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Shut Up! (Part 1)

How to soundproof a rehearsal room

How to soundproof your demo room


Tim Oakes defeats the decibels.

Picture the scene of domestic bliss: Dad and Mum sat beside the fire watching Corrie St., the dog asleep on the mat, the cat resting sleepily on the sofa. Dad sips his Scotch, Mum pours another cuppa. Meanwhile, next door, the Archdemons of Megadeath (a noted local folk/rock band) are setting up. On the count of three... the cat is scorched by the fire ensuing from Dad's whisky landing spot bang in the middle of the flames, the dog gets a Typhoo shampoo - and tempers get just a bit frayed...

A common scenario, this moment of Streuth! It happens almost everywhere that musos try and rehearse, and unless you're rich and/or live miles from anywhere the chances are that the volume of your rehearsals is going to be much too loud for those who live around you. Rehearsing at home is impossible for most bands, who usually revert to using a rehearsal studio or hiring a local hall. Both of these alternatives are expensive though, and in the long run, if you have the space available to dedicate a room for rehearsals (almost) permanently, you can reduce the spill of your sound outwards for less than £100 for an average 15'x10' room.

THE INS AND OUTS OF SOUNDPROOFING



Recording and rehearsal studios have quite different requirements, letting alone such differing factors as how close they are to residential areas. The recording studio aims to create an acoustic environment which is free from outside influences, such as traffic noise or passing jets. The room will probably also be treated for internal sound quality: for instance, the use of gloss flat surfaces to 'liven up' the sound as an alternative to the usual 'dead' tones caused by the soundproofing. A rehearsal studio, however, is used for live rehearsals, and so the sound levels within are much greater than in its recording counterpart. Where the rehearsal studio is near, or in, a residential area, then the objective is to stop the sound from getting out of the room.

Different materials have different capabilities when it comes to killing sound. The ability to absorb noise is called the 'Absorption Coefficient' and is measured from 0.0 (no absorption at all) to 1.0 (100% sound absorption). Thus 0.75 means that 75% of the sound is absorbed. This rate changes according to the frequency of the sound, too, with some some materials better at low frequency damping or mid or high frequency absorption. Table 1 shows a few relevant examples - but to interpret that lot, you have to realise the difference between sound absorption and sound insulation. Take the example of a material that gives good sound absorption and which 'kills' 75% of the sound. What's left of the sound comprises 5% reflected back from the surface, and 20% which gets right through to the other side. In real terms, this is only a 7dB drop. The figure on the right of Table 1, N.R.C. (Noise Reduction Coefficient) gives the average ability of the material to insulate sounds.

Ability to absorb noise across frequency

Material Frequency (Hz)
125 250 500 1000 2000 4000 NRC
Cork tiles 22mm 0.05 0.1 0.2 0.55 0.6 0.55 0.38
Carpet 9mm tuft, felt back 0.08 0.08 0.3 0.6 0.75 0.8 0.43
Curtains (solid behind) 0.05 0.1 0.15 0.2 0.25 0.3 0.17
Plaster tiles 0.45 0.7 0.8 0.8 0.65 0.45 0.74
Mineral Fibre 50mm 0.4 0.6 0.8 0.9 0.95 0.9 0.81
Chipboard 19mm 0.1 0.15 0.2 0.1 0.1 0.05 -


Fascinating stuff. And obviously the best way of letting Granny sleep is to line the entire rehearsal room with Mineral Fibre - if you can afford it! This material costs in the region of £750 for our average room! Steep. So I've given the alternatives to fibre, and very cheap they are, too! Table 2 gives a rough approximation of the capabilities of each one, for comparison with Table 1.

Alternatives to Fibre

Material Frequency (Hz)
125 250 500 1000 2000 4000 NRC
Brick/Plaster 0.1 0.1 0.15 0.08 0.1 0.15 -
Polystyrene 25mm 0.1 0.15 0.25 0.2 0.1 0.15 0.2
Egg trays 0.15 0.3 0.2 0.15 0.1 0.1 0.15
Curtains over air gap* 0.05 0.2 0.4 0.5 0.5 0.5 0.35

* This isn't material stretched flat, but 200% material in drapes
While the individual average NRC figures aren't all that good, the different materials have capabilities in different areas, and thus a 'picture' of absorption is built up.


HOLES IN THE WALL



First off, let's take a look at the room itself. The best rooms are obviously those with no shared walls, ceiling or floor. A box in a field is perfect - but we don't live in a perfect world, and there's likely to be at least one surface which adjoins another room. This wall, floor or ceiling will need special attention.

