What is an NRC rating?
With a plethora of specialty absorption products available on the market today, how do you determine which on is right for your situation? Although aesthetics are important, that should not be the only factor considered before making a decision. Generally speaking, most people want the bare minimum amount of coverage to solve their specific noise issue.
This is where NRC comes into play, it is an abbreviation for Noise Reduction Coefficient, which is essentially a numeric representation of how much of the sound energy that hits the product is absorbed. NRC is very helpful but is not a perfect rating system. The absorption coefficient of a product is different depending on which frequency we are looking at. NRC is simply and average of the absorption coefficients at the frequencies: 250 Hz, 500 Hz, 1000 Hz, 2000 Hz. These frequencies are exactly one octave apart from each other so there is not much variation in what is actually being analyzed with this rating system. A newer standard, SAA (Sound Absorption Average), is slowly replacing NRC and should be preferred when available. Never the less, NRC is still a viable guideline for determining a products absorption capability. An NRC value of 1.0 means that 100% of the sound energy that hits the product is absorbed (good) and a value of 0.0 means none is absorbed that all of the sound energy is reflected back into the room, contributing to additional echo/reverberation in the space (bad). For example, two acoustic panels with an NRC of 0.5 each will be about equivalent to a single panel with an NRC of 1.0.
Make note that these absorption products can only absorb energy that actually reaches them, this is why coverage is so important. Installing only a couple acoustic panels in a large auditorium won’t make any hear-able difference, this is simply due to all of the surfaces that are still left untreated. There are, however, other methods to increase or decrease absorption without changing the coverage. For example, mounting an acoustic panel 1 inch off of the wall will leave some airspace behind the panel allowing for air pressure to flow behind the panel as well. This can result in NRC values above 1.0. “Over 100%??? How is this possible?” you may ask. The NRC rating really only takes into account the front face of an acoustic panel. However, the panel can also absorb through it’s sides and backside, essentially creating new small sections of absorbing surface area your wall or ceiling didn’t have before. Cool stuff right? But that isn’t all. In addition to creating an air gap behind the panel, increasing the thickness of your absorptive material usually increases the absorption as well. Generally speaking, thicker panels have a higher NRC and are much better at absorbing low frequency (bass-ey, boom-ey) sounds, and also on the other hand, if you reduce the thickness of an absorption panel this will inversely affect the performance in a similar fashion. Material density also plays a large role in changing the characteristics of absorption. The most common and affordable absorption material by is fiberglass insulation. Check out the figures below for a comparison of fiberglass panels and how absorption coefficients are affected by varying thickness and density.