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Practical Studio Design: Part1

The Principles Of Soundproofing
By Paul White

SOS begins an in‑depth series on practical studio design. This month, the emphasis is on measures you can take to soundproof your studio or practice room.

Whether you run a home studio, a commercial studio or simply a MIDI sequencing system, the end result emerges from a pair of loudspeakers — and that's where your problems start. One man's music is another man's noise, and the likelihood of the former being categorised as the latter increases as day runs into evening. Professional studios employ elaborate soundproofing techniques to cut down on unwanted noise, both getting out and coming in, but there's a lot you can do for yourself if you're prepared to try your hand at a little DIY.

It is vitally important not to confuse soundproofing with acoustic treatment — sticking foam tiles or egg boxes to your walls may produce a better sounding room, but they won't make any significant difference to the level of noise escaping. However, room acoustics is an important topic in its own right and we shall be covering it as soon as soundproofing has been dealt with. Actually, the term soundproofing is rather misleading, as it could be argued that nothing is truly soundproof — the correct term is sound isolation.

Sound

Sound is simply vibrational energy using the air as a medium, but it can also propagate in liquids and solids. The law of conservation of energy tells us that energy can neither be created nor destroyed; it can only be converted from one form to another. Sound energy is absorbed by surfaces, such as walls, furniture or people, and also by the very air that it passes through. The sound energy is converted to heat by this process, though the amounts of heat involved are extremely small.

Sound Isolation

Efficient soundproofing relies, in part, on providing an effective way for the sound energy to be converted to heat before it escapes. What keeps sound in also keeps it out, and the simplest form of sound attenuator is a solid wall, which will both reflect and absorb some of the sound. Of course, any reflected sound must eventually be absorbed somewhere, and if too much sound is reflected, the room becomes excessively reverberant. Reverberation is simply the audible result of the sound energy bouncing from surface to surface, the reverb dying away as the energy is finally absorbed.

Figure 1: Single wall.Figure 1: Single wall.Unfortunately, for a wall to be a good isolator of sound, it has to be heavy (Figure 1) — if you double the mass of a wall, you'll roughly halve the amount of sound transmitted. Also important is the fact that soundproofing becomes less effective at lower frequencies. For every octave drop in frequency, the isolation provided by a wall is halved — which is why, when you walk past a night club playing loud music, most of what you hear is bass.

Because the acoustic attenuation of a wall or partition is frequency‑dependent, sound isolation is often measured in dBs for a range of frequencies averaged over the range 100Hz to just over 3kHz; the resulting figure is termed the Sound Reduction Index or SRI of the material. Tables giving figures for the most common building materials are available; for example, a single brick wall provides around 45dB of isolation while a double thickness, solid wall provides around 50dB (Figure 2). While 45‑50dB of isolation is a useful figure, there's no way you'd actually call it soundproof — as you'll know if you've ever lived next to noisy neighbours separated only by a single wall.

Double Walls

Figure 2: Double-thickness Wall.Figure 2: Double-thickness Wall.If a single wall can provide a reasonable degree of isolation, would two walls, separated by an air gap, be better? After all, if you add the 45dB for one wall to the 45dB of the other, you'd end up with 90dB of isolation — which should keep just about anything in or out. As usual there's a catch; to get this sort of figure, the walls have to be spaced far enough apart so that the air between them doesn't couple the energy from one wall to another (Figure 3). Far enough, in this case, means several feet. Obviously there's a choice of sacrificing some isolation and building the walls much closer together, or losing a lot of space.

Figure 3: Spacing two walls apart.Figure 3: Spacing two walls apart.In professional studios, corridors around the studio area are often designed as part of the sound isolation strategy. This allows a large wall spacing to be used while still allowing the space to be useful. Most practical approaches to soundproofing rely on some form of double structure with an integral air gap. Later in this series, I'll be giving constructional details of batten and panel partitions which may be used to build or divide studio areas within an existing room or to increase the sound isolation of an existing wall.

Weakest Link

Before getting carried away with the idea that dungeon‑like walls will solve all sound isolation problems, it must be realised that the room as a whole is only as soundproof as its weakest link. Most sound leakage will occur through doors and windows because these are built of lighter materials than the surrounding walls. To a lesser extent, the same is true of ceilings and suspended floors. Sound will also escape through any hole, no matter how small, so ill‑fitting windows and doors, unfilled holes for pipework, even keyholes, will seriously compromise the result of your work.

Double-glazed windows offer some improvement in sound isolation, though a significant amount of sound will still pass through. If you don't need the window, consider filling the space with sandbags and boarding it up, but if you appreciate natural light, consider fitting double-glazing to the main window with secondary glazing on the inside, maintaining as large an air gap as is practical. What applies to walls also applies to windows — the more weight you can add, the better the sound isolation, so use the thickest glass you can. Heavy curtains also help but are of little use on their own.

