Understanding Sound Transmission Class

Soundproofing is measured in North America using Sound Transmission Class or STC. The STC scale is a logarithmic measure of sound reduction throughout the frequency range of human hearing. Confused already? Don’t worry, we’ll explain all this below.

Lets start by explaining dB. Decibels are a measure of sound intensity; that is, the magnitude of the fluctuations in air pressure caused by sound waves. The louder the noise the more intense the wave, picture a supersonic boom. The decibel scale is also logarithmic like the STC scale, meaning that it is a measure of the % change in sound reduction.

A simple scale is linear, meaning that an increase of 10X would always result in a change of 10Y. In a logarithmic scale and an increase in 10X leads to an exponential increase in the Y axis. Take for example Normal speech that is around 60 dB in volume, with a logarithmic scale a 70 dB noise such as a household blender is 10X louder than normal speech and a 100 dB noise such as a lawnmower is 100,000 times louder than normal speech.

Typically, achieving a 10 dB reduction in sound is considered to be doubling the soundproofing of a wall or ceiling. This is easily achieved with wall assemblies that include SONOpan.

The STC rating is a standardized way of measuring DB reduction in different building materials and puts all materials on a level playing field. Drywall and SONOpan have similar ratings for example but when combined they achieve a soundproofing result that is basically double that of each material alone and significantly better than doubling up on each specific material. Explaining how this happens allows us to expain how soundproofing works.

Every material has an STC rating, meaning that every real life wall or ceiling will differ slightly based on the exact materials being used and the methods of the installer. We only focus on the 5 materials that are specifically used for soundproofing, since these are the materials that have a significant influence on soundproofing results. These 5 materials are laid out below.

Insulation – This stops resonance in an empty cavity and helps capture certain frequencies. The human ear cannot hear the difference between specialty, fiberglass, wool or cellulose type insulation so it is our recommendation to simply use the appropriate size insulation for the cavity.

SONOpan – Our panels are dual function, they create a solid barrier to stop sound while also absorbing a wide range of sound frequencies, including low bass. Our Noise STOP Technology is the key to this, SONOpan has over 17,000 impressions in both sides of the panel resulting in a varying density throughout. When it comes to soundproofing, each frequency range has a corresponding sound wave where the peaks and valleys of that range are a certain size. The peaks and valleys in SONOpan are what make it extremely effective at soundproofing.

Drywall (mass) – Mass is also an important element in soundproofing. There are several options when it comes to mass but the simplest is drywall, ideally 5/8″ thick drywall and in extreme soundproofing situations up to 2 sheets can be used on each side of the wall or on the ceiling. Don’t let that intimidate you though! Most situations will do just fine with a single sheet of 1/2″ drywall.

De-Coupling – We counted this as a material but there are several ways to de-couple. Essentially what this does is break the connection between two building materials which reduces the transmission of sound vibrations from one material to the other. This is not always a necessary addition to soundproofing but it is quite effective when heavy vibrational frequencies are present.

Air gap – The final soundproofing strategy. Though not a material, This is a factor to consider when soundproofing. Generally speaking sound transmission diminishes through air, the larger the space the better but don’t go less than 1″ if doing a double wall assembly.

From a most basic assembly to building a sound studio, you will always find some combination of these 5 elements in any soundproofing assembly.