John Sayers' Recording Studio Design Forum Archive

Hi everyone,

I'm helping out a friend who owns a bar, and has a serious low-mid reverberation problem.

SIZE & MATERIALS:
The room is 10.40/15/5.73 meters (W/L/H).
Walls are made of painted brickwork. Ceiling and floor are concrete.

LOCATION:
Not an isolation issue.

BUDGET:
Roughly 1000$US

OVERVIEW:
The original idea was to create acoustic hangers made of fiberglass, but this was overruled due to 'interior design' problems...

So, my next idea is to suspend acoustic panels from the ceiling (horizontally).
The main concept is to use sealed wooden boxes, with rigid fiberglass glued to the bottom (from the inside).
Boxes will be suspended off the ceiling.

Is anybody experienced with large rooms, and give ANY input on this?

How thick should a box be?
What type of fiberglass is recommended for this application?
What would be the recommended distance from the ceiling?

Any thoughts are gladly welcomed...
Thanks.

Jonessy,

Why not looking in the yellow pages of whichever part of the world you're from (it's usefull you enter a location in your profile) and look for acoustic ceilings?
Guaranteed within a relative small radius you find several of them specialized in placing such ceilings.

There are countless options to choose from.
Lammella ceilings, perforated ceilings, finished Rockwool and fiberglas board ceilings, invisible caulck things, etc. all fulfilling all regulations in function of fire and building codes for public places.

Is there a reason to go the DIY route?

Thanks for the reply, Eric.

Eric_Desart wrote:Why not looking in the yellow pages of whichever part of the world you're from (it's usefull you enter a location in your profile) and look for acoustic ceilings?

...

Is there a reason to go the DIY route?

Actually... Yes.
We considered acoustic ceilings, but these are:
1. Expensive
2. Ugly
3. Not versatile

Using a conventional acoustic ceiling would mean that the entire ceiling is lowered by about 2 meters.
Using the suspended boxes should allow us to:
1. Overcome the lowmid problem, but still maintain spaciousness in the overall design and looks (which is important to the interior designer).
2. Add an 'industrial' feel to the place (which is the major design concept).

This is the real challange - to make a place both look good and sound good...

I have the experience and education in small room acoustics, but have never actually dealt with larger spaces (such as this project).
So the DIY shouldn't be a problem, I was just wondering what other people think, and if anyone is experienced with larger spaces...

Do you think that this type of absorber could be effective in such a large space?

jonessy

I had terrible luck here.
After an extensive reply I pushed the submit button, and the forum replied with a critical error.
I even couldn't recover it from my cache.

Just a short version.
I have experience with any kind of room (from small to concert , pop-concert, theaters, huge tv studios, factories, whatever) but that doesn't answer your question.

Main difference:
Small room acoustics: strongly defined by modal acoustics.
Large rooms: more defined by geometric acoustics (modes themselves are very dense in audio range)

The formulas you know (Sabine, Eyring, whatever) you can better apply for geometric than modal acoustics. So there's no problem.

Basically you can use any wool as for small room acoustics.

Important is:
The further you distance a board from a wall the better low frequent absorption becomes.
However when you continue increasing the distance this effect disappears and reverses.
The same board at very long distance will act as a board in a diffuse field showing poor low frequent absorption.
As a rule of thumb: A board in a diffuse field will show comparable absorption as a 50% thick board directly mounted against a wall (from low to mid).

Further important:
The low frequent gain by diffraction/edge effect (mostly in studiogroups wrongly confused with edge surface absorption) will diminish also in a free field versus closer to wall mounting.

In other words: absorption in a free/diffuse field is less to much less efficient for low to mid.

Your boxes with backing substitute the lack of a close by reflecting wall which is good if you hang them far from boundaries.
I should use ceiling tiles in those boxes with a cavity. Advantage: you can buy them in lots of finishes, they are self supporting until large sizes etc.
You can partly fill the cavity with cheaper wool if you like.

The total thickness or cavity depth is to be defined in function of the absorption spectrum you want.

Look at Bob Golds page:
http://www.bobgolds.com/AbsorptionCoefficients.htm

As long as you don't give a location (as I suggested) it has no sense for me or others to give possible links to possible producers or materials.


jonessy:
I just remembered that knightfly put it even in the rules:

knightfly wrote:This means -
No location, no answer.

It helps yourself and others to read them.

Thanks for this, Eric.

Eric_Desart wrote:As long as you don't give a location (as I suggested) it has no sense for me or others to give possible links to possible producers or materials.


jonessy:
I just remembered that knightfly put it even in the rules:

knightfly wrote:This means -
No location, no answer.