Doors may be uprated by fitting airtight seals and by making the door heavier. Some of the heavier fire doors are ideal for this purpose. As with walls, two are much better than one, and speaking realistically, you'll never get adequate sound isolation from a single door, no matter how thick you make it. Pro studios use an airlock system with several feet between the two doors, but significant improvements can be made simply by fitting two doors, one on each side of a standard 9‑inch wall.

Floors

Solid concrete floors provide adequate isolation for most domestic and project studios, though even these can be improved by building a so‑called floating floor on top. Again, details of floating floor construction will be provided later in this series, though it is worth mentioning the general principle. Because sound travels happily through solid materials, solid walls and floors can carry sound to other parts of the building. And floors get more than their share of vibration because most of the noise‑making equipment rests directly on it.

A floating floor usually consists of a second layer of concrete separated from the first by a resilient layer. The resilient material helps prevent vibrations travelling between the two layers.

Wooden floors are far lighter than concrete and so tend to pose a more serious leakage problem. In a domestic environment, much can be done to improve the situation, but being realistic, no practical measures will make it possible to, for example, play a drum kit directly above a domestic room without the amount of noise being intrusive.

However, a few simple measures may be enough to make life tolerable for the rest of the household if you're running a small recording or MIDI studio and monitoring at sensible levels.

Ceilings are less straightforward to treat, though there are several approaches which can improve matters. Whereas a pro studio might make use of tons of concrete slabs, woodwool, sandbags or multiple layers of plasterboard, the least disruptive measure is to fit a heavy, hairfelt underfelt to the floor of the room above. In a basement room where there is access to the ceiling joists, there are false ceiling designs that use up the bare minimum of space, whereas in a room with lots of headroom, a complete false ceiling could be fitted. None of these jobs is beyond the capabilities of a competent DIY enthusiast and the materials are readily available from most builders' merchants.

Floating Rooms

Professional studios invariably make use of the so‑called 'room-within-a-room' construction technique; a separate inner room is constructed, usually from batten and plasterboard, which is isolated from the walls, floors and ceilings of the original room. Though this is impractical for most home studio applications, it can be applied to low‑budget commercial studios and project studios. A further advantage of the 'room within a room' system is that it helps simplify internal acoustic treatment.

While it is possible to achieve very high levels of sound isolation, the law of diminishing returns dictates that the last few dBs of isolation will be very expensive to achieve. Invariably, there is some compromise between cost, disruption and the degree of sound leakage that can be tolerated. In most cases, tackling the weak areas such as door and windows, and perhaps uprating the odd wall, will produce tangible benefits for very little outlay.

Choosing Suitable Studio Premises

Home studio owners often have little choice regarding their premises, but if you are seeking premises in which to set up a commercial studio, it is vital you take into account the existing structure of the building, its location and the proximity of any outside noise such as railways, traffic or heavy industry. Furthermore, don't assume that the quiet cottage industry in the next industrial unit won't suddenly be taken over by a rather noisier business.

A ground floor location in a brick or concrete building is a good starting point — access is good, you can start with a solid floor and there's no worry about upsetting the people below you. If there's no choice but to take an upstairs premises, then check out what is happening above, below and to either side of you, and don't assume that things won't change in the future. Studios tend to be most anti‑social at night so ensure that there are no regular night‑shift workers or residential buildings nearby. In either case, make sure you have plenty of headroom if you feel that a false ceiling might be necessary.

Calculating SRI

The Sound Reduction Index or SRI of a solid wall based on the mass per square metre of the wall material. Note that the figure is frequency dependent.

R = [20 log(fm) ‑ 47] dB

f is the frequency of the sound

m is the mass of the wall measured (kg/m2)

R is the Sound Reduction Index being calculated

Because the result is frequency dependent, this information is presented in table form, with SRI figures for 150Hz, 250Hz, 500Hz, 1kHz, 2kHz and 4kHz.

Sound Isolation Main Points

  • High structural mass is required for good isolation at low frequencies.
  • Double‑layer construction is more effective than one thicker, single layer.
  • Air‑tight door and window seals are essential.
  • It may be necessary to isolate structure‑borne sound to prevent leakage through pipework, steel building joists and so forth.

Note: soundproof also means airtight, so some consideration must be given to ventilation.

Practical Studio Design: Part 1 The Principles Of Soundproofing

Practical Studio Design: Part 2 Soundproofing Doors And Windows

Practical Studio Design: Part 3 Building And Improving Walls & Partitions

Practical Studio Design: Part 4 Floors & Ceilings

Practical Studio Design: Part 5 Room‑Within‑A‑Room Construction

Published August 1993