John Sayers' Recording Studio Design Forum Archive

I'm looking at building 6, 4" panels, mounted 4 inches off the wall, to treat my first and second reflection points (left, top and right), as well as 3 bass traps in the corners (the forth corner of my room is cut at an angle, where the door is, so there's no 90* angle there, just two 135* angles at each side of the door).

I was planning on building membranes in the corners, after watching an episode of Pensado's Place, the acoustician on there said that superchunks really aren't effective in the lowend in small rooms, and that membranes are the only way to go. I'm still undecided, because there is a serious lack of membrane plans on the internet, but I'd kind of prefer not to build superchunks, because they are going to be very expensive, and potentially won't do as much as they would in a larger, more open room.

I have access to:

4" Roxul RHT80 (8pcf) - $1.30/sq ft
4" OC703 (3pcf) – $2.62/sq ft
4” OC703 FSK (3pcf?) – $2.59/sq ft
4" OC705 (6pcf) - $4.01/sq ft
4" Thermafiber SAFB (2.5pcf) $0.92/sq ft
4” Thermafiber Safing-unfaced (4pcf) – $1.28/sq ft

All panels come in 4"x24"x48" dimensions. 2" thickness is available in everything but the Roxul.

I was leaning more towards the Roxul, until they told me it was RHT80 and not 60. Is 8pcf too much for this sort of application (or would it work better in the application as a bass trap, more than a broadband absorber)? Otherwise, I'd probably go with Thermafiber on second choice, because it's significantly cheaper than the Owens Corning stuff. Unless the OC is really going to give me a huge improvement over the other two, I'd rather save some money and be able to get more of it if needed.

I've attached pictures of my room below, please note - The dimensions don't actually take into account the width of the walls, the dimensions you see are the INSIDE dimensions of the room.

As you can see from these images, I've got quite a bit in the room with my desk, as it also functions as my practice/jam room - The speakers in the corners next to my monitors are my PA system and I have 2 4x12 stacks along the long wall, with a small 2x12 cab along the short one. Obviously, the biggest item in the room is the drum set though. It's not the ideal setup, but it's what I have to work with for now until I move next year.

To be honest, my mixes translate fairly well and I don't have a huge issue with any particular frequencies (that I'm aware of), but I figure it's time to do some treatment, especially right now when I have dispensable income. The biggest problem I have right now, is that since I moved everything around (my desk used to be on the 13ft wall, with the 2 4x12s on either side, the 2x12 behind it, and the drumset in the corner where the 2 10ft walls meet), I've noticed a lot of unwanted echoing in vocal tracks I record, from what I assume is coming from all the reflection points from the bare walls.

Which would you buy for each application (absorber/bass trap), and why?

Hi "MatrixClaw". welcome.

Please check the forum rules for posting: you seem to have missed a couple of important ones! (One of them is the size of pictures and images...)

I was planning on building membranes in the corners, ... after watching an episode of Pensado's Place, the acoustician on there said that superchunks really aren't effective in the lowend in small rooms,

Well, there's several things wrong with that approach. Although the acoustician is correct that superchunks are not effective in the very low end, he is wrong about that only applying to small rooms: it applies to ALL rooms. No matter what size room you put a superchunk in, it will be just as effective (or ineffective). What matters is the depth of the device, or more accurately its thickness and distance from the wall. Distance from the wall defines the lowest frequency it can treat for waves arriving normal to the surface (perpendicular), and thickness defines effectiveness. (For non-normal waves, the frequency goes even lower and the device becomes more effective.) So room size has no effect. Not sure where he got that idea from!

But that isn't the issue at all, for several reasons.

