Accoustic Screens

[Mark Shayle]

Can anyone offer any advice on how to construct acoustic baffles or screens?

 

I’m going to be working in a rehearsal room that is adjacent to a church. Although the party wall is thick old stone there are also a couple of large (say 3 metre square), high frosted windows. I want to place a screen on the rehearsal room side to help diminish the noise crossover between the two spaces, and help to take some of the echo out of the rehearsal room.

 

I have vague recollection of making up acoustic screens for orchestra pits years ago, made from ply on a timber frame, covered in something like loft insulation with canvas stretched over it. Also another dim recollection of one west end pit where all the surfaces were covered in rockwall (??) with perforated metal covering it, making a space so dead the noise from your mouth never made it to your ears!

 

Added complication, the building’s listed, so whatever I do will have to leave no trace and be removable……….that said I should get away with a few screws here and there. Also, I would like it to not be too expensive to construct and be more effective than hanging up some heavy drapes.

 

You collective thoughts very welcomed!

 

Best

 

Mark

[Simon Lewis]

Mark,

 

Your ideas for reducing the reverberation time in the rehearsal area are OK - you could make up frames stuffed with rockwool (and covered with hessian to stop fibres falling off), or you could consider hiring / buying theatrical drapes and hanging these on the walls.

 

The issue of noise transmission between the rehearsal space and the church is more problematic, compounded by its listed status.

 

Basically, you need to stop transmission through these windows, (if they are common between the two spaces). Typically, we'd use non parallel triple glazing with different glass thicknesses, but I doubt if you can do that! Short of building a large carefully constructed airtight baffle over the windows, its hard to see how to stop this problem.

 

You must also be prepared for flanking transmission being an issue.

 

HTH,

 

Simon

[Ekij]

Robustness may be an issue. How long do the baffles need to survive? What sort of exposure to damaging influences (children) will they experience?

 

Plywood on stands covered in egg boxes are good. If you can get the egg boxes free this is fairly inexpensive.

Pegboard (where the peg holes go through the wood to make lots of little holes) can be good too and a lot easier than drilling hundreds of holes yourself!

 

Drapes hung in rows so they look "2 dimensional" to the sound source are much more effective than you would intuitively think (you expect if you can see between the baffles then the sound can get between the baffles, in practive sound reflects around a lot and therefore gets 'caught' in the baffles).

[Mark Shayle]

Simon and Ekij, firstly thank you so much for your considered replies. Can I ask you both for clarification on a couple of points:

 

Simon, do you think the ply rockwool combination will only help the room reverb and do little to stop the transfer between the glass? For me the problem I would like to solve is the transfer of noise between the spaces. The room reverb is secondary in importance, but would be nice to address.

 

The triple glaze idea is great, but realistically an unlikely prospect.

 

Forgive my ignorance, what is “flanking transmission”.

 

Ekij. Re robustness, the frames would be well above head height and once installed will not come into contact with any prying little fingers!

 

The pegboard idea is a good one. The perforated metal I was describing looked just like aluminium pegboard. If I recall it was installed for Tommy in the pit at the Shaftesbury My recollection of it was that it was extraordinarily good at providing sound insulation.

 

The two dimensional drapes you mention……….am I correct in imagining drapes hung “from the rafters” at a right angles to the wall arbitrarily spaced? Just trying to picture this. Maybe catenery wires stretched across the room with drapes at each end against the walls? The room has a high ceiling and the windows are also very high off the ground, so this may be a fairly good way to go.

 

With everyone’s indulgence I may try to take a picture of the room for a bit further musing.

 

Once again, thanks for the input

 

Mark

[Brian]

...do you think the ply rockwool combination will only help the room reverb and do little to stop the transfer between the glass?

Correct. The only way to stop noise transmission is to use mass. Masonary and lead are good, drapes are bad.

 

Forgive my ignorance, what is “flanking transmission”.

Where sound travels by an indirect route.

 

In this case your direct route is via the window and wall between the two spaces. The indirect, or flanking, path might be from the dividing wall and then down a side wall. Or via a ceiling then a wall...

 

The pegboard idea is a good one.

To work correctly you need the right amount of 'hole'.

 

 

As Simon alluded to, when people talk of acoustics there are really two parts to the problem. First is true acoustics - which is what happens to sound in a space. The other is properly called noise control which is keeping unwanted sound in or out of a space. The two things require completely different and usually independent approaches.

