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Practical Studio Soundproofing: Part 1

Tips & Techniques By Paul White
Published February 1998

Practical Studio Soundproofing, Part 1

Waking up the neighbours with the latest turbo folk masterpiece is a major concern for many project studio owners. Paul White explains that monitoring with headphones and moving house are not the only solutions...

While making music is enormous fun for those actually doing it, it's often the case that other members of the household and neighbours are less appreciative, so it's often necessary to consider some sort of soundproofing to maintain the peace. The purpose of the first part of this series is to explore problem areas and establish a few basic physical facts about soundproofing. From next month, I'll be looking at solutions to the most common soundproofing situations.

Egg On Your Walls?

The story still circulates that sticking egg‑boxes to walls will work as soundproofing, but this is quite simply not true — I tried it in my college days, and though it makes a slight improvement to some aspect of the acoustics of a room, it has virtually no effect at all on sound leakage. However, there are practical measures that will have an effect in a typical project studio, although in most situations the word 'soundproofing' is rather misleading — the best you can hope for is to improve the situation. Eliminating all sound leakage is virtually impossible without a custom‑designed building.

Most of the practical measures I'm going to describe are within the scope of a competent DIY enthusiast, and nearly all the materials are available from your local builders' merchant. There are some specialised materials that must be purchased from speciality suppliers, but I'll include some addresses for you to contact before the end of the series.

Sound Theory

Sound is essentially a form of energy that propagates by mechanical vibration through gases, liquids and solids. Energy cannot be destroyed, only converted to another form, so to 'lose' sound energy you have to make it do work that will convert the energy to heat. The reason why sound doesn't simply continue forever is twofold:

  • Firstly, the so‑called inverse square law means that sound reduces in level the further it travels from the source (simply because it is being shared over a larger area).
  • Secondly, sound is progressively absorbed (and converted into heat) by any surfaces that it encounters and by the air that it passes through. (By the way, because we don't need a lot of acoustic energy to produce a subjectively loud sound, the heating effect of sound absorption in a typical studio can be considered to be negligible.)

The challenge in designing effective soundproofing is to convert as much of the unwanted sound to heat as possible. The simplest way to attenuate sound is to put a solid wall in its way, and one of the fundamental rules that you should try to remember is that every time you double the mass of a wall you'll roughly halve the amount of sound transmitted. This means that to halve the sound leakage through an existing wall you'd have to double its thickness.

Another keystone of acoustic theory is that as the sound frequency is reduced, the isolation provided by a structure also falls. In fact, for every octave drop in pitch, the sound isolation is halved. From this, it's easy to see that soundproofing against high frequencies is not too much of a problem, but deep bass is very difficult to contain. You only have to walk past a nightclub to hear the amount of bass that can escape through solid brick walls!

Because attenuation is frequency‑dependent, the effectiveness of a particular sound‑absorbing partition design or material is generally measured in dBs, for a set of frequencies averaged over the range 100Hz to just over 3kHz. This figure is called the Sound Reduction Index, or SRI. A single brick wall might, for example, have a quoted SRI of 45dB, while a double‑thickness wall made from the same material might be rated at around 51dB. This latter figure represents a lot of attenuation, but if you're producing levels of around 100dB on one side of the wall, around 50dB will still make it through to the other side — and remember that this figure will be worse at the bass end. If you're directly adjoining a neighbour and have just a solid brick wall between you, it's unlikely that the degree of isolation will be adequate if you monitor loudly, and lightweight partition walls or breeze block will fare rather worse.

To give an example of typical SRIs, a light panelled internal door has an average SRI of around 15dB or less, and at low frequencies it will be significantly worse. On the other hand, a brick cavity wall, plastered on the inside, can have an average SRI of better than 50dB.

Double Benefit

Wall construction and sound isolation.Wall construction and sound isolation.If a single wall can reduce the sound leakage by 45 or 50dB, what happens if we use two walls separated by an air gap? You might, not unreasonably, think that 45dB for one wall added to 45dB for the next would give a 90dB figure, which would be terrific. However, the maths doesn't work out quite so simply, and, furthermore, unless the walls are separated by a considerable gap, 'air loading' between the walls reduces the efficiency of the isolation.

Even so, approaches to sound isolation based on multiple barriers separated by air gaps tend to be the most successful, and a double structure will invariably perform significantly better than a single‑layer barrier of similar mass.

Problem Areas

It's all very well looking at how to build soundproof walls, but in most real‑life situations the walls are the best‑designed parts of the room from a sound isolation viewpoint. There's little point in trying to improve the walls if the doors and windows leak like sieves. Even double‑glazed windows offer only a limited amount of sound isolation compared to a solid wall, though they are far better than single‑glazed units. DIY improvements in this area might include extra internal glazing with a large air gap and heavy glass — or, if you don't need the light at all, you could fill the window space with sandbags and board it up. Heavy curtains are a minor help, but the difference they make isn't great, especially at low frequencies.

A soundproof room is an airtight room.

If you don't need to open a window, at least you can make it airtight very quickly by using a mastic gun and some frame sealant. Doors, though, have to open, and it's surprising just how much sound gets through and around a typical internal door. Modern interior doors are very light, and significant improvements can be made by increasing the mass of the door, either by replacing it with a heavier one, or by adding material to one or both sides and fitting good door seals. However, even the heaviest single door will perform significantly worse than a double‑door structure with an air gap in between.

