Acoustic panels: how far to mount from wall?

Ren Man · Post by **Ren Man** » Mon Jul 05, 2004 3:07 am

This is my first post. Mostly I just browse this forum and have learned quite a bit from both the questions asked and the answers offered back.
Since I'm in the HVAC business, the material of choice I used to build 2'x4'x4" 'midband" absorbers for my studio was Johns Mansville Permacote Linacoustic R-300 rigid fiberglass duct liner board. This is 3lb density fibergalss with acoustic absorbtion ratings almost identical to Owens Corning 703.
I've spent a lot of time going through Ethan Winer's 'Real Traps' site and studying his materials- a virtual treasuretrove of useful information. Had I known about Ethan's Real Traps Before going to all the trouble of building all of my own acoustic panels, I would have ordered them from him.
Anyway, I've noted that Ethan specifies mounting his Real Traps four inches from the wall (in an effort to increase low frequency absorbtion).
My question? I've mounted my panels only one inch from the wall surface, and am wondering if re-mounting them four inches from the wall would increase their low frequency absorbtion significantly. For that matter, since I would be re-mounting them anyway, would six or eight inches increase the low frequency absorbtion even more? Or does distance from the wall reach a point of diminishing returns with a four inch thick panel?
There are studio pictures on my website if this would be helpful- www.nrpstuudio.com
I've made some errors in the design of my control room. By adding the restroom and isolation booth at the back of the room 1) it's not symmetrical and 2) my room proportions (and modal distribution) have been altered. The end result is bass problems. I have a pretty severe null at the mix position. My hope is to aleviate this problem without ripping out walls.

Ren Man · Post by **Ren Man** » Mon Jul 05, 2004 3:09 am

Oops. The web address above is misspelled. It should be www.nrpstudio.com

knightfly · Post by **knightfly** » Mon Jul 05, 2004 10:48 am

Christopher (or Chris, any preference?) first, welcome; second, nice site, and a good vibe (from what you can tell from pix) -

There are several things we'd need to know that aren't really clear from pix - a floor plan (and elevation if ceiling isn't flat) along with exact dimensions and locations of speakers, your head, walls, etc, would get us closer to some helpful suggestions - there's one null you can't get away from and still stay symmetrical, that's the axial side to side one - so knowing actual dimensions and what frequency your null(s) are at will help with that. Absorption in the right places can help with the side-to-side nulls - the rest can usually be tamed by positioning. (Don't forget to list things like distance from the centers of woofer cones to both floor, ceilings, and walls (all walls) - if using a sub, where/what is it? (same information needed)

When you mention Ethan's name and traps in the same sentence, we tend to assume panel type traps - can you be more explicit about your trap construction? (pix don't show that part either, you can cover ANYTHING with cloth)

I'm kind of surprised that Ethan suggests off-wall mounting, I thought most of his trap designs were membrane types, which work best at boundaries.

What's your ceiling like? Don't worry about long-windedness, you can't describe complex things with a few syllables...

The pix aren't quite telling me what you've done with your front corners - again, a drawing would help here. It looks like you have something diagonally across the corners; if so, what exactly?

Again, welcome - and, if you haven't noticed, I for one don't believe in brevity if it impacts understanding

Steve

Ren Man · Post by **Ren Man** » Mon Jul 05, 2004 12:45 pm

Steve,
Thanks for thr reply. I prefer Christopher- My wife's name is Chris and to quote an old business associate, "Christopher's got more Columbus to it" (I was trying to come up with a name for an art gallery I was starting up back in 1977. Ended up calling it Christpher Gallery)
Talk about long winded...
I'll get to work on my homework assignment rigth away. I'm not sure how to insert jpg images into this thread. I'll have to figure that out.
Thanks, Christopher

