Small Studio Acoustics (Part 2)
Walls have Ears...
DIY soundproofing and acoustic treatment for walls.
Despite limited resources, the home studio owner can go quite a long way towards soundproofing and acoustically treating the studio. This month we concentrate on the soundproofing the walls.
As intimated in last month's article, studio walls are not usually the greatest problem when it comes to soundproofing, if they're made of brick or stone, but they can present difficulties if one wall is a studding partition or if you want to build a new internal wall. The same is true if a next door neighbour is only one layer of breeze block away from the drum booth. But even if your walls are all brick, a further improvement can be made using the techniques described here.
In the event that a new solid wall needs to be constructed, whether internal or external, use a cavity construction (with few or no ties bridging the cavity) consisting of brick or concrete blocks if at all possible; breeze blocks are too light and consequently offer poor attenuation. Once all your solid walls are in place, you can then construct additional studding walls or line the existing walls in a similar manner.
Because a basic studding wall doesn't have the necessary mass to be a good absorber, we have to resort to a few tricks to improve the situation. Well, two thin walls are more effective than one thicker one so double skinned structure is the order of the day if you have the space to accommodate it. The first step is to construct a frame from 2" x 4" studding. But you have to be careful how you fix this in place. Sound travels quite happily within solids so it's essential to ensure that the frame doesn't make a good acoustic contact with the floor, the walls or the ceiling. Sadly, gravity is against us on this one so some degree of acoustic coupling is inevitable. The best compromise is to insert ¼" thick (or thicker) neoprene sheeting (a synthetic rubberlike material) between the frame and the floor to support it and to use a similar approach where the wall touches other walls and the ceiling. If you can't find a local supplier of neoprene, check Yellow Pages for rubber, foam and plastics suppliers; they should be able to point you in the right direction. Some screws will have to pass through the neoprene to hold the studding wall in place, and of course these will transmit some sound, but then there has to be a compromise somewhere. If you're lining an existing wall, then the cavities within the frame between the existing wall and the new skin should be filed with Rockwool or glass fibre and then a 3/8" plasterboard skin nailed in place. Rockwool works rather better than fibreglass, even though it costs slightly more, and your local builder's merchant should be able to get it for you by the roll.
It's vital that the plasterboard doesn't reach the floor, ceiling or the existing walls. Leave a small gap which can be filled with a silicon rubber sealant or mastic of the type used in bathrooms. Next, fix another plasterboard sheet directly onto the first. Some acousticians say a different thickness is best, but others say it makes no difference so long as you get as much mass in place as possible. Again make sure that there is clearance all the way round and seal it with more of the silicon compound. It's important that the joint is quite airtight. Figure 1 shows this construction method applied to an existing wall and Figure 2 shows a free standing studding wall built on the same principle. If you're stuck for space, a single thickness studding wall with a Rockwool filling may suffice but obviously, a double one will offer better insulation.
The board should be skimmed or plastered to increase the mass, and a strip of beading should be fitted along the edge (not touching the walls) so that the plaster doesn't bridge the gap occupied by the sealing compound and the neoprene sheet. Even when all this is done, the attenuation at low frequencies will be less than that offered by a solid brick wall, but substantial nonetheless.
No two rooms are quite the same, and an understanding of the basic principles will help you to apply some simple soundproofing techniques to your situation. Firstly, remember that the greater the mass, the more work the sound has to do to move it, so a large mass is definitely good news for attenuation. Sound also travels well through solids, so anything you can do to isolate one solid section from the next one; incorporating rubber, air gaps or other absorbant materials will help.
If constructing two thin walls, it's wise to do what you can to stop one layer being in actual physical contact with the next, otherwise the sound will tend to travel through the solid part of the structure. Acoustically absorbent material such as glass fibre or Rockwool is a great help when placed between partitions, and of course, don't forget to make sure that everything is absolutely airtight. But whatever you do, don't be tempted to put the Rockwool or glass fibre in plastic bags for convenience's sake or it won't do its job. Using open weave cloth bags will work, though. Remember too that glass fibre and Rockwool are irritants, so wear gloves and a face mask whilst doing the job. If you don't remember to start with, you will soon enough when you start to itch! It is a good idea to use the loose weave cloth bag method from a safety point of view to prevent stray fibres form getting into the air.
If building an internal wall, try to position it so that it's not quite parallel to any of the other walls of the room (if you have the option), as this will help when it comes to acoustic treatment. Parallel walls cause flutter echoes and comb filtering effects and are to be avoided if at all possible. Professional studio designers take this into account and calculate the optimum angles for all the walls, but at home, you're generally stuck with what you've got so you just have to make the best of it.
The methods used in these articles follow a DIY approach; any professional studio designer would be able to budget for a more sophisticated solution, but even so, the underlying theory is exactly the same.
If you'd like to know more about the subject, there are one or two books I can recommend (though some will have to be ordered from the USA).
Next month I'll be seeing what can be done about doors and windows.
The Master Handbook of Acoustics by F. Alton Everest (TAB 1296) published by TAB books Inc, Blue Ridge Summit, Pa 17214.
Acoustic Techniques for Home and Studio by F. Alton Everest (TAB 646) published by TAB books Inc, Blue Ridge Summit, Pa 17214.
The Handbook of Sound Recording Practice by John Borwick. Disributed by APRS, 23 Chestnut Grove, Chorleywood, Herts WD3 4HA.
This is the only part of this series active so far.
Feature by Paul White
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