CR Rear Wall Question

Well, things are moving along. I framed up the rear wall of the Control room Saturday. The structural part of it, that is. And I laid-out the framing for the slot resonators, and the up-lighting "bench". The area for the slot resonators are a maximum 24" deep, and a minimum 14" deep and about 48" across. Seems kinda big to me. Attached is a picture of the base plates laid out on the floor. I'll end up with a rear wall height of about 11 feet. The functionality of the slot resonators will sort of "end" at about 9' high. It'll still appear to go from floot to ceiling, but that additional 2 feet will actually allow me to install the return air vents for the CR in that void. The bottom of the slot resonators are of course sealed by the floor, and a piece of 5/8" plywood will seal the top at 9'. Again, above that is the void for the return air vents. Sound like a plan? Any thoughts on the dimensions of the Slot Resonators?

Another view.

...Feel like I'm talking to myself here.... So, I ran the helmholtz resonator spread-sheet, and with that size, and those dimensions, I should be able to attenuate frequencies down to about 200 Hz. Not bad, I guess.... I was hoping the increased distance from the wall would allow for a little lower frequency attenuation... but apparently not.

Hey Mike, Don't forget, the resonator was design for midrange freq's. The hanging bass traps whereever you plan to put them is for the bass freq's. Bryan Giles

Yeah. I know. I was just thinking that the 24" would attenuate a little lower. I haven't really seen anything that will calculate low frequency attenuation.

Michael, sorry it took so long to answer - I've been waiting for a reply from John, because I'm a little confused as to your use of slots in that location. From what I've seen of your plan, it looks to me like you're kind of close to the mix position to use reflective or diffusive treatments there, yet you're showing both in your plan. As to variable depth/slat/slot widths giving a wider frequency range - Barefoot disagrees with that concept; the wavelength at 200 hZ is 5.65 feet and low frequencies are basically non-directional, so a resonator that's less than probably 2 wavelengths or more in dimension would tend to just average out over the surface of the resonator. Not that the resonator wouldn't work, just not as assumed. You could, if you add "separators" between sections inside the trap, create separate resonators with different average depth of cavity if there's a need - one tricky part is deciding ahead of time just which frequencies are going to be a problem. With splayed walls, modes are still there but not as easy to predict pressure zones, etc - the only way I know to get close to what frequencies may be problems is to take the AVERAGE dimensions in all three axes and calculate modes from that. Any modes that are supported in more than one axis would be my first choice for trap tuning. If your head will be less than 10-12 feet from the rear wall, I would either use straight broadband absorption (703 across the front of the cavity) or keep the slats but put 1" 703 panels in FRONT of them (spaced by at least an inch) to avoid early reflections back to the mix position. Hope that helped, maybe you could clarify your intentions??!? Thanks... Steve

Yeah - I agree with Steve about being to close to the mix position to use slots on the rear wall. Maybe something more like this?? cheers john

OK. You guys are freaking me out here, so before I have a panic attack :) let me clairify a few things to make sure we're on the same page, and that I'm not butchering the terminology. Let's start with the control room size: Front Wall 13'-7" Side Walls 11'-10" Splayed 12 degrees at that point the 12 degree angle reverses and continues for 6'-6" to the rear wall. Length From front wall to rear wall 18'-0" Mix position 6'-6" from the FRONT wall. (Leaving 11'-6" from my head, to the rear wall) These are actual "As-Built" dimensions. Now, some terminology: When I say "slot resonators" I mean these: Now these appear to be set, in rear corners, at 45 degrees. The ones I have in mind would be at 22 1/2 degrees. My original plan, the one on the Johns Studio's Under Construction Site, shows an RPG type diffuser on the rear wall. I'll concede that that probably won't work, and in its place will be an angled absorber - Cloth covered 703. I see control rooms designed with dimensions similar to mine in length that DO use slat resonators on the rear wall. So, why not here? Maybe my "Studio Vocabulary" isn't where it should be?