Ren Man · Post by **Ren Man** » Mon Jul 05, 2004 2:53 pm

Couldn't figure out how to post an image so I published them on a temporary page on my web site. Just scroll down to the very bottom of the home page and click on Building Plans.
I'm almost embarassed to make these drawings public. They are poor quality plus, in publishing them I reveal my mistakes to the world. I wish this forum and others like it were around before I broke ground on my control room back in October of 1993. Oh well, "it is what it is" as they say.
The top image is an isometric of the controm room and studio. This gives you a feel for the complex geometry of the control room (and the unfortunate asymmetry of same).
The second image is the West studio elevation showing the RPG Diffusor Blox.
Third image is the studio floor plan. the bottom of the image shows the control room window and door(s) into the control room
Forth image is the control room floor plan with dimensions showing the aproximate mix position.
The last image is control room West wall elevation showing the ceiling height at the wall and at the center of the peaked ceiling.
My monitors are placed symetrically along the north wall (right side) of the control room. Centerline of the Genelec woofers are 60" in from the side walls, 26 1/2" out from the North (back) wall and 50" off the floor. The Tannoy dual concentrics are 72" in from the side walls, 22 1/2" out from the North wall and 49 1/4" off the floor.
My mix position is 79" out from the back wall and centered between the side walls. The same height as the monitors. All of these dimensions are to the wall proper- NOT taking into consideration the thickness of the wall panels and the fact that they are mounted one inch from the walls.
The wall panels are two 2" layers of the R-300 3lb density per cu ft fiberglass (as mentioned above) covered with Guilford fabric- 2'x4'x4". The front and sides are exposed acoustically (no frame work boxing them in). Only a 3" wide luan "picture frame"-like panel covers the perimiter of the back side with the center of each back panel exposed. This luan frame gives me the means to attach them to the walls and ceiling.
There are a total of four 20" diameter ASC Tube Traps (two stacked in each corner). These made a big difference but there are still low frequency problems.
Hopefully this is enough info to at least begin a dialog. Sorry about the poor quality of the drawings. As always, Thank you.

Ethan Winer · Post by **Ethan Winer** » Tue Jul 06, 2004 12:49 am

Steve,

> When you mention Ethan's name and traps in the same sentence, we tend to assume panel type traps <

Just to clarify: Note that Christopher mentioned my company's site, which implies traps that do benefit from an air gap.

--Ethan

knightfly · Post by **knightfly** » Tue Jul 06, 2004 5:01 am

Ethan, point taken - hadn't been to your site in a while, finally noticed the vent slots in the sides (duh) won't ask what's inside (wink wink) but looks like a very nice non-DIY solution...

Chris, I resized your pix and posted them here; that way, if you want to remove the "embarrassing" link from your site it's cool. BTW, unless you're on a MAC or Linux machine, the simplest way to post pix is to make a jpg of them (or tif), resize (paint works for this) so the width is under 800 (preferably 750) pixels, and then click on the Browse button below the text box - find the file on your hard drive and select it. This works for single pix. You're allowed up to 150k file size, so bmp images won't work. tif sometimes screws up colors more than jpg -

knightfly · Post by **knightfly** » Tue Jul 06, 2004 5:09 am

And the other two -

To post more than one pic at a time, click the browse button; find the file on your drive, and double-click it. This puts the info into the browse window - now, click the "add attachment" button, wait til it refreshes - now the browse box will be empty - repeat this for each pic.

This was the first time I'd used the multi-pic thing, and it only allowed three pics on a post for some reason - seems like I'd seen others do more than that, so there may be a way - otherwise, just post again with the rest.

Hope that helped; gotta go play outside til dark, I'll try to dig into this more in the next day or so... Steve

