70Hz range, which is the 3/4 wavelength of my listening position, where I have a measured null. There will be cloth-covered R38 above it extending to the ceiling. 1. Is the surface area and/or placement of the proposed slot absorber enough to make a difference? 2. Are 2x4's adequate for slats? If spaced 1/16" apart, that should theoretically hit the low 70Hz range. 3. Is fluffy pink insulation ok for behind the slats, or should I use OC703? 4. Am I insane? Thanks, Seamus

Hello there Have you looked at membrane resonators? They are easier to build plus you can get even deeper absorption using another design, and you'll save an expensive resonator with 2x4 :shock:

Thanks for the reply! My first thought was a membrane absorber, actually. It just so happens that I have a bunch of extra 2x4's, though. However, if a membrane absorber would be more effective, I would go that route, instead. I did some more measurements last night. After some speaker repositioning I got the nulls smoothed out a little. They (Quested S8r) are 1.5' from the front wall and 27" from the side walls. The high pass filter is on, which helped a great deal. As it is now, I have a 62Hz null (-16dB), as well as some various nulls between 95Hz and 150Hz (@ -12dB). There is also a -16dB null at around 350Hz, which is weird. I should probably use some sine waves and an spl meter to try and find out where they are happening first before I bother building anything. Just for the sake of giving more information, the only bass treatment in the room at the moment is that the entire front wall is covered with 12" FRK faced R38. The whole wall except for the 24" hollow core door in the center, that is. This helped out a lot with some huge problems that we used to have. I was planning on Covering the entire back wall with 18" of pink fluffy (2 layers R30), as well. Right now, there is only an 8' x 8' section of 4" OC703, which is not doing a whole lot, I suspect. I am, also, planning on doing a 2'W x 8'H section of R38 on the side walls right next to the speakers. The side walls are plaster and lathe on brick. The front wall is just one layer of 5/8 sheet rock. By one layer, I mean that I built an inside-out wall with the 5/8 rock on the outside of the room with the 12" R38 facing the room. Thanks, again! Seamus

Some of your problems might be solved with "standard" treatment, saying, corner traps (a bunch of insulation stuffed or thick panels straddling the corners), also place some absorption at your first reflection points (you know, you should do the homework too), and then measure again. If you still have a deep null then consider treating it accordly Thats my advice

Thanks, again. I do have first reflection point treatment on the side walls and the ceiling. I've been ruthlessly studying acoustics for about a year, now. I've just finally had time to put it into a tangible form. Seamus

Make some superchunks then (insulation triangles stuffed at the cornesr, ideally all 4 of them, covered with nice cloth), and then measure again

Yeah, I decided not to go with the superchunks quite a while ago for money and time reasons. I'm going to stick with the low density fiberglass, for now. It's cheap, quick and easy to install and seems to have excellent specs. It, obviously, takes up a lot of room, but done strategically, isn't a big deal. If I still have the nulls, I'll look into the tuned traps more. I'll be able to just remove sections of the fiberglass and replace them with whatever trap will work out best.

You know a easy and cheap way to find out? Go buy some insulation packages, and just stuff them in your corners, with the plastic cover and everything. And measure

seamus wrote:

Yeah, I decided not to go with the superchunks quite a while ago for money and time reasons.

"Right now, there is only an 8' x 8' section of 4" OC703, which is not doing a whole lot, I suspect." There's your material<looks up> for your super chunks.

Ha! Yeah, that's true. There is still the time and the "where do I cut all that fiberglass?" part, though. Also, I had kind of ear-marked those panels for the live room. I'm getting the feeling that you guys think that the superchunks would be worlds better than low density fiberglass soffits (roughly 2' x 2') in the corners...

jbassino wrote:

You know a easy and cheap way to find out? Go buy some insulation packages, and just stuff them in your corners, with the plastic cover and everything. And measure

That's true, as well. However, those bales are compressed quite a bit in the plastic. I fear that it wouldn't give me an accurate idea of what's going on.

Im sure it will. give it a try

Yeah, ok. I'll try it. Although, I only have two bales of R30, so it'll only be 4' high. I'll put them against the wall (next to the speakers) and measure. Then I'll do the R30 in 8'H x 2'W x 18"D frames on the sides of the speakers, and measure. I probably will end up doing the superchunks, if I need to. I do have the OC703, as it was helpfully pointed out. The live room can wait. There are some structural limitations to the back wall that would prohibit me from doing all of the corners, but there is some room here and there. I'll probably try a tuned trap as a last resort. Thanks so much for the advice and attention. Seamus

So, a little update. I built front wall/side wall corner soffits that are 2' wide, 8' high and 18" deep. After repositioning the speakers, the problems between 95Hz and 150Hz improved quite a bit, but that 62Hz is a killer. I also built a back wall absorber that is 6' wide, 8' high and 18" deep. It didn't change the frequency response at all. This really frustrated me. I hadn't covered the side corner soffits with fabric, yet, so I pulled the insulation out and measured again. There was nearly no change. It was the speaker repositioning that had improved the 95Hz - 150Hz, apparently. There may have been a slight change in the lower 100Hz area, but not enough to justify eating up all that space. The decay time in the room (by ear) increased, however. Anyway, I'm left with a back wall absorber that doesn't do anything for the response and no corner treatment. I'm going to do the superchunk idea in the front corners and use the pink fluffy that I have left to extend the back wall trap up to the ceiling. We'll see if those two things can touch the 62Hz. Thanks, Seamus

I think you might be working under false assumptions: Absorption cannot eliminate a room mode. It will always be there. You can damp it somewhat with absorption, but it won't go away. To eliminate it entirely, you have to change the geometry of the room somehow. You can break up standing waves with angled surfaces, which is one reason to not have parallel walls in your room. You can also suck some of the energy out of it with resonators of one type or another, but absorption alone can't do the job. Maybe that membrane resonator might be a good idea, after all? I'm also not sure where that 62Hz issue is even coming from! Based on your dimensions, you do not HAVE any modes near there! You have one at 59.4, and the next one is at 68.2 (both tangential), but nothing at all near 62. Are you sure your dimensions are accurate? Are you sure your frequency measurements are accurate?

