Soundproofing Principles

[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.

Effects of Mass

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)

flanking wall

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.

  1. Increase the mass of the partition (use thicker drywall).
  2. Add decoupling where there previously was none ( use whisper clips).
  3. Add absorption where there previously was none, or increase the level of absorption (add insulation).
  4. 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.


Customers Questions and Answers

1) Mike C: I am building some lodging for clients and am framing the bedrooms with a double 2x4 wall. Is there a recommend ed minimum spacing between the 2 walls? The walls are also load bearing but I do have the option of stagger framing the studs on the separate walls but would need to keep the offset within 2 inches of each other. I spoke with my insulation contractor and he suggested we blow in the insulation between the wall for more density. I would like to keep the walls at 7 1/2 inches thick as the rooms are fairly small. Any suggestions would be appreciated. Thank you

Trademark Soundproofing Reply: Hi Mike, As long as they are fully decoupled from each other even a half inch will suffice.

2) Mason: Hey I have a project and In need to make a sound proof box what can I put in the sides to make it sound proof

Trademark Soundproofing Reply:

Mass Loaded Vinyl.

3) Aloke S: I am trying to construct an open-air theatre as a service to the community. The area granted to me is surrounded with roads having heavy traffic. As a first step I need to sound-proof the area. I am looking for cheap and effective solutions. This may not be your area but can you help?

Trademark Soundproofing Reply:

First off an open air area will never be soundproof. That being said you can still block the sound significantly however it won't be cheap. You should consider soundproof curtains.

4) Pete P: In a commercial building we have a room 25'x35' with normal 2"x4"wall with 1\2" drywall on both sides. The ceiling is a hanging ceiling with 24"x24" ceiling tiles. We filled area above ceiling tiles with acoustical fiberglass insulation batts and had walls filled with expanding insulation. In this room is an exercise company who plays loud music. In the next room is a massage studio looking for quiet and relaxation. The insulation helped a lot but the massage studio still says they hear the music. I've read your articles and have learned much. A construction company recommended very expensive drywall. Do you have any suggestions?

Trademark Soundproofing Reply:

Hi Pete. You can add a layer of regular drywall to the existing wall and sandwich the Green Glue Damping Compound in between. Add Mass loaded vinyl above your drop ceiling tiles (below the installed insulation).

5) Andrei D: Which of these materials (Acoustic Foam, Foam Blocks, Soundboard, and Gypsum Wallboard) is better for insulating sound and why?

Trademark Soundproofing Reply: Gypsum wallboard will"block" sound better than the others.

6) Dale: I'd like to create an enclosure around our outdoor AC unit, both to protect it and to dampen the sound as it gets pretty loud and we hear it inside our room. I have about 6" of workable space with between the AC unit and our house. I have sheets of mass loaded vinyl; would it help to use Green Glue between two sheets and attach those sheets to the inside of the enclosure? Could you give me some other helpful ideas, we really need to dampen the sound for my wife's sanity's sake and we're on a bit of a tight budget.

Trademark Soundproofing Reply:

Green Glue will not work well between 2 layers MLV. You can try the MLV on it's own or get professional outdoor sound blankets.

7) Gani: Hi, I’m trying to sound proof a room that has noise coming from the window side of the room. I’m planning to build an entirely new wall on that side to block out the noise. How thick should The wall be? Any formulas would help so I can compute.

Trademark Soundproofing Reply: As long as the wall is totally decoupled from the first wall (not attached to the 1st wall at all). Than it can be a 2x4 wall with insulation, double drywall and a damping compound in between.

8) david: i am adding a separate partition to an existing demising partition in order to provide better sound attenuation from a new tenant on the new wall side. we only have access to one side. existing partition is 5/8" gpy on 8" metal studs with batt insul. new wall is held 1/2" off of existing wall. we are adding an EVA mass loaded barrier (wall blokker) directly to the existing drywall, then the 1/2" gap to new 6" metal studs, then another layer of wall blokker, then one layer 5/8" gyp. question is... does having two cavities help or hurt the sound attenuation of the entire double partition? or is it better to remove the inner layer of existing drywall?

Trademark Soundproofing Reply: Better to remove the inner layer of existing drywall. See our article regarding the triple leaf effect.