Understanding How Decoupling Works

If you have done some research on the topic of soundproofing, you have almost certainly come across decoupling in many forms. Decoupling means mechanically separating the two sides of a wall to make it harder for sound to pass through the wall. Here is a simple example of decoupling two sheets of drywall.

And here we show a simple sketch of how wood studs couple the conventional wall, and how products like resilient channel or modern sound clips can decouple it and improve performance.

The stiffness of wood studs couples the two sides of a conventional wall. As a result, sound can easily pass from one side, through the studs, to the other side.

Sound can pass through this type of wall without going through the air and the insulation, so insulation has only a limited effect on the single wood stud wall.

By including a resilient decoupling mount, the sound that tries to pass to the other side via the structure is thwarted, and performance can improve.

Because sound cannot easily pass through the structure, it has to pass through the air cavity, and insulation becomes far, far more effective.

Part 2 – Decoupling and Resonance

When you move those two sheets of drywall apart you do improve things, but only at some frequencies.

What happens is that the air in the cavity of the now-decoupled wall acts like a spring, and like all mass-on-a-spring systems, this results in a resonance. At (and around) this resonance point, the performance of the now-decoupled wall is actually considerably worse than if no decoupling had been used. Like this:

In theory (and in lab tests and in the field), decoupling only helps well above the low frequency resonance. In this case, with a resonance of 63 Hz, the decoupling only helps above 100 Hz, and hurts performance below this frequency. Let’s take a look at an actual lab test exactly as above.

One of the lines to the left is 4 layers of 1/2” drywall as a solid slab with no air cavity.

The other line is the same drywall, but with 2 layers on each side of a 2x4 wall with resilient channel and insulation.

Exactly as predicted above, what we observe is large gains at higher frequencies, but performance losses at lower frequencies.

This is caused by the resonance intrinsic to decoupled walls. To attain both the tremendous high frequency performance of decoupling and good low frequency performance, we will have to address this resonance problem somehow.

Attaining good low frequency performance part 1 – lowering resonance frequency.

To attain good low frequency isolation with a decoupled partition, we will have to handle the resonance somehow. The first method that we can use is to lower the resonance frequency of the wall. To do this we have to:

a) Add mass to one or both sides of the wall (heavier decoupled walls have lower resonance frequencies)

b) Increase the depth of the air cavity

c) Add insulation (if no insulation is currently present)

The benefits of lowering resonance are two fold:

1) The frequency range where the wall performs poorly moves down
to frequencies that the human ear can’t hear as well, which reduces disturbance.

2) The frequency range where the decoupling is effective becomes
wider – your decoupling efforts work to a lower frequency.

Notice how, as the frequency of resonance does down performance at frequencies important to theater, music, traffic, aircraft, and the human ear are greatly improved?

It’s a lot better to have a 20 Hz problem than a 40 Hz problem, or to have a 40 Hz problem than an 80 Hz problem!

If you get your resonance too high in frequency, your decoupling efforts may make things WORSE, not better. For example, you would be better off for almost any application to use two layers of drywall sandwiched tightly together than you would to use two layers of drywall separated by one inch.

Later in this document we will go over some basic tips for lowering resonance and attaining good all-around performance. Next, we’ll look at our other option for dealing with resonance – damping it with viscoelastic materials.

Attaining good low frequency performance part 2 – viscoelastic damping.

We saw above that conventional decoupled systems have considerable performance dips at resonance. It has been demonstrated in lab tests that products like Green Glue, or factory-damped drywall panels from makers such as Suppress Products have a substantial positive effect at resonance on conventional, coupled, walls. The same benefits are observed to the same extent on decoupled walls such as double stud walls. By an enormous margin, pound for pound the best wall that you can build is a viscoelastically damped double stud wall.

Table – choosing the right decoupling system for low frequency performance.

Best decoupling selections for walls	For conventional (non-damped) drywall	For well damped drywall (such as Green Glue damped board)
Best type of decoupling	Double stud wall	Double stud wall
2nd best	Modern sound clips like RSIC or ISOMAX	Staggered studs
3rd best	Staggered studs	Sound clips
4th best	Resilient channel	Wood or metal furring channel (not resilient)
5th best	Wood or metal furring channel (not resilient) perpendicular to the studs at 24” on-center	Resilient channel

Best decoupling selections for floor/ceiling constructions	For conventional (non-damped) drywall	For well damped drywall (such as Green Glue damped board)
Best type of decoupling	Separate ceiling joists (true room within a room)	Separate ceiling joists (true room within a room)
2nd best	Spring ceiling hangers	Spring ceiling hangers
3rd best	Modern sound clips	Sound clips
4th best	Resilient channel	Resilient channel or non-resilient wood or metal furring

Tips that are invaluable when planning a decoupled partition:

1. Use as much mass as possible on each side of the wall
2. Always use double drywall on at least one side of the wall
3. Use as deep an air space as possible
4. Use insulation. Fluffy fiberglass insulation is as good as anything
5. Select modern sound clips such as ISOMAX or RSIC over resilient channel (lower resonance point)
6. Select double stud walls over anything (lowest resonance point)
7. Utilize viscoelastic damping if possible
8. Choose thicker, heavier drywall over thinner – i.e., use all 5/8” drywall if possible

Construction diagrams

These should be helpful to anybody wondering what, for example, a double stud wall is.


Conventional wall (coupled)	Staggered stud wall. Shown is 2x4 studs offset on a 2x6 base plate	Double stud walls. Two separate rows of studs with drywall only on the outside

Resilient Channel

Resilient channel is a thin, flexible metal channel that is screwed to the studs. Drywall is then screwed to the channel, and the flexibility of the channel creates a decoupled wall.

Sound Clip

Sound clips are typically rubber-containing devices that screw to the studs of your wall. Metal hat channel is then inserted into the clips, and drywall is screwed into the hat channel. These provide decoupling by virtue of the resilient rubber component of the clips that ultimately supports the weight of the drywall.

Conclusion:

Decoupling is an extremely powerful tool, and can raise the STC (a measure of mid and high frequency performance) extremely efficiently. Ultimately, every truly great sound isolation scheme will utilize some type of decoupling.
However, decoupling is not effective at all frequencies. In fact, decoupling a wall results in reduced low frequency performance around its resonance point, and is only effective at frequencies much higher than resonance. Thus, to lower the frequency where wall performance is weak and maximize the frequency range over which decoupling helps you, you should strive to have resonance as low as possible.
The combination of viscoelastic damping and decoupling is extremely potent – giving the benefits of decoupling without the drawbacks.