Angled walls..

audioeric · Post by **audioeric** » Tue Oct 28, 2003 11:11 am

Hey does anyone have any "hard" evidence about how much angle a wall should have.. I believe I read on here that 12 degrees should be the minimum, with 24 degrees being the main angle or something like that.. Just wondering what algorithms or anything anyone has..

The other engineer at the studio is trying to say that like a 2 - 3 degree (barely ledgible on paper) is enough to not get any flutters or anything.. basically saying that since they aren't parallel that the effect parallel walls would have are completely gone..

You see, we're trying to decide between a few different variations of a design.. maybe you all could help a bit..

Here's a plan that John had drawn up.. (THANKS SO MUCH JOHN!!)

Here's the other guys plan.. Now, this is about the least that I could draw as far as angle.. This is about right, maybe a bit less angle.. Now, the actual numbers are wrong on the sides.. We measured it all out, and the total length is about 45'.. So, I went ahead and taped off some stuff, and having the other Iso booth (Iso Booth 1 in John's) just isn't feasible, as the total length of the studio is 5' less than he thought it was.. (sorry)

So, I went ahead and made some changes.. I tried to keep the control room as large as possible in this, as it's really not scale here.. it's actually going to be a bit shorter than the live room is.. So, imagine the live room a bit longer, and the control room a bit shorter.. Sorry, my drawing skills really suck..

Anyway, I figured this would fit our needs the best, as we usually just need more than one room when doing drums and scratch tracks for everyone else.. So, having the drums in teh live room.. bass and guitar in ISO booth.. or viseversa.. and singer in the "lobby" or wherever the guitar and bass are.. or if doing just a live demo, putting the drums in the ISO booth, and use the live room for amps and all..

anyway, any help would be appreciated on the angled wall thing.. Really looking for some "hard" evidence.. He's not one to accept, because it needs to be that way.. And, I'm not either.. He says he has some recording studio design book that has an algorithm and he's figured it up that only 3 degree angle is all thats needed to stop flutters, echos, and standing waves..

eric

barefoot · Post by **barefoot** » Wed Oct 29, 2003 2:57 am

Eric,

This is actually a rather complex question you are asking. There isn't a simple equation that can tell you the modal distribution of arbitrarily shaped rooms. Analytical solutions to the acoustic wave equation are only available for simple shapes like spheres, rectangular boxes, and cylinders. Complex shapes must be calculated numerically. I don't have numerical modeling program on hand, but I'll see what I can come up with.

In the mean time, by simplifying the situation, you can get a good idea why 3 degrees is insufficient. First of all, you must realize that ALL room shapes have standing wave modes. Certain shapes like rectangular boxes, however, reinforce a small number of specific frequencies. Parallel walls are particularly bad because the higher order modes are all harmonically related. Complex shapes, on the other hand, tend to distribute the modes weakly across a wide frequency spectrum - and in many cases enharmonically. The question then becomes, how complex is complex enough?

Lets consider a room with parallel front and rear walls, parallel floor and ceiling, and side walls splayed at 3 degrees. The width of the front wall is 15' (W1) and the distance from front to rear is 24', so the width of the rear wall is:

W2=15' + 24'*tan(30deg) = 16.26'

So, the standing wave modes that will be excited across the width of the room are distributed across a frequency range relating to the range in widths.

Just considering the fundament modes where the wavelength is twice the width we get:

F1 = c/W1/2 = 1128/15/2 = 37.6 Hz
F2 = c/W2/2 = 1128/16.26/2 = 34.7 Hz

So, the fundamental room modes setup between the non-parallel walls range from 34.7 Hz to 37.6 Hz over the length of the room. This is actually a very narrow frequency spread. Musically it's about a half step interval. Of course, the frequency spread in the vicinity of the listening position is even tighter since the wall distance spread is just a fraction of the total. Room modes just a fraction of a tone apart will easily reinforce one another.

Try the experiment with a pair of acoustic guitars. Tune the bottom E strings a fraction of a tone apart. Dampen the other strings, bring the guitars reasonably close together, and pluck the E string of just one of the guitars. Do you think the E string of the other guitar will start to vibrate? Of course it will.

The room I described will have a fairly strong room mode centered at about 36 Hz, plus its harmonics. This mode would be slightly stronger if the walls were parallel, but only slightly. Increasing the splay angle will distribute the resonances over a wider frequency range and, hence, they will be less strongly interactive.

Thomas

knightfly · Post by **knightfly** » Wed Oct 29, 2003 5:11 am

Eric, Thomas nailed it for you there - I imagine where your friend got his 3 degree value could easily have been from Everest's Master Handbook of Acoustics - I have both the hard copy and the E-book version, but still disagree with Everest's general dismissal of non-parallel walled rooms.

In one place, Everest states that 1 foot in ten, or even 1 foot in 20 is enough. The 1:20 works out to just under 3 degrees, so that's where your friend could have gotten his number.