[paulears]

The snag really is that everyone is talking in acoustics terms about what non-acoustics people call 'sound leakage'. Without boring you with the theory, older buildings with thick walls are pretty good at stopping noise getting out - but, the snag is that there are always places where noise - in particular low frequencies get out. So your windows are probably the weakest link - kick, low toms, bass guitar will simpy pass straight through. Lightweight fabric as in curtains isn't much better than glass. The key is density - so heavy velvet type curtains are a bit better - but still not as good as the walls. Very often the best you will do is reduce it, but I'd bet not enough to please locals who will want silence - and they won't get it unless you have a budget to match!

 

You need to concentrate on the windows first - they need covering with something pretty dense to work. so a frame that secures to, or is at least sealed to, the building is crucial - air gaps of any kind mess it up. timber frame, with mastic type sealant, covered with layers of roofing felt, insulation board, ply, mdf etc etc in layers will work to some degree, but will be really heavy - and need some method of keeping upright!

 

It takes peoplea long time to get qualified in acoustics - they're worth their weight in gold! Me - years of trial and error working out which of my crazy ideas work (and those that don't) is all I have to go on.

[Mark Shayle]

Thanks for all the feedback. I'm getting a good idea if how to go with this. The weight issue of the screens might be a bit of an issue. I'm thinking of making 3 separate panels for each window hanging the top on well fixed split battens the bottom resting on the frame, if there is enough room, if not, split batten again.

 

The panels themselves constructed from 3x1 on edge, with a sandwich from back to front of 8mm (?) mdf, rockwool, foam sheet, Hessian fabric.

 

There is in my mind a trade off between mass (taking on board everyones comments) and cost / weight in terms of building and securing.

 

I've added a couple of pics of the windows to my flickr account that you might like to look at here

 

Thanks for ongoing advice

[Nick S]

I have just finished working at a temporary venue at the Fringe with a somewhat similar requirement - a room with an entire wall of glass onto the main (and very noisy, with bands/PA at night) entryway/hall.

 

We did something similar to you - 8mm MDF, rockwool, but instead of the foam sheet and hessian fabric we used plasterboard (screwed into the wooden frame). Although not perfect, it worked pretty darn well providing us with minimal leakage when a full PA was on the other side. I think (can't quite remember) the board was between 9-12mm thick.

 

If you can afford it, go with the plasterboard - having used both in similar situations, I can tell you the board does a far better job.

[Simon Lewis]

Mark,

 

sorry not to reply earlier - been away over the weekend.

 

Most of your queries have been dealt with, but I would add:

 

1) don't use eggboxes - they will make it sound worse!!

 

2) if you cannot build a high mass sealed frame at the windows, try building two lighter but non connected ones. They must not physically touch unless it is via a flexible connection (e.g. neoprene).

You will probably get some ideas from "build your own studio" articles or books.

 

The idea is to have (say) 12mm Gyproc or mdf, then a space (or even lightly filled with glass wool) then another sheet of material ( which could be 24mm to increase the impedance mismatch). You must ensure they connect only by flexible fixtures.

 

Simon

[Bobbsy]

Sound advice (I've been waiting for ages to use that one!) from Simon Lewis.

 

Just to try and pull together some of the advice given in here:

 

There are two distinctly different aspects to the art of acoustics. The first is isolation (sound proofing) which is for keeping unwanted sounds inside or outside the room in question. This is achieved by a mixture of two techniques: mass (i.e. solid, heavy walls, floors and ceilings) and decoupling (i.e. creating double walls with NO physical connection between them, floating concrete floors, etc.). The use of soft furnishings, rockwool on walls, etc. etc. does very little to prevent the transmission of sound.

 

The second aspect is acoustic treatment to control reflections within the room. This is where the various soft treatments come into play. Done by a trained acoustic specialist, this is an art form, with different treatments included to absorb specific frequencies to remove most room effects but not make it into an anechoic chamber (which is VERY strange to listen to!).

 

In this particular case, since the OP mentions a thick, old stone wall this probably provides the necessary mass to prevent too much transmission via that route. The windows are probably the area with the most possibility for improvement, and Simon's suggestions seem good. Somebody mentioned using Gyproc and this does indead work well. Depending on the size you need, one hint would be to use a double skin of thinner Gyproc for each layer with any seams offset so there are no "through" cracks. Similarly, if you manage to do two complete walls, don't have them physically connected in any way...neoprene as Simon suggests is a good joint. Be aware though...done properly this will be heavy and probably not suitable for putting up and taking down!

 

Bobbsy