Brick is a good insulator of sound, but, sadly, people like to get in and out of rooms and so put holes in the wall for doors. This idiotic move is often accompanies by misguided attempts to introduce fresh air and light, thus adding other holes to form windows. These are prime areas for leaks in your insulation, so here are some guidelines to help you with these problem areas.

Doors


Modern doors are rubbish. They are generally made of thin sheets of plywood over an air gap, and so they're lightweight and thus poor insulators of both heat and sound. However, if the top of the door is removed the air cavity can be filled with fine-textured straw. This has similar properties to Mineral Fibre, and is about 90% cheaper! Pack the straw (not stubble type, but hay) quite tightly into the gap, and the result is a good acoustic insulator. Old solid wood doors are good at sound insulation, but can't compete with a well-stuffed modern door (careful! - Ed.). The next targets are the gaps around the door. These are hard to fill, but two layers of 5" wide carpet strips all round the outside edge works quite well.

My own rehearsal room has two doors, one opening out and one opening in, with the hinges on opposite sides. The inside door is a standard house door with padding (see later), while the outside door is a modern type with straw-filled air gap. This type of door can be picked up second-hand for about £5, but MFI also have them at a reasonable £19.95. Having two doors creates an air gap, and the sound insulation is superb. Also, you can buy window sealing strips of self-adhesive foam rubber at about £2.50 for 30m (it comes on a roll), and this can be used in the corners of the door jamb. It compresses right down when the door is closed, forming a good seal.

When working on a doorframe, make sure that the frame itself will stand up to the strain of compressing foam material (this puts stress on the hinges and clasps) and the weight of the door(s) used.

To complete the insulation, a layer of absorbing material should be placed on the inside of the inner door. This can be the same material as the walls, but a good alternative is a second-hand child's cot mattress. These measure, typically, 4'6"x3', and can be tacked on to the door direct. Alternatively, a sheet of foam rubber covered with cloth and nailed to the door with brassheaded upholstery pins looks nice and gothic, and costs about £5.

Windows


While all this activity is sealing the door tight, you might find the room becoming airless. For this reason, don't seal the windows permanently - make sure they can be opened when the room isn't in use. The best bet is a smaller version of the hollow door, made from 2"x2" timber with plywood panels affixed on both sides. Fill it with straw first!

This can then be hinged to fit into the window space, or just held there on a shelf with four small bolts, two top and two bottom, to hold it fast. Seal up with the window-insulating strips, and this will cut down leaks to a minimum. For very basic soundproofing, an old mattress can sometimes be used to seal a window. But this is crude, and the mattress can't be cut to fit. It'll also take up a phenomenal amount of space, and flollop about all over the place anyway...

Floors


Basically, if your potential rehearsal room is above another room you have problems! Carpet and wood are fairly good at mid-range absorption, but they let a lot of sound through and are poor for low frequencies. Drums and bass guitar have a tendency to be the loudest and most aggravating noise for neighbours, so basically a good low-frequency insulator is required. As Table 1 shows, plaster tiles (Gyproc, for example) are excellent in all frequencies - but they cost like crazy. Alternatives include removing the carpet and laying down a 3" layer of newsprint. Unfortunately, this is hard to make flat, and will be tricky to lay carpet over. Other methods include a secondary floor, giving an air/straw gap, but this is prohibitively expensive, and a better bet is simply to lay cheap carpet on to the bare boards (the carpet can be a jigsaw of allsorts, even carpet squares), on which is laid large plywood boards (five ply minimum). To prevent the edges lifting, tack them with 3" largehead panel pins all the way round. On top of this is then placed another carpet (the original one?). In this way, there is an air gap just in the pile of the lower carpet, which will reduce both the sound travel and the direct vibrations from drums and bass cabinets.

Well, that's enough to get you pondering and planning for one month - next issue, we'll be looking into walls, eggboxes, fire and damnation!


Series

Read the next part in this series:
Shut Up! (Part 2)



Previous Article in this issue

FX on Fire!

Next article in this issue

Competition


In Tune - Copyright: Moving Music Ltd.

 

In Tune - Dec 1985

Topic:

Acoustics

Construction / Build


Series:

Shut Up!

Part 1 (Viewing) | Part 2


Feature by Tim Oakes

Previous article in this issue:

> FX on Fire!

Next article in this issue:

> Competition


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