If you have that book, or can borrow it, you'll see that NOWHERE in it does Everest show a SYMMETRIC, splayed-wall room, only non-symmetric shapes. This would preclude any kind of symmetric sound field for mixing, and for a control room I wouldn't recommend that either.

However, there was one place where Everest admitted that a symmetrically splayed control room would help with upper frequency reflections (in other words, RFZ) -

Thomas is exactly right in saying that this isn't a simple choice. There is more to it than just killing flutter. I've done primitive ray tracings for control room front ends that took much steeper angles than even 6 degrees on a side to get reflections to go where I wanted them to. In fact, I just finished a basic drawing to see if putting the mix desk in a corner of a small room was better, and still don't know for sure if I want to do it.

John has built (and supervised) numerous rooms, and he's told me that 6 degrees per side is the least he's found to be sure of killing flutter. Again, that's flutter, NOT reflections back to the mix position.

Oh, and speaking of "hard" evidence - the stiffer your outside walls are, the more bass trapping you'll need in the room - what better way than those "wasted" areas behind your (properly) angled walls?

Yeah, we're ganging up on ya - you know you like it, hmmm??!?

Steve

audioeric · Post by **audioeric** » Wed Oct 29, 2003 11:38 am

okay.. that's it.. it's official..

you guys fucking rock!!!

Thanks so much, and yes, I do believe that's the book he has.. We were also talking about what to do with the computer monitors, and he wanted to put them overtop of the window in the control room.. yes, that's right.. the bottom of the monitor would be at about 6 feet 6 inches.. I have no clue how in the world anyone was supposed to see it.. You see, he hates computers, but doesn't really like to do a lot at once.. So, he'll start a mix on the analog board, and then leave it that way for a week, and expect no one to mess with it.. Um, hello, the studio is my place of work, I need to work to get money, the board is how all signal gets recorded through, I think it's just a tiny bit necessary.. RANT RANT RANT.. okay.. breathe..

thanks so much again guys, these are PERFECT!!! you just have no clue!!

Now, yeah, gang up on me all you want, you guys have great advice, and can back it up with logic and numbers.. And yeah, that was the idea as well is to use John's plans to make bass traps in there, as 2 of those walls, the left and top, are cement walls.. which we'll be covering with a built wall, but will also be properly angling all of that and doing bass traps in the corners..

again, thank you so much.. if you guys are ever in Tampa, FL just let me know, drinks are on me

eric

knightfly · Post by **knightfly** » Thu Oct 30, 2003 1:48 am

Glad we could help - ain't partners fun? (just kidding) I had a 21" monitor sitting up high for a while, now I'm working on a design that puts 3 19" LCD's BELOW the level of the 3 front monitors - its so much more comfortable to look DOWN slightly - it's been a bear, but I think I've finally got a design going that will let me still get my knees under the desk, have room for two mixers on the desktop, 3 monitors behind those, about 15 tilt-out rack spaces under each side, and speakers still at ear height. Man, am I glad I'm not an "Ergonomicist", or whatever they're called, for a living - a guy'd need a rubber room after a few weeks of that... Steve

audioeric · Post by **audioeric** » Thu Oct 30, 2003 3:05 pm

Yeah, ergonomist.. They'd definitely have a heart attack in the studio.. especially ours right now.. sitting in the smallest seat possible, the keyboard tray hits my knees, so i have to lift it up about 3" and set it on my knees.. and its about the size of the top of a stove, maybe bigger.. our monitors are actually a good distance away, we're thinking of using a tray to slide them toward and away.. it should work well.. I'm not sure if you saw my post awhile ago about this..

http://www.johnlsayers.com/phpBB2/viewt ... ight=#4761

these were ideas for CRT monitors, but would be probably 100x easier for LCD's..

anyway, thanks again guys.. looks like we'll be going with this design

I've got more questions.. but I need to be up in about 3 hours.. trying to help out some friends and do studio stuff just isn't working well right now..

later all, and thanks so much again.. we'll be sure to take lots of pictures

eric

telefunken · Post by **telefunken** » Wed Dec 03, 2003 8:28 am

in regards to the 12 degree total, and 6 on each side, could you have one wall angled at 9 and the other side splayed at 3 for a total of 12? and so on and so forth. or that wouldn't work?

knightfly · Post by **knightfly** » Wed Dec 03, 2003 10:35 am

That would be OK for anything but a Control Room, where you need (vertical) plane symmetry between left and right halves, centered on the mix position - otherwise, you end up with different reflection patterns from side to side, which will shift imaging toward the side with the shorter paths (psycho-acoustic panning - the earlier signal dictates where the sound "comes from") -

It's also necessary in the case of floors/ceilings - sloped floors get kind of tricky to keep equipment from rolling away

so we take the entire 12 degrees or so at the ceiling, leaving the floor flat and boring

... Steve