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Soundproofing: DIY “Room Within a Room”
Home studio acoustics Part 1
Home Studio Acoustics Part 2 – Making HF Absorbers

Soundproofing: DIY “Room Within a Room”​

You will be very lucky to get complete sound proofing for your studio or rehearsal room, but for basic home recording or practising you will get away with partial soundproofing, unless your family/neighbours are really nasty, or you need to cut out the noise from low flying aircraft. When I was at college my landlady complained about my constant saxophone practising so I used to get inside a wardrobe to practise. This worked to a certain extent but was not very pleasant.

Note that acoustic tiling, foam tiles or eggboxes will only cut down reverberation inside the room, they do practically nothing to stop sound from being transmitted to or from the room.

The most basic type of soundproofing is to add solid matter to existing walls and ceilings: you can do this by fixing layers of thick plasterboard, however this will only cut sound transmission slightly. Double glazing will also help but if you need to be a bit more serious then you need a “room within a room”, ideally a “floating” room made from stud walls and ceiling with at least two or three layers of thick plasterboard on each side. This is basically a box “floated” on rubber so as not to transmit sound from the walls down to the floor of the house and then to all the other rooms.

To float the room it should have it’s own wooden floor that is sitting on a layer of neoprene rubber or specifically designed soundproofing mat. A cheaper solution is to try a couple of layers of rubber carpet underlay, but bear in mind that this will be the hardest bit to replace if you get it wrong.

For windows use thick glass (at least 1/4″), ideally double glazed and set so that they are not exactly parallel in order to cut down resonance. A single pane of very thick glass is better than standard sealed double glazing.

For a door get a 30 minute (at least) firedoor and seal around it with rubber, including the bottom so it will need a sill.

Now for the important bit:

Ventilation​

OK, so now you have created some very effective soundproofing, but you will suffocate in this studio unless you either open the door every few minutes or work out some kind of ventilation system with baffles (see below). Air conditioning is a very useful but still needs ventilation or frequent door opening. The quietest air conditioners are those that gradually ramp up and down with the thermostat rather than the on/off type, I have a Sharp AY-XO95E which is quieter than my Macpro.

Ideally you need two of vent holes with fans, one for air in and one for air out. You can get by with just a fan on the extraction hole. Normal kitchen fans are quite noisy as they are bigger and faster than necessary for normal airflow as they are designed for extracting smells very quickly but if you get a 12v fan (available from Maplins, Tandy or Radio Shack), you can control it’s speed with a cheap all purpose power adapter that switches between 3v – 12v.

Here is a basic diagram of how to make a vent baffle that will help cut down sound transmission. The whole thing can be made out of 1/2 inch MDF and is about 4 feet by 2 feet by 2 feet. All the internal surfaces should be lined with rockwool (RW3) and sprayed with varnish or paint to stop dust and bits of fibreglass. If you don’t have room for this type of baffle you need to make sure the vent holes are as small as possible and that the ducts are as long as possible or lead into something like a chimney. As long as you don’t light a fire.

Home studio acoustics Part 1​

This is a brief outline of how I have carried out some basic DIY acoustic treatment to my studio. Although I call this a home studio it needs to be capable of professional quality recording and mixes for my work writing and producing music for TV and film. I am lucky enough to have a live room which sounds great without needing much at all in the way of acoustic treatment so I have concentrated on the control room.

As the studio is solely for my own production and is not hired out, I do not need the control room to sound perfect from every position as you would if there were different people listening from various positions. (It’s very annoying to be sitting at a desk only to have a band member pipe up from the back of the room asking you to turn the bass down, and have to say “come over here and listen to it as it’s always too bassy back there”). Sooner or later I shall work on getting the room more accurate from all angles, but I decided my priorities were to get it right my normal mixing position: sat in front of the speakers with me forming one point of an (approximate) equilateral triangle.

Some of this is science, some is trial and error. In my previous studio I employed a consultant, Howard Turner (Studio Wizard) to give me some advice, which was basically how to set up the monitors and to dampen the high and mid frequencies as the room would be too reverberant otherwise. I also considered bass traps, however I was lucky in that the bass was not a real problem apart from right at the very back of the room. I also always try to get feedback from mastering engineers to find out if they had to do much EQing to my mixes. (NB. sending a mix off to be mastered by an experienced mastering engineer is a great way to check how well your control room is behaving).



My aim is to make the ambience of the control room feel similar to a typical living room. If the control room is too dry I would probably add too much reverb to the mixes and they would sound too wet on a typical home stereo or on a TV. Similarly if the control room is too reverberant my mixes would end up too dry. Having done a mix I try to listen in as many different environments and systems as possible. It’s probably impossible to get one mix to sound right on every system, but you soon get an idea if there is a basic fault with the room – if your mix sounds OK in the control room, but lacks bass, is muddy or is too dry in all other situations you still have some work to do.