First, membrane traps (panel traps) are tuned devices: They treat only the specific frequencies to which they are tuned (simple mathematical equation), and no others. Next, they are fairly high Q devices as well (although not as high as Helmoholtz based devices can be), which is exactly what you DON'T want for treatment in a small room. Third, you can make them cover a slightly higher range of frequencies, sure, but at the cost of effectiveness: you flatten the Q in order to broaden it. Fourth, you cannot do a panel trap in a corner anyway! By definition, in order to resonate only at its tuned frequency, it must be over a cavity of constant depth, which implies that it must go flat against a wall. Fifth, panel traps work on the pressure component of sound waves, which is maximum at the wall plane. If you angle a trap across the corner, then it is not in the maximum pressure zone, and therefore is not working efficiently. Etc.

So while the comment is partly true and may be well intentioned, replacing corner absorption (of any type, not just superchunks) with a panel trap would be pointless.

On the other hand, superchunks (and other corner absorbers) are broadband devices, that cover a broad chunk of the spectrum, without being tuned to any particular frequency. They are velocity-based devices (not pressure based, like panel traps), so they work well when spaced away from walls. However, since they are "permeable", they work for waves traveling in all directions, even for those traveling almost parallel to the surface of the wall, not just for waves traveling perpendicular to the surface.

Finally, there's the issue of how you would go about tuning your panel traps, even if you could figure out a way to give them constant cavity depth: Would you tune them to specific modes of the room? If so, how would you place them? It would be pointless placing a tuned pressure device in a pressure null for the frequency that it is tuned to!

There's plenty of research that shows superchunks and other forms of absorption to be effective at treating small rooms, even down to low frequencies. Yes, they are less and less effective the lower you go, but that doesn't mean they have no effect at all.

You have a very small room, so that room is going to need LOTS of broadband bass trapping, which it would be impossible to do with panel traps: there simply isn't enough wall area to be able to treat all your modal issues.

Based on your room dimensions (and ignoring the angled door), these are your first thirty room modes:

43.5 hz (1,0,0 Axial)
56.5 hz (0,1,0 Axial)
70.6 hz (0,0,1 Axial)
71.3 hz (1,1,0 Tangential)
82.9 hz (1,0,1 Tangential)
86.9 hz (2,0,0 Axial)
90.4 hz (0,1,1 Tangential)
100.3 hz (1,1,1 Oblique)
103.7 hz (2,1,0 Tangential)
112.0 hz (2,0,1 Tangential)
113.0 hz (0,2,0 Axial)
121.1 hz (1,2,0 Tangential)
125.4 hz (2,1,1 Oblique)
130.4 hz (3,0,0 Axial)
133.3 hz (0,2,1 Tangential)
140.2 hz (1,2,1 Oblique)
141.3 hz (0,0,2 Axial)
142.1 hz (3,1,0 Tangential)
142.6 hz (2,2,0 Tangential)
147.8 hz (1,0,2 Tangential)
148.3 hz (3,0,1 Tangential)
152.1 hz (0,1,2 Tangential)
158.2 hz (1,1,2 Oblique)
158.7 hz (3,1,1 Oblique)
159.1 hz (2,2,1 Oblique)
165.9 hz (2,0,2 Tangential)
169.5 hz (0,3,0 Axial)
172.5 hz (3,2,0 Tangential)
173.8 hz (4,0,0 Axial)
175.0 hz (1,3,0 Tangential)

While many of those are close enough to each other that several could be covered by one panel trap, you'd still need around a dozen different traps, tuned to a dozen different frequencies, to treat them all: where would you put them all? Some of them would have to be pretty deep, too, in order to treat the very low frequencies. And even if you did manage to figure all of that out, you'd still have another problem: A room lined with solid, hard, massive reflective surfaces. You might treat the lows well, but the room would be a nightmare for the highs!

The speakers in the corners next to my monitors are my PA system

Regardless of how you treat the room (panel traps or absorption) those will have to go elsewhere, as they are exactly in the most effective place for bass trapping: the tri-corners.

I don't have a huge issue with any particular frequencies (that I'm aware of),

Maybe yo should run an acoustic analysis of your room with REW: I think you'll be shocked to find out just how bad it is!

