We’ve already talked a bit about sound absorption coefficients. To understand NRC, you have to understand the sound absorption coefficient. The coefficient describes how much sound energy that a material absorbs. Theoretically, a coefficient will vary from 0 (100% of sound is reflected) to 1.0 (perfect absorption). In practice though, labs report coefficients higher than 1.0 all the time due to the testing methods and sound diffraction. It’s normal, and not something to worry about.  

Materials with coefficients of 0.50 and higher are generally classified as “sound absorbing”, while those with coefficients at or below 0.20 are “sound reflecting”. There can be a wide variety of materials in a single room, all with different coefficients. As a general rule, for a human to be able to tell a difference at all between two materials, the sound absorption coefficient must be at least 0.10 different. By the time the difference is 0.40 or greater, you have a really large difference that will give substantially different results.

Let’s say you’re in your garage working on a project and you’re firing up some power tools. Once you rev up the circular saw, sound energy explodes outwards and starts reflecting off of every surface in the room. Since the typical garage’s walls are drywall and the surfaces inside are reflective too (a metal table, a car), very little of that sound energy will be absorbed. It’s bouncing all over the place. Now, let’s imagine it’s a hot day and your garage isn’t climate-controlled, so you open the garage door. The effect inside the garage will be MASSIVE, because you’ve just replaced a wall made of metal (your garage door) with open air (perfect absorption). If you can’t open the garage door, you can accomplish the same effect using acoustic materials.

The (unrealistic) goal of many people looking to install acoustic materials is to “soundproof a room", and so they go looking for soundproof foam. While these materials can reduce the noise level a bit, they aren’t really designed to keep sound out or in a space (soundproofing). Make sure you’re getting the right material for the right job.

Sabins are units of measurement for sound absorption. Simply put, take each absorptive material in the room’s square footage and multiply it by its sound absorption coefficient at that frequency. Add it all up an you get the total absorption for the room (at that frequency).

Total absorption (sabins) = Σ (surface area x sound absorption coefficient)

We love math, so let’s throw an example out there:

• Install 50 square feet of acoustic panels for home with a sound absorption coefficient of 0.80 at 500 Hz. Fabric wrapped panels and ceiling acoustic cloud panels will deliver 40 sabins of absorption for sounds at the 500 Hz frequency.
• Now let’s imagine that same homeowner also installs 50 square feet of acoustic foam in that same room. With a sound absorption coefficient of 0.50 at 500 Hz, another 25 sabins have been added to the room (at that frequency).

While NRC ratings measure how effective a material is when it comes to absorbing sound energy, sound transmission class (STC) ratings measure how effective a material or structure is at preventing sound from passing through it.

While the two ratings are both used for sound related projects, they measure different things and serve two fundamentally-unique purposes. For someone looking to improve speed intelligibility, improve the quality of sound, or determine how to reduce echoes in a room, they should be concerned with the NRC rating. For someone looking to prevent sound from traveling between rooms, they should be thinking about the STC rating. If you're concerned with impact noise being transferred through a floor, that is measured by IIC rating.

While NRC ratings and the science behind them can be a lot to digest, they are actually a pretty simple tool. A higher number means more absorption, and less material required to improve the sound quality in any room. If you have questions about acoustic materials or need help solving a problem in your home, business, or vehicle, reach out to us! We’re here to help.