Ren Man · Post by **Ren Man** » Thu Jul 08, 2004 11:13 am

Knightfly,
I was reading through my July 5th post and realized that my terminology would likely cause confusion.
When I said "My mix position is 79" out from the back wall" I was indicating the wall directly in front of (not behind) the mix position- the North wall (or the right hand wall in the supplied drawings).
I did read yor July 6th and prompty removed the "offending" pictures from my web site. I appreciate you taking the time to look at my situation with your detatched analitical approach.
I read in other posts where complex interior room geometry can complicate the process (or even make it impractical or near impossible) of calculating room modes and nulls and/or predicting their positions accurately. I've become convinced that building this complex room geometry created more problems than it solved. In the long run it's been a much more sensible approach to build a rectangular room with acceptable proportions and treat it acoustically (such as with the RPG Diffusor Blox and Skyline diffusors as I did with my 1996 studio addition).
But I still hold out hope that my control room can be treated without tearing out walls or other drastic measures.
I had also considered the possibility of opening ports to tap into the air cavity between the control room joist ceiling and outer truss ceiling of the outside structure (even though it will compromise the sound isolation of the room). As far as a sceintific approach to determining if it would workat all, let alone what size, location and quantity of openings, and the affect of the depth of the sleeves required to penetrate the ceiling; I wouldn't know where to start.
Anyway, I hope you haven't become disenchanted with my quandry after seeing the drawings. Thanks again, Christopher

Ren Man · Post by **Ren Man** » Thu Jul 08, 2004 11:38 am

The more I read this, the more I realize how little information I've communicated about the room.
The walls and ceilings are double layered 5/8" drywall on a wood frame structure with 1/2" Celotex (some call it Homasote board) sandwiched in between the layers of drywall.
I've treated the wall directly in front of the mix postiion (and 3 1/2 feet out on either side) primarily with the 2'x4'x4" panels described above. There are smaller 2'x2'x2" panels intermixed with the larger panels in a grid pattern. all of them are mounted 1" from the wall.
My approach was to leave the side walls reflective or live as this is the way we reference ourselves in "normal" rooms; while at the same time reducing the interfering reflection of the wall directly in front of the mix position by making it almost entirely absorbtive.
There are a few panels on a rear sidewall (next to the keyboard rig) to tame a reflection.
A portion of the ceiling above the mix position (and around the perimiter of the north half of the ceiling) is treated with the same 2'x4'x4" panels- 9 around the perimiter and two above the mix position.
The "L" shaped area formed by the entry doors has two of these panels on the "non-door" wall.
Hope this helps.

knightfly · Post by **knightfly** » Fri Jul 09, 2004 1:20 am

Christopher, after looking at your pix and drawings there seems to be some disparity - the pix (depth perception is tricky with 2D) seem to show some distance between your nearfields and the wall, where the drawings show them touching.

Your slight splays in sidewalls (calculates to about 3 degrees each side) isn't enough by itself to ensure control of flutter echo, but your absorbers are probably making up for that.

The splays, however, are going the wrong way (and aren't nearly steep enough) to help with creating a Reflection Free Zone - to do that, they would need to be wider at the mix position than at the front corners, and by enough to make it so you can't see your speakers in a mirror placed anywhere on that wall that's not absorbed.

The other problem the reverse splays will make worse is bass buildup in those acute front corners.

Regardless of whether your speakers really are against the front wall (if so, there should be heavy absorption directly behind them (at least 4", and spaced as far from the wall as you can afford, say 8-12" or so) the first thing I'd try is using as wide absorbers as you can (4 feet is good) diagonally across your two front corners - 4" thick 703 or equivalent works well. You need to get these corners to stop acting as "amplifiers" and start acting as broadband absorbers.

Edit - just re-looked at your pix, finally realized where your tube traps are. Those corners need as much absorption as you can get, I'd move the tube traps (maybe beside you, since they don't absorb really low) and use 4" fiberglass across those corners as I mentioned in the previous paragraph.

Before you do that, however, I would first take a tape measure and a helper, and measure EXACTLY all dimensions - from side walls to speakers (always measure to center of woofer cone), speaker-to-speaker, speaker-to-front wall, head to each wall, speaker to floor and ceiling, head to floor and ceiling, etc - we need to know ALL these dimensions (measure to hard surface, not surface of absorbers) to the nearest inch -

Once you have those numbers, I'll show you a neat little spreadsheet I found that will help you build a "grid" showing where good (and bad) places are for your head and speakers - this isn't absolute, but saves a LOT of time moving things and trying them out. Usually, a movement of as little as 2-3 inches will make an audible difference.