Monitors​

I use JBL LSR4328 active monitors. These are especially useful as you can “tune” them to a particular listening position in the room. Choice of studio monitors can be very subjective, but they should of course be designed to allow you to hear plenty of detail without flattering the sound. Hi Fi speakers are often designed to enhance the sound at some frequencies which may cause you to under-emphasise those frequencies when you mix. I built some stands for them which are basically columns made from 1/2″ thick MDF 3 feet high by 10 inches by 10 inches and filled with sand to isolate the speakers. I based the height of the stands on having the tweeters at the same level as my ears when I’m sitting down.

Absorption & Diffusion​

Once the sound is out there in your room you have two main considerations, Absorption and diffusion. My aim was to cut down the reverb time of the room using absorbers, but to leave enough sound reflection so that I did not compromise by adding too much reverb to the mixes. Luckily the room is not completely square, parallel walls can cause the worst problems as the sound waves will bounce back and forward.

Diffusion​

Dedicated diffusion panels will help to diffuse waves which might otherwise bounce around, however I decided to wait and see if there were any problems once I’d sorted out the absorption. As it happens everything seems to be quite good. Once you get some furniture and a few irregular surfaces such as bookshelves and ornaments you may find that is all the diffusion necessary. In fact I seem to remember a Sound On Sound article which mentions that a bookshelf (ideally with different size books) can make a great diffusion panel. Purpose built diffusion panels are available but I shall be researching some DIY alternatives soon…

Bass Absorption​

The thicker and more solid the walls are, the more likely you are to get a problem with bass frequencies (something to be aware of when soundproofing). Once again I was quite lucky with this room, the back of which is formed by a bay window with sealed double glazing units. These act as a bass trap, but only at certain frequencies, mainly around 200hZ. I am about to build some bass traps, however this is not a priority as any bass problems here are at the back of the room and do not affect my listening when sitting at my normal mixing position. I am currently working on a design based on these.

High and Mid Frequency Absorbers​

In my previous studio I mounted absorbing panels either side of the mixing area and on the ceiling above it. When I moved to my new studio the ceiling was much higher and the floor is hardwood, so I needed quite a few more panels. I now have two panels behind the monitors, one on either side, two large ones on the ceiling above the mixing position and two smaller ones on the ceiling at the back of the room. I was going to make some more, but after adding a sofa and a rug into the equation the room seemed to be about right, and once again, no complaints from mastering engineers. In the next section I’ll explain how I built the HF absorbers

Home Studio Acoustics Part 2 – Making HF Absorbers​

The absorbers I built are basically wooden boxes lined with Rockwool with a frame across the front over which is stretched some acoustically transparent material, in this case hessian. You need some basic tools and very basic DIY knowledge, e.g. how to use a saw and screw two pieces of wood together. Expert DIYers and carpenters can probably ignore the instructions and will probably do a much better job than I did, however I’m very happy with the results which look good and most importantly – they work very well. The sides of the boxes are timber 4″ x ½”. The back is hardboard with a couple of battens across for strength, although the backs could just be thicker wood or MDF. I chose hardboard and battens for lightness. Here I shall outline how to build a box 2’ x 4’

UK SUPPLIERS​

Rockwool is better than foam as it absorbs more frequencies. You need the rigid stuff that comes in 2 inch thick slabs. Available from the insulation section of any builders merchant or large DIY store, e.g. Wickes or B & Q. You must get the compressed insulation that comes in slabs, not the soft stuff that rolls up.

Materials​

• Back: hardboard 2’ x 4’
• Sides: timber 12’ x 3½” x ½”
• Rockwool 2’ x 4’ x 2″
• Wood batten 1½” x ¾” x 12’
• Very small battening, approx 4’ x ¼ ” x ¼ “
• A few 1½” wood screws
• Some ½” panel pins
• Wood glue
• Hessian 4’ 6″ x 2’ 6″
• Polyester batting 4’ 6″ x 2’ 6″
• Paint (optional)

You will also need at least a saw, a sharp Stanley knife and straight edge, a staple gun and staples, a drill, sandpaper and a screwdriver. I have suggested starting with this size as it’s easy to get hardboard already cut to these dimensions. You can make the absorbers whatever size you want of course. If you need to cut the hardboard you can either saw it or cut with the Stanley knife by scoring a few times along a straight edge.