Michael, panic attacks are for AFTER you've built too much and it's wrong - I've gotta play "honey-do" for about an hour, but I'll get back to you early this evening at latest - first, your CR is right at 20 milliseconds delay from your head back to your head off the rear wall. Second, your surrounds will cause early reflection problems with slats there. Third, what John drew will fix it all. More in a bit... Steve

OK, honeydew done 4 now - Here's a quick markup of your CR, with the two reflection paths I was concerned with added in. I converted your CR drawing to vector, but only cleaned up the angles that matter when doing a crude ray-trace, so most of it might look kinda wierd - the reflection angles, however, are exact. I only drew one side, but the other would be identical. Note that the path length we're concerned with is from your head to the back wall and return, which as I said earlier would give about 20 ms delay. This is right at the go/no-go area as far as smearing is concerned, and I personally wouldn't take the chance. In the case of the surrounds, the path we're concerned with is the difference in length between the direct path and the reflection path, which are only about 4.6 feet, or 4 ms, different. This is definitely too soon for reflections to be happening. So far, all the 5.1 mixers I've known of are tending to absorb any wall that has a speaker aimed at it, then putting the ambience back with DSP. This is why I'm looking at reversible inserts for the front half of my CR, absorbent side for 5.1 and reflective panel trap side for stereo, for RFZ. That's about all I can think of at the moment, I hope I clarified my concerns. Had I noticed any of this earlier, I would have brought it up sooner. Hopefully it's not too late to still consider it a relatively minor "whoops"... Steve

Wow, Steve, thanks a lot. I'm glad I asked. Saved me a ton of headache and heart break. So... no slot resonators in my CR. I guess.... I thought I really had this all figured out, then along come you guys. Now I feel really stupid! :oops: Now, John's pic shows bass hangers... behind what? Cloth? And no 703 in the bass hanger cavity? I'm doing a quick re-charge on the camera. Maybe I can get you to look at something else? In the live room. I'm probably beating another dead dog in there! I'll post a pic......

I have to add this, no Slot resonators on the REAR wall of your control room. On the side walls at mix position is fine. Justr not on that rear wall. That framing you laid out for your resonators.... I'd do what I did in the rear of my control room. That would be all absorptive with4' Mineral wool covered with fabric. This is what I did. I left a piece of fabric off in the pic for demonstration purposes. It's basically the saem thing John and Steve are sying just my interpretation. http://www.johnlsayers.com/Studio/Image ... _wall1.JPG That is a portion of my rear wall and you will notice the frame and the mineral wool in the frame with the cloth stapled to the front of the frame. Bryan Giles Sorry should've moved the chair. :)

Yeah, I've been checking out your progress too giles, thanks for the input. Steve, In the live room I have these colums built. They're there for elec. conduit, lighting, and outlets. Having these colums eliminates any penetrations in the exterior wall. That's the basic functionality. On the face of these colums I was going to add RPG style diffusors, and between them (Against the wall) I'm going to build some of John's Varible bass absorbers/diffusers. This first photo shows the wall with colums that would basically be the .... how should I say this... non-attack side for the 7'-2" grand I have... so the lid would open AWAY from that wall... basically into the live room. The second pic is the OPPOSING wall - located some 30' away. You can see that it has only ONE column which is centered between the other two on the opposite wall. Same treatment here. If you notice, in the first pic, theres a small "problem area" between the column and the wall separating the live room from the drum room. It's just kind of a small niche or corner created by the column and the wall. Any thoughts on that, and with the general scheme of treatment for this portion of the live room. Remember, there's a large grand going in here. OK. Pic #1

And pic #2

OK one more pic. This is that "problem area"created by the juxtaposition of the column and the live room wall. Right there where that plank is leaning against the wall just behind that red basket.