You're right about the rear of the room; that alcove is likely as much a culprit as your two front corners. If nothing else, maybe a thick, movable 4" rigid fiberglass gobo that could "live" against the wall between restroom and alcove when not in use...

Got a couple dozen posts to answer in Construction, if you can get me those dim's I'll work on a "grid" for you. The other stuff you're already familiar with building... Steve

Ren Man · Post by **Ren Man** » Mon Jul 12, 2004 11:49 am

Knightfly,
I apologize for taking so long to get back to this but have been extremely busy both at work and in the studio- story of my life.
To answer a few of your inquiries...
Disparity between pics and drawing- The drawings were done quite some time ago and don't reflect the current positioning of the monitors.
I've copied a portion of my July 5th post with some editing for clarity and the addition of a few missing dimensions-
My monitors are placed symetrically along the north wall (right side in drawings) of the control room. Center line of the Genelec woofers are 60" in from either side wall, 26 1/2" out from the North wall (right side in drawings), 50" off the floor, 47" from the ceiling, 75" center to center. The Tannoy dual concentrics are 72" in from the side walls, 22" out from the North wall, 49 1/4" off the floor, 47" from ceiling, 50" center to center.
My mix position is 79" to 80" out from the North wall and centered between the side walls- aprox 93" to either side wall allowing 6" for the width of my head (since the walls are splayed this dimension changes as I move around). Allowing for different heights of people working in the studio, for all practical purposes the ear is the same height as the monitors off of the floor. All of these dimensions are to the wall and ceiling proper- NOT to the acoustic wall panels. The Genelec S30-C's are my main monitors and are positioned to form an equilateral triangle with my head in the apex. The acoustic panels you see behind the monitors are the 4" thick fiberglass panels previously mentioned and are spaced out from the wall 1".
The splays in the sidewalls being directly across from one another were intended to be taken as a sum whole for a total of 10" in an 8' run. Apparently this is incorrect thinking on my part. However, in reality there is no problem with flutter echo (even before I moved the equipment in and the room was completely empty).
Yes those are 20 diameter tube traps stacked floor to ceiling (positioned in front of the 4" fibergalss wall panels) in the two front corners.

PRR · Post by **PRR** » Fri Jul 16, 2004 9:18 am

Offensive comments removed by author.

Eric_Desart · Post by **Eric_Desart** » Fri Jul 16, 2004 10:07 am

PRR wrote:> For near-complete absorption, the absorber needs to be at least a half wavelength thick: about 28 feet for 20Hz, 11 feet for 50Hz.

In many cases we do not need complete absorption. Even a little loss will take the edge off of rings and nulls. You get around 50% absorption with 1/10th wavelength of depth: 5 feet for 20Hz, 2 feet for 50Hz.

PRR

Clear is that you are very intelligent (mean that).
But where do you get such stories, because stories it are.
Knowing more than others also calls for some responsibility.

Can you also distinguish between straight and random incidence?

For STRAIGHT incidence NOT ONE SINGLE absorber in whatever anechoic room in the world comes even close to 1/2 wavelength, and they are measured in impedance tubes at straight incidence, systematically giving lower absorption than reverb room measurements at random incidence.
And as per standard such absorbers reach unity with a very narrow tolerance (specially expressed a R factor to detail the spectral related deviations between 0.99 and 1 - below 0.99, NOT Sabine, but real energetic absorption, they are rejected).

Please distinguish what you believe from what you know.

You can have a look to this link:
http://www.bobgolds.com/AbsorptionCoefficients.htm
and
http://johnlsayers.com/phpBB2/viewtopic ... highlight=

Best regards
Eric

Eric_Desart · Post by **Eric_Desart** » Fri Jul 16, 2004 9:47 pm

Edited

To prevent misunderstanding:
This message never contained txt but was meant to reactivate the previous one which was extended by editing