Floors And Ceilings

Concrete floors are good news from a sound isolation viewpoint because of their mass, but wooden floors can be a real problem. Even if you build a 'floating' floor above the original, the sound leakage will still be worse than that through a solid brick wall. Without major structural work, it's very unlikely that you would be able to use a real drum kit in a wooden‑floored room without causing some disturbance to those below. This can be a major problem in commercial premises or flats, especially if there isn't room to accommodate the additional floor height, but in your own house, where some noise leakage may be acceptable, there are strategies that can be used to improve the situation without too much structural upheaval.

If floors are a pain, ceilings are 10 times worse, because whatever soundproofing material you add, you're going to have to find some way to hold it up there — with floors, at least gravity is on your side. Short of suspending wood‑wool or sandbags over your head, or building a substantial false ceiling below the original, there's not a lot you can do that's really effective, but a couple of layers of thick underfelt below the carpet in the room above can help a lot.

The Pro Approach

One studio designer I know always tells me about the story of the guy who didn't want to spend much on soundproofing because he only used cheap musical instruments in his studio. Sadly, physics is no respecter of budgets, and 110dB of sound obtained by hitting a dustbin is just as loud as 110dB from a top‑of‑the‑range guitar amplifier.

Just to illustrate how difficult the problems can be, a professional design would usually involve building a completely separate inner room inside an existing room, isolated from the original floor by blocks of neoprene rubber. Aside from the obvious cost factor, most people who have home studios simply don't have the space to do this — but just in case you're in a position to try it, I'll be covering the basics of room‑within‑room construction later in the series. A further advantage of this system is that adding internal acoustic treatment is often simplified, as a properly designed inner shell makes room acoustics more predictable.

The Pragmatic Approach

The laws of physics are most definitely on someone else's side when it comes to keeping sound in or out, but don't let that put you off. Various sound‑isolation methods lend themselves to a low‑cost, DIY approach, and it could be that tackling just the weakest areas brings about sufficient improvement. By taking a common sense approach to applying the principles outlined in this series, you should be able to make noticeable improvements at minimal cost.

At its most basic, to achieve good sound isolation you need structural mass and airtight seals around doors and windows, but you also need to consider structure‑borne sound and find ways to avoid it. This is important because sound travels very efficiently as mechanical vibrations through solid structures, such as wooden joists or steel girders. There's little point in getting everything else right if your soundproofing is rendered ineffective by an ill‑considered structural feature.

If you're lucky enough to be deciding on new studio premises you can save yourself a lot of time and money by taking particular note of the existing structure of the building. You also need to think about its location and any noisy industrial activities that may be taking place close by. Perhaps the easiest location to deal with is a ground‑floor premises in a heavily‑built brick or concrete building with a solid floor. However, if the ceilings aren't heavy, you'll need to know what is going on above you in the building, and whether its use may change to something less studio‑friendly in the future. Also listen for low‑frequency rumble from traffic or trains — even with a solid concrete floor, you may have to resort to building a floating floor to keep outside noise to a minimum, and if this is the case, does the room have the necessary height?

If you're looking at an upstairs room, find out what is happening above, below and to either side of you. Some businesses may close down at night when studios are traditionally busy, but can you afford to have your hours restricted, and are any of the neighbours likely to go in for sudden extended overtime? If you are planning to do any serious acoustic treatment or build a 'room‑within‑a‑room' inner shell, you must allow plenty of space for the acoustic treatment. Even a simple inner shell will need a couple of feet of free space above it to work effectively. A simple floating floor may be between three and six inches deep, depending on how you build it, and most rooms will stand this. However, if you're putting in a false ceiling, you need to allow almost as much room as for an inner shell room, and certainly not less than about 18 inches.

For most project studio owners, major construction is out of the question, so you'll need to rely on uprating what already exists. You may also need to compromise on the amount of noise you make. If you can't get the noise down as much as you want by soundproofing, you may have to find a compromise that keeps all parties happy. For example, the majority of private studio owners record part‑time, so it may be possible to record drums or other loud instruments when the neighbours are out or, at any rate, not at night when they are trying to sleep.

In Part 2, we'll make a real start, with details on how to improve your room's weak spots.

Air Studios?

A soundproof room is an airtight room, so you need to think about how you'll get fresh air into the studio. Can you get by with opening the doors between takes, or will you have to install an air‑conditioning system? Simple air conditioners just cool and recirculate the existing air, but a serious studio air conditioner that brings in fresh air from outside needs silencer baffles, large ducting, anti‑vibration mountings and so on. This is likely to cost more than most complete home studios, so a compromise approach is most likely.

Sound Reduction Index

It's possible to work out the approximate Sound Reduction Index, or SRI, of a single solid wall if you know the mass per square metre of the wall material. The answer is frequency dependent, which is why frequency has to be fed into the formula.

R=20 log(fm)‑47dB

  • f is the frequency of the incident sound.
  • m is the mass of the wall measured in kg per square metre.
  • R is the Sound Reduction Index (in dBs), we are trying to calculate.

Materials that aren't completely solid behave differently from solid ones, and actual measurement is often the only reliable way of checking actual performance.