Michael, I'm thinking of two different possibilities for that corner - it would kind of depend on exactly where your piano is going to "live" for most of your recordings, as there may be some modal considerations for micing to get the most even frequency response. One possibility (and probably the safest, considering the effective room shape variations, is simply that corners are where ALL modes build up, so the safest thing should be just making that entire corner into a broadband absorber by enclosing it with 3-4" 703 or mineral wool. The other might be more interesting, but easier to screw up on - how well this would work would depend mainly on the actual positioning of the piano within the room, and could be really sweet or really crappy with very little movement. It would involve tuning a panel trap (not deep enough for either a slat resonator or absorption type bass trap) for one of the length modes in order to lessen the depth of its node, so that the piano could sit in a relatively even sound field. With your vaulted ceiling, mic height will likely be at optimum at different heights depending on the distance the piano is away from the wall, due to ceiling modes. This could be next to impossible to predict though, and I'm not enough of an acoustician OR recording engineer to even try to BS my way through it. Still, it could be a fun experiment if you were more into that than finishing the studio... I've marked up an approximate modal chart for your room, showing some possible positions for piano and mics relative to room boundaries - the LAST place you want a mic would be at the lowest points on the mode curves, the GREEN lines are possibilities. Note that I've used 12/6" for ceiling height, which should be about right for where (I think) you're putting the piano. On the Height chart, the green lines I've drawn are possible heights for lower and upper mics if you mic the piano that way. An XY or Blumlein pair might not like the horizontal position I drew on the length chart, I've only used XY pairs a bit and it was before I even KNEW there were wrong places in rooms for mics. Mainly just ramblings, if push came to shove I'd probably just put 4" of absorbent in front of that corner and see what happened. Same with the OTHER corner, between the other column and the Mech room... Steve

You know, it never ceases to amaze me how that when the level of detail, and the factoring of conditional variables increases, my understanding of acoustics in general decreases. :cry: Its a very interesting study. I wish I knew more, but I really just wanna make and record music. No art without science though. I think I understand, albiet it on a limited basis, the graphs you posted. The ideal sweet spot for that room would be at a point where all modes intersect? Of course, the graphs are 2 dimensional, and the room is 3 dimensional, I'm having a little problem visualizing the intersection of the modes in 3 dimensions, particularly for the height, but in general, I think I get it... and I'm an engineer. :shock: Thanks again, Steve. You never fail to educate.

Interesting design on the rear wall treatments, John. Something like that wont change any of the framing I've already done. I'm curious on the facing of the area that houses the hangers though. Are those just cloth covered wall studs? No plywood on the backside? No 703? No "port to bass"? How do you "client proof" something like that? Suppose someone casually leans up against it? They'll push right through that cloth. These things are about 4' long and at least 9' high.

Here's a pic of the beginings of it, all jigged up.

Michael, join the club, it's not an easy concept to visualize. In the end, your ears are the authority. All this may do (besides confuse the issue) is suggest a place to START with instrument AND mic placement. I'm close to being late for the "DDJ", but I'll get back to you on this more tonight, and maybe some comments on the back wall construction if John doesn't get back to you first... Steve

knightfly wrote:

"... In the end, your ears are the authority. All this may do (besides confuse the issue) is suggest a place to START with instrument AND mic placement. ... Steve

Yes. Based on the modal response of the TREATED room. How cool is that! :idea:

Any thoughts on how to client proof a cloth covered wall? At the very minimum, I'm going to put a 12" "kick panel" at the bottom.

Michael - you make a 2" x 1' frame with cross pieces at 2' intervals - no one will go through it - believe me :):) Back wall of Left Bank is exactly that kind of contruction. cheers john

2"X1 foot? :shock: OR 2"X1"?

Michael Jones wrote:

Any thoughts on how to client proof a cloth covered wall? At the very minimum, I'm going to put a 12" "kick panel" at the bottom.

John's suggestion is good, but I would staple large-hole chicken wire of the front of it just below the cloth, because then an elbow or a mic stand won't puncture the cloth either. Well, staples is probably a bad idea but the concept is conveyed. I would think that 1" hole chicken wire of a thin gauge steel would do the trick and not hamper sound at all. What do you think John and Steve? I'm all for client-proofing. Its usually worse than baby or cat proofing!