[This article is a more in depth explanation to the science of sound deadening. For a basic idea please see our soundproofing 101 article.]
Sound isolation is science, not magic, and as such it is possible to outline a foundation of basic principles that define noise reduction in any given situation. In this case, there are 5 basic principles that govern the sound isolation of any partition.
Before we get started, it’s important to mention what we call rule number 1 of sound isolation – seal quality. If your partitions are not sealed, then a lot can be lost and a high level of performance cannot be attained. With that mentioned and emphasized, lets get started.
Principal Number 1 - Mass
The first principle of sound isolation is mass. Mass impedes the transmission of sound in a simple way - it’s harder for the sound to shake a very heavy thing than a very light thing, no different than saying it’s harder to push a shopping cart full of bricks than an empty cart. However, to make large changes in performance you have to make very large changes in mass. In theory doubling the mass of a panel without an air cavity will improve things by 6dB. Typically, on the common single wood stud wall, doubling the number of drywall layers yields 4-5dB of improvement.
As this series of tests show, just adding layers of drywall to the common wood stud wall or wood joist ceiling yields only a small benefit. To really improve your wall, you have to not only add mass, but also improve some of the other 4 Principles.
Considering the fairly high cost of drywall installations, simply adding layers is perhaps the least efficient way to improve sound isolation available to builders today.
Principle Number 2 - Mechanical decoupling, or mechanical isolation
One of the most familiar of the 5 basic Principles is mechanical decoupling. Sound clips, resilient channel, staggered studs, double stud walls. All of these function by inhibiting the movement of sound from one side of the wall to the other through mechanical paths (like studs or joists). Instead, the vibration has to pass through the air cavity in the wall, where some of it will be lost, and through the insulation/absorbing material, where (at some frequencies) much of it will be lost.
What most people don’t understand about mechanical decoupling is that it is frequency-dependent. When you decouple, for example, two pieces of drywall, you create a resonance, and only well above that resonance does the decoupling help you – below about 1/2 of an octave above that resonance it actually makes things worse. Like this
De-coupling is a very powerful tool, but one must plan around this resonance and the low-frequency performance problems it can cause.
In our next tip, we’ll go through some basic steps for handling resonance in walls, and constructing good walls in general.
[Read more on decoupling in this soundproofing article].
Principle Number 3 - Absorption
Installing insulation in a wall or ceiling cavity increases the sound loss due by eliminating/removing/destroying some sound. Another benefit of insulation in a cavity is to lower the resonant frequency of decoupled walls. All that noted, insulation loses its effectiveness at very low frequencies. Put some fiberglass in front of a center channel and you’ll hear badly muted dialogue (the insulation is effective), put it in front of a subwoofer and you might not hear any difference at all.
The soundproofing war isn’t won or lost by what’s inside the walls, so don’t get too caught up in fretting over what insulation to choose, just make sure to use something. Fortunately, common fiberglass routinely used in construction has been shown to be as effective as any other insulation type, particularly at low frequencies. Absorption is most effective in decoupled or damped walls, in a conventional 2x4 wall, sound can easily pass through the studs and doesn’t need to go through the insulation.
[Read more on insulation soundproofing here].
Principle Number 4 - Resonance
This works against the good things done by Principles #1, #2 and #3 above by making it very easy for sound to vibrate a wall. At resonance frequencies even a massive decoupled wall with insulation will vibrate very easily (as we saw above in the decoupling section). Since a vibrating wall vibrates air on the other side, resonance increases the ease with which sound is transmitted. This is not a good thing.
There are two basic ways to deal with resonances
1. Damp the resonance - This reduces their magnitude and therefore reduces the sound exiting the wall on the other side. Green Glue is the highest performing Visco-elastic damping compound available. As a side note, limp mass materials (MLV, Mass Loaded Vinyl) are not effective at damping the resonances of walls.
2. Move the resonance point - If we lower the resonance frequency of a wall by employing Principles # 1, 2 and 3, we are less likely to encounter sound at those lower frequencies (a wall won't be exposed to a 70Hz sound as often as a 100Hz sound). This will reduce the opportunity to have that wall resonate.
Principle Number 5 - Conduction
The last important principle of sound isolation is conduction. Conduction plays a role in keeping common wood stud walls (not decoupled walls) from attaining a high level of performance. The drywall conducts vibration to the studs, which transfer it to the other side, keeping performance low.
Outside of the performance of walls, conduction plays a large role in flanking noise – noise traveling from one room to another by some path other than the direct.
To reduce the conduction of a structure, you can either insert mechanical breaks (like cuts), or raise the damping of the structure (damping dissipates the energy as it travels, lowering conduction considerably, often to the point where it is irrelevant)
To minimize the amount of vibration that becomes “structure-borne” it is important to treat the surfaces on the sound source side.
No partition can ever perform better than the level of Flanking Noise. In the scenario to left, no modification you can make to the wall will help improve sound isolation. This is because it isn’t the wall that is failing; it is because the adjacent surfaces are failing.
The only way to improve on the situation is to increase the transmission loss of the flanking pathways.
Getting the most out of these 5 Principles
Well above we saw the five basic Principles of sound isolation. We can distill these 5 Principles into 4 basic areas of improvement that can be made in a partition. To improve sound isolation.
- Increase the mass of the partition (use thicker drywall).
- Add decoupling where there previously was none ( use whisper clips).
- Add absorption where there previously was none, or increase the level of absorption (add insulation).
- Add mechanical damping to the system (use green glue between two constrained layers).
Anything that doesn’t accomplish one of those 4 things will not help your cause, and in general, anything that doesn’t make a large improvement in one of those won’t help your cause very much. For example, if you already have R19 insulation in a ceiling cavity, adding 2” of mineral fiber won’t make a dramatic improvement.
Return to our Soundproofing Homepage.