The decay time in the room (by ear) increased, however.

I guess I must be misunderstanding what you said: The RT60 time INCREASED after you put a whole bunch of absorption all over the room? That doesn't make sense!

Thanks for the reply. Yeah, I don't think that the 62Hz issue is due to room modes, actually. If I take a measurement from a foot or so closer to the back wall, it moves up to around 70Hz. To me, that would suggest that it's a reflection from the back wall, I guess. Sorry for the confusion. I meant that once I took out those front corner soffits, the RT60 increased noticeably. It occurs to me that it might be proper to include a link to another forum that I have been getting help from, as well. http://www.gearslutz.com/board/studio-b ... ion-3.html Seamus

Seamus, I noticed from the other thread that you have not yet sealed your walls, that the have gaps around the edges and holes in them. It occurs to me that this might well have something to to do with your problems. Not quite sure what it might be doing, but it wouldn't surprise me at all that it is doing something to mess up your room acoustics. Maybe it would be a good idea to finish that first, before continuing with your testing? - Stuart -

Thanks, Stuart. I'm going to go and seal it up tonight. I think that you are right. I wonder if it would act as a Helmoltz to some extent. Thanks, again. Seamus

You know, maybe that 62Hz null is modal in nature. I do have that 1,0,1 at 59.4Hz that you mentioned, Stuart. When I moved the mic back, maybe the null wasn't moving in the frequency range, but it was moving in amplitude. In other words, maybe I just moved out of it, and there was a different problem rearing it's head in the new spot that is unrelated. What's the margin of error on this when taking into account things like structure and/or materials? Could it be +/- 2.5Hz? It seems like it could very well be.

When I moved the mic back, maybe the null wasn't moving in the frequency range, but it was moving in amplitude. In other words, maybe I just moved out of it, and there was a different problem rearing it's head in the new spot that is unrelated.

That would actually make sense, since room modes / standing waves are, by nature, "fixed" in the room. As you move about the room, you'll go from peak to null of the same mode. So you might well be right: as you moved the mic to a different spot, you were looking at a different mode, or indeed an entirely different problem. I think I'd be more concerned about what is happening at your mix position, than what is happening elsewhere in the room. If you have a major null at peak at the rear wall or side window or anywhere else isn't an issue: for example, you will ALWAYS have major problems in the exact middle of the room anyway, no matter what, which is why it's a really bad idea to put your mix position there. So I reckon I'd concentrate on what is happening at the mix position. It's hard to say what the margin of error might be: that depends on may things, not least of which is the accuracy of your physical measurements (room dimensions) and the accuracy of your test measurements (mic, pre-amp, meter, test tones, etc.). Also, there is the issue of the logarithmic nature of the musical scale: a difference of 2.5 Hz at the bottom end of the scale is nearly two entire notes, but at the top end it isn't even a tiny fraction of one note: It is totally irrelevant. At the frequency of your problem, the difference between 59.4 and 62 Hz is nearly an entire note: it's the difference between an A# and a B, so it IS a big issue in that octave. If it were the a couple of octaves higher, the difference between 259.4 and 262, it would be irrelevant, but that low in the scale, I'd say it matters, and precision is important. So if you think it might be a "margin of error" thing, then perhaps you should re-measure your room with great accuracy, and also use very accurate single-frequency tones (1 Hz. accuracy), rather than sweeps or pink noise, to scope it out in better detail. But either way, it seems like you might need to be thinking about a tuned trap to deal with that issue. IMHO! - Stuart -

Yeah, it occurs to me that the room may be shorter than I think it is. You can't hire good help these days. After repositioning the speakers, I got that 62Hz up to -12dB. I also have a 115Hz and a 155Hz (approximations) null at -12dB. Good extension down to 30Hz, though. Here is a copy/paste from that (ahem) other forum that I posted: "I spent a few hours moving speakers around last night. I got the response to +/- 12dB from 30Hz to 300Hz. -10dB at 30Hz -12dB at 60Hz ish -12dB at 115Hz ish -12dB at 150Hz ish An interesting position was that at the farthest possible place from the front wall and the side walls (while still having them a couple of feet apart) yielded a FLAT response down to 80Hz... and then it dropped like a rock. So much so that at 60Hz it was already -30dB.. and it kept on falling. The 1/4 wavelength of 60Hz is about 4'8", which is about how far they were from the side walls during that particular test." Well, I am intending to build superchunks in all the corners that I can. If that doesn't touch the 60Hz thing, I will build a tuned trap of some sort. I assume that the superchunks will help the 100Hz-200Hz area pretty well. I would actually rather build a slot type, but if a panel trap would work better for the 62Hz, that's what I'll do. You totally right about the 2.5Hz on the musical scale. My brain doesn't link acoustics with music as much as it should. Thanks a lot, Stuart. Seamus