Making The Box​

1. Cut the sides and top of the box. Two long bits for the sides, 4’ long and two shorter bits for top and bottom (23″). (As the top and bottom fit inside the sides, which are ½” thick, the external dimension of the top will be 2’). If your timber is slightly thicker than ½” you need to make the top slightly shorter or else you will need to adjust the size of the hardboard for the back.
2. Drill holes in each end of the 4’ side pieces so that you can screw into the shorter top & bottom pieces. You need to be quite accurate so the screws go into the middle of the top and bottom bits. As the screws are going into and along the grain of the top & bottom they should not need a guide hole to be drilled, but there is no harm in drilling a small guide hole if you like. A countersink can be used so the screw heads do not protrude.
3. Glue and screw the sides, top and bottom. At this stage it will be quite flimsy so be careful. A professional carpenter would have used a clamps and a set square but the next stage will firm it up and ensure square corners as long as you do it before the glue sets
4. Lay the hardboard back over the four screwed pieces, make sure the corners fit squarely.
5. Glue the back on and pin at approx 6″ intervals
6. Once the glue has set, sandpaper the box to smooth off the sawn edges.
7. Paint or varnish the box to taste. (Bare wood can look good depending on the quality of your carpentry)
8. Lay the rockwool inside. It’s easy to cut with a sharp knife but keep it away from bare skin as it is an irritant. (Ideally wear a mask). Cut it just slightly larger than inside of the box so it fits snugly. If the boxes are going on the ceiling it would be a good idea to glue it in with some contact adhesive.
9. Covering the rockwool with batting
   Lay some polyester batting over the top. This is what people make duvets out of and is much nicer stuff. I use it because its HF absorption is also very good and it will stop any stray fibres from the rockwool getting into your room. If you do this you can either tuck it in between the rockwool and box sides, or staple it to the sides. If it all fits snugly it will stay in place, although you might want to fix it in place with staples for the ceiling boxes

The Front of the Panel​

You could of course leave it like it is, you now have a fully functioning HF absorbing panel, but it will look much better with a screen on the front. This is basically a frame made from wood battening with an acoustically transparent material such as hessian stretched across. The screen can other be the same size as the box and screwed onto the front (as in this illustration of the ceiling panels), or it can be made a bit smaller and set just inside, which I think can look just a tad more professional: To do this you need to make the external dimensions of the frame the same as the internal dimensions of the box, minus a little bit to allow for the thickness of the hessian. As the exact thickness of timber you used for the sides of the box may vary depending on your supplier, I won’t give the exact dimensions here, but once you have the material you can experiment to work them out. The method below is for the screens mounted inside the box.

1. Cut the four pieces of battening for top, sides and bottom, glue and screw. As with the box this will be flimsy until it’s finally mounted so be careful. You can add an extra batten across the middle for strength.
2. While the glue sets, sit the frame inside the box, wedged tightly with card so that it sets nice and square. Wood glue takes only about an hour to set.
3. Once the frame has set, Lay the hessian out and place the frame on top
4. Wrap the hessian around and staple the top two corners. You must stretch the hessian so that it doesn’t sag, but be careful not to pull to tight or it will pull the frame apart which is still quite fragile.
5. Stretch again and staple the bottom two corners.
6. Staple all the way along each side at intervals of 1″ to 2″, stretching gently and carefully as you go.
7. Apply some wood glue along the hessian where you have stapled it and allow to dry. (Use enough to soak through the material to the wood beneath)
8. Pin and glue some thin battening inside the box as supports for the frame (see illustration).
9. Drill two holes in each side of the box and screw the frame in securely. If you find the hessian is too tight and is pulling the frame inwards, more screws from the sides should straighten it up.
10. You can make these look a bit nicer with some plastic screw caps that fit into cross headed screws. Available from all good hardware stores.

Mounting the Panels​

Mount the panels using some bent brackets

You could just screw them to the wall but it’s advisable to mount them at least 2″ from the wall as they will work much better. To do this I fixed some angle brackets onto a piece of batten along the back of the absorber. This is fixed about 6″ from the top of the panel, glued to the back and screwed into the sides. I then bent over the ends of the brackets with a pair of pliers to hook over some battening on the wall. This has a couple of bits of smaller battening glued on top as lips for the brackets to hang on. My walls were brick, but if you have a stud wall you will need to find a timber stud to screw into or use plenty of plasterboard fixings.



Spacer at bottom to hold away from wall

I then put another batten at the bottom of the panel and used a piece of 2″ wood as a spacer which does not support, it just holds the panel away from the wall. Mounting the ceiling panels was a bit tricky as you must find a joist to screw into, I wouldn’t trust a plasterboard fixing on a ceiling – if you do on your own head be it. I used a hook in each corner and some strong link chain. The panels above the mixing desk are angled slightly upwards towards the middle of the ceiling, and have some extra rockwool and batting laid on top for extra absorption.

The mobile high frequency absorber:



Coming soon part 3: Acoustic diffusers. (As soon as I've made them)