I've noticed a lot of unwanted echoing in vocal tracks I record, from what I assume is coming from all the reflection points from the bare walls.

Ahhhh! So you DO have problems! : What you are hearing is exactly what you say you don't have: "huge issues with particular frequencies". You are hearing the room, and since it is a small room you cannot possibly be hearing room reverb: you are hearing the modal response of the rooms, plus reflections, plus flutter echo, plus probably comb filtering, phasing issues, and other fun stuff. Surprisingly, all of that can be treated with (guess what?) absorption! Deep, thick bass traps in all four vertical corners (yes, all four, even the door), as well as perhaps in some of the other 8 corners. And also add the usual treatment for a control room: thick absorption on the first reflection points, behind the speakers and on the rear wall. And maybe also add some checker-boarded treatment on the side walls, if necessary to deal with possible flutter echo issues across the room. If all of that takes out too much of the high end, then you might need to wrap some of it in plastic, or use some type of panel in front of it. I would probably also combine that with something special around the drum area, but I'd wait and see how the first round of treatment goes before deciding on what to do there, specifically.

Which would you buy for each application (absorber/bass trap), and why?

I would buy a whole bunch of plain old 4" 703, since it is very effective across a broad range of frequencies, is relatively easy to work with, and there's plenty of research and supporting literature to help you design your treatment plan using it. I would NOT use the Roxul RHT80 or the 705, as they are both way to dense to be useful for low frequency treatment (although they might be OK for treating just highs, if you happen to need that: perhaps on the ceiling above your cymbals, for example).

Anyway, for what it's worth, that would be my approach to treating your room: And once it's done, I think you'll be greatly surprised at how much better it sounds, and how much better your mixes translate!


- Stuart -

Sorry about the issues with the post, I believe I've fixed them all now

Thank you for the the explanations. I've been trying to get answers to these questions on other forums, and yours was by far the most informed and useful response I've gotten, thank you! People keep confusing me, telling me the Roxul would be best as bass traps, because the high density is better for trapping lower frequencies, but since you seem to actually know what you're talking about from a mathematical standpoint, I'd rather take your word over their's!

I was really hoping to not go with the 703, if possible, just because I can get 2 to 3 times as much as the other materials for the price of one panel of OC703. I definitely don't mind spending the extra cash if it's going to be that big of an improvement, but I could nearly cover the entire surface area of the walls at the same price of the 703, if I go with the Thermafiber (not that that'd be good, but it's just an example of how much money I could save) I did manage to find this online, which lists the absorption coefficients of the 703, Thermafiber and Roxul I have access to. I've attached a spreadsheet below so you can more easily see the differences.

As you'll see, the RHT 80 says it's 5.9 PCF on this site, and not the 8 PCF I found elsewhere on the internet. It does seem to be most effective in the 125HZ range, while the plain 703 is best in the 250HZ. However, out of all of them, the 4 PCF Thermofiber seems to be the most consistent across the board, and has an average coefficient not far off from the 703 at 1.03 vs the 1.06 of the 703.

703 clearly seems to be the best in the 250-500HZ range, while the RHT 80 takes the cake in the 125HZ range, and the thermofiber is probably the most consistent across the board (the only place where it really lacks is in the 125HZ range, which isn't too much lower than the 703, had it not had this lower rating, it would have a better average coefficient than the 703). Is there any validity to these numbers?



I guess the next step would really be to run REW and see where I really need the most treatment, and go from there? I just ordered one of the Behringer mics so I can test it out.

My measurement mic finally came in, and based on the graph, it looks like most of my treatment needs to be in the <125Hz range, so I might go with the RHT80 in the corners (since it has the best rating at the 125Hz range) and Thermofiber on the walls (since it's fairly close to 703 everywhere else, and is half the price).

Unless someone else sees something I'm not seeing and thinks I'd really benefit a large amount going with 703 over everything else...?