How to Stop Sound

Sound is a form of energy that travels as a wave – when that wave hits a barrier as in a sheet of drywall, that barrier will vibrate and cause the air on the other side to vibrate, thus transferring the sound through the wall. There is no one-shot soundproofing technique that will stop noise in its tracks; rather we try to employ a combination of solutions to achieve solid performance.

The four most important elements of soundproofing are:

  1. Absorption
  2. Damping
  3. Decoupling
  4. Mass

Absorption can be achieved by installing fiberglass of R-11 in 2×4 walls and R-19 in 2×6 walls and ceilings. Although fiberglass is not a major player, only adding 3-4 points, it is still a step that should be done in any serious soundproofing project

Damping is the process of turning your barriers (drywall, sub floors) into dead panels that do not vibrate, thus preventing sound from passing through them. Damping is achieved by applying a damping compound in between two constrained layers.

Decoupling is meant to separate the attachment of walls from the studs, thereby breaking the direct path of sound, which can be done using resilient channel, and more importantly – with resilient sound clips. This decoupling technique actually adds another component called “resilience” to the walls. [Click here for an in depth explanation to decoupling].

Mass simply means creating a heavier wall by using more (another layer) and/or thicker material.

How Is Noise Tested?

Let’s take a look at a very popular term called STC (Sound Transmission Class) which is an industry standard to calculate sound transmission through barriers. Many building codes now require condos and townhouses to meet a STC of 50; luxury hotels and condos should be looking to reach 60 and higher.

STC Performance Description
20-25 Very Poor Low speech audible
25-30 Poor Normal speech understood
30-35 Fair Loud speech understood
35-40 Good Loud speech heard, but hardly intelligible
40-50 Very Good Loud speech heard faintly
50-60 Excellent Loud sounds barely heard

A standard 2×4 wood stud wall with 1/2″ drywall 16″ O.C. has an STC of 32. An important point to keep in mind is that STC testing only tests on frequency ranges from 125 – 4000 Hz. One problem with this type of classification is that a lot of the common noise issues are due to sounds that fall below or above this range. Examples of such sounds are bass drums, deep male voice, trucks, etc. – all of these can start at 50 Hz. High pitched female voices, shrill ringing of phone and auto horns are good examples of sounds that reach above the 4000 Hz range.

This will explain why many big projects, after having paid tens of thousands of dollars for acoustical engineers and getting thousands of pages of tested soundproofing materials with lab testing of STC’s of 50 and above, still receive complaints for poor acoustical performance.

For good measure we will add one more reason for poor performance results: the fact that although material can test to a STC of 25 for instance, does not mean that when added to a wall assembly of  32 (like a standard 2×4 wall) that the end result will be a STC of 57 (25+32=57). In reality, it may only add 2-3 points to the wall bringing it to a STC of 35. A perfect example of this phenomenon is the much touted Mass Loaded Vinyl and all the other brand names that it is sold under.