It helps yourself and others to read them.

The location is Israel. I know most of the suppliers and prices around here, hence I decided to disregard this rule.
Sorry for that.

Anyway, I made a quick sketch of the boxes here.

Do you mind taking a look?
The desired bandwidth is 100-400Hz.

Also, any idea of what would be the best way to safely mount these heavy things?

Thanks again, and sorry for the location...
jonessy.

jonessy

I not sure if I interpret this drawing right.

What is this hole at the top?

My first idee was from the visible bottom to the top

1) Some open cloth or fabric for eastetic finish
2) Your 4" fiberglass 48 kg/m3 is OK
3) What is this fireproof fabric ? Why is that there? This looks so thick?
4) Empty cavity of 4 to 8" totaling 8 to 12"
5) than wood without a hole. Why did you drafted that hole? (some kind of Helmholtz attempt?)

In normal applications in front of a wall people should advice you to leave this backpanel out in order to get better low frequency absorption.
In this case this ceiling is too far therefore the backpanel substitutes a closer by wall. Hence it should be closed.

All surrounded by a closed or perforated frame (closed is OK)

I assume this are freehanging elements?
Wat is the line at the bottom. Not wood I assume? But just the fiberglass board?


Wat is the size you plan: 1.2 x 1.2 m or 0.6 x 1.2m? I should prefer a larger one.

What is the height below those absorbers?
Floorsurface and volume bar?

Thanks again.
The sketch was corrected and uploaded again.

What is this hole at the top?
Silly me. Please ignore the hole...

3) What is this fireproof fabric ? Why is that there? This looks so thick?
The fabric comes with the rigid fiberglass boards. It may be ignored, or removed if neccessary.

I assume this are freehanging elements?
Yes.

Wat is the line at the bottom. Not wood I assume? But just the fiberglass board?
It would be easier to construct with a bottom wood panel (as well as the top one), but a framed fiberglass board can do as well.
What is better in your opinion?

Wat is the size you plan: 1.2 x 1.2 m or 0.6 x 1.2m? I should prefer a larger one.
The designed size is: 2 x 4 m with a total depth of 30cm.
This means that each box should weigh above 100kg. Is this doable? I mean in practical construction terms?

What is the height below those absorbers?
Approx. 4.23m

Also, do you think that instead of hanging 3 absorbers at the same height, it would be recommended to make, lets say 6 absorbers at different heights?

Thanks again for your help.

jonessy wrote:Thanks again.
The sketch was corrected and uploaded again.

What is this hole at the top?
Silly me. Please ignore the hole...
OK

3) What is this fireproof fabric ? Why is that there? This looks so thick?
The fabric comes with the rigid fiberglass boards. It may be ignored, or removed if neccessary.
It can remain there but is more logical at the bottom

I assume this are freehanging elements?
Yes.
OK

Wat is the line at the bottom. Not wood I assume? But just the fiberglass board?
It would be easier to construct with a bottom wood panel (as well as the top one), but a framed fiberglass board can do as well.
What is better in your opinion?
Here I think we still misunderstand one another.
If you put the wool in a completely closed box (bottom + top) then this works as some kind of panel damper, but that wasn't the idea is it? (should be a bad idea anyhow).
Basicaly it works as a framed fiberglass board but with extended edges and a top panel which act as the ceiling.
You can interpret it as a suspended ceiling in a box without bottom.

Wat is the size you plan: 1.2 x 1.2 m or 0.6 x 1.2m? I should prefer a larger one.
The designed size is: 2 x 4 m with a total depth of 30cm.
This means that each box should weigh above 100kg. Is this doable? I mean in practical construction terms?
Yes, you have the cavity above the fiberglass giving space for wooden strengteners/stiffeners.
And what the roof can carry you can ask the architect.
What is the height below those absorbers?
Approx. 4.23m

Also, do you think that instead of hanging 3 absorbers at the same height, it would be recommended to make, lets say 6 absorbers at different heights?

That's more a matter of eastetics and design. I think the architect should decide that.
In as far it has an acoustics impact that's difficult to predict, but not important enough to divert the architect from his design wishes.
Thanks again for your help.

Hope my thoughts could be of use.

Thanks for your comments and thoughts.
Greatly appreciated...

jonessy

Eric, good to hear from you again; I hope the world is treating you at least reasonably well. As always, your advice/counsel are much appreciated.

Your "west coast" friend (me ) is about to become a gran-papa for the third time, love the kids but hate the further proof of "old fart" status.

May you have a very happy holiday season, and take care... Steve