How to Soundproof: Basic Principles

The four elements of soundproofing. If you understand how to apply these, you can learn how to soundproof a room.

When you first consider installing soundproofing, it seems a simple enough concept: You put up some soundproof material between you and the source of the noise, and voilà! — instant quiet! Then you go on the internet and start looking for good materials to use, or you go to a home improvement store and talk to some of the sales reps about your do-it-yourself (DIY) soundproofing project, and the picture starts getting fuzzy. There are a whole host of materials that claim to be good for soundproofing, at prices all over the map. Then your brother-in-law tells you to forget all those fancy, pricey products; all you need is to hang some carpet on the walls, or some egg cartons, and maybe some heavy drapes over the windows....

The reality is that soundproofing isn't a simple topic, and there is a lot of misinformation floating around, but it's not a black art either. There are just a few key soundproofing principles, and all materials and techniques that are effective accomplish their task using one or more of these principles, which are solidly based on the physics of sound transmission.

First, let's clear up one very common point of confusion. Sound absorption is not the same as sound blocking. Absorption is indeed one of the elements of soundproofing, but it actually contributes only a minor effect. Most materials with sound-absorbing properties are intended to improve the acoustics of a room, not to prevent sound from coming in or going out. They reduce reverberation and echoing of sound already present in the room, but they do not block sound from entering or leaving. So a product having excellent sound absorption is not necessarily useful for soundproofing. If you're looking for effective soundproofing, don't spend your time looking at products that are designed for acoustical room treatment.

Now, on to the four principles:

1. Mass

Any solid object that is thick and heavy will block sound. Drywall and mass-loaded vinyl are examples of good soundproofing materials that use this property. This approach is most effective against airborne sound, such as voices, and not so useful for impact noises such as footsteps, which are transmitted primarily through the building structure.

2. Damping

Tap a wine glass with a fork, and it will ring. Now clamp it with your fingers, and the sound abruptly stops — that's the effect of damping. In soundproofing, damping is accomplished with damping compounds such as Green Glue Noiseproofing Compound, a viscoelastic adhesive. The special property of the damping compound is its ability to convert sound energy into heat, so that the sound abruptly stops, as with the damped wine glass.

For the damping compound to work, it needs to be applied between two stiff panels (drywall, plywood, or subflooring, for instance), which are then screwed together, forming a Constrained Layer Damping system. When sound hits the system, it causes shearing forces between the stiff panels which create friction in the damping layer, thus converting the sound energy into heat.

Damping is the most effective of the four elements against low-frequency noise, such as the booming bass beats of music or noise from construction machinery.

3. Decoupling

Some sound is transmitted through the building structure. With decoupling, gaps are introduced into parts of the structure, preventing the sound vibration from continuing along on its path.

Decoupling is most easily accomplished during a building's construction, for example by staggering stud beams so that the two wall panels are supported by two different sets of studs, rather than having both walls use the same studs and thus being connected by them. Decoupling can also be incorporated later, using resilient sound clips and furring channel, although the original walls, ceiling, or floor will need to be removed first. Decoupling is also at work in windows that have non-parallel panes so that they don't vibrate together as a unit. And decoupling is a key element in the construction of a "room within a room," an advanced soundproofing technique that is highly effective.

Because it requires tearing out existing structures, decoupling is used less often than the other three elements when soundproofing is added to an existing structure. Also, it must be planned carefully and installed correctly, because it can actually worsen low-frequency noise, by creating a new resonance chamber.

4. Absorption

Absorption does have a role to play in soundproofing, but it has the least effect of the four elements. Acoustical ceiling tile and loosely packed fiberglass insulation are examples of good sound absorbers, and they provide a small measure of additional soundproofing. (Your brother-in-law's egg cartons and heavy curtains would also fall in this category.)

For Best Results, Use All Four

These four elements work independently of each other. This means, for example, that if you've already applied plenty of mass but no damping, you can gain significant improvement by adding a damping element to your system. So if you have already have some soundproofing installed but it's not doing enough to reduce the noise, think about which of the four element(s) are weak or lacking in your system, and that's where you might look next to gain the most improvement.

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