If you’ve found yourself looking into NRC rating, there’s a good chance you’re working on a project that requires sound absorption to reduce echo and reverberation in a room or space. Perhaps you’re starting a podcast or building a sound studio in your home. Maybe you are trying to improve the acoustics at your workplace so people can concentrate more easily. Whatever project you’re thinking about, understanding what an NRC rating is and how it works will help you find the best acoustical product solutions you’ll need. Here are the basics of NRC.
A Noise Reduction Coefficient – commonly known as NRC – is a single number rating which represents the average of sound Absorption Coefficients of a material at specific mid-range frequencies (tested at 250, 500, 1,000, and 2,000 Hz octaves). The purpose of an NRC rating is to provide a simpler way to determine how well an acoustical product absorbs mid-range sound (generally thought of as the range of speech frequencies). NRC ratings vary from product to product and are affected by the type of absorbent material, its thickness, its density, and its mounting method.
By definition, the NRC rating is a mathematical coefficient and should range only from 0.0 to 1.0. A 0.0 rating might represent something like a smooth-finish concrete wall, where sound is completely reflected off the surface. A 1.0 rating might represent something like an open window, where all the sound passes through the window opening and doesn’t reflect back into the space.
However, you may come across materials with NRC ratings higher than 1.0, which can be confusing. This doesn’t mean that the material can absorb more sound than that arriving at the material; instead, it’s an issue with the testing standards used to determine the NRC rating. The perimeter and thickness of the material being tested will cause an “edge effect” (diffraction), which can result in errors in the calculation used to arrive at the NRC rating, yielding results above 1.0.
Standards organizations are currently engaged in researching changes needed for the Noise Reduction (and Absorption) Coefficient methodologies to be more accurate for all cases of product ratings.
It’s not as easy as saying the higher NRC rating the better. It also matters how much of that material is present. For example, carpets have a relatively low NRC rating (0.15 – 0.30) but often cover an entire floor, which can have a large influence on reducing sound reflection strength within a room. However, carpet alone is usually not enough to reduce echo and reverberation to a desirable level, or it may not be feasible to use it in a space, like in a gymnasium. This is where acoustical products designed to effectively absorb sound, thus having a high NRC rating, are necessary.
So, what is a “good” NRC rating when looking at acoustical materials? When you want to take meaningful steps to reduce echo and noise in a room, a good starting point would be materials with NRC ratings above 0.7. Noisier rooms may require a higher NRC rating, or more material. Keep in mind that frequencies below about 200Hz (low frequencies) and above about 2,500Hz (high frequencies) are not included in the NRC ratings – if these frequencies are part of your echo and noise problem, asking an expert would be advisable. You can always contact us – we can help you find the ideal solution for your situation.
This can be a tricky question. While products with a higher NRC rating can absorb more reflected sound energy, adding too much absorptive acoustical treatments in a room will be less than ideal. It can make the room sound “dead,” or lacking in the ambient reflections needed to make sense of the space, and may even deter people from interacting or speaking at normal volumes. When building a sound studio or vocal isolation booth, both a proper amount of absorption and diffusion treatments should be considered. When working with the areas of your home where people gather, you’ll want a mix of reflective and absorbent materials.
To understand NRC better, it’s important to know a few facts about the test method that determines the values used in the calculation. The ASTM (American Society of Testing and Materials) C423 test protocol is the standard method for determining the sound absorption of a material using a reverberant room – a highly-reflective room with near-zero sound absorption. These test rooms are constructed with thick soundproof walls, ceilings, and floors, and heavy soundproof entrance doors.
The C423 test first measures the empty room’s reverberation times at specified frequency ranges and within specified temperature and humidity ranges; then, after confirming consistent temperature and humidity, reverberation times are measured again, this time with the necessary amount (determined by test room volume) of acoustical material in the room. Then, the two results are compared to determine the amount of reverberant energy absorbed by the test materials. The measurements are done across a range of frequencies, usually in one-octave or third-octave bands from 100 Hz to 5000 Hz, using multiple microphones and loudspeakers.
When the total area of the added acoustical material is considered, the Absorption Coefficient can be determined for each frequency band measured. With this information, the NRC rating can be calculated. The NRC rating is simply the arithmetic average of the Absorption Coefficients at 250, 500, 1000, and 2000Hz octave bands rounded to the nearest multiple of 0.05.
While the NRC rating can be very useful for providing a quick representation of the performance of an acoustical product, it is not perfect. The NRC rating can work well for representing acoustical absorption in the mid-range speech frequencies, but if your issue is an industrial building with machines that make a high-pitched noise, or a club where low-frequency bass is a nightly occurrence, then the NRC rating isn’t as helpful. As previously mentioned, it does not tell you how well the material absorbs sound below about 200 Hz or above about 2500 Hz.
The NRC rating is currently being phased out but is still a required calculation so current ASTM C423 tests can be compared to older ones. Its replacement is the Sound Absorption Average (SAA), which takes the average Absorption Coefficient over a greater range of frequencies, and with more frequencies in-between; 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, and 2500 Hz. If your concern is low or high-frequency noise, you may still have to look beyond the NRC or SAA rating. Our knowledgeable sales staff can help you find the best performing solution for your unique needs.
In doing your research, you may have come across both NRC and STC ratings. While they both have to do with the acoustical properties of a material, the NRC Rating tells us how much sound is absorbed by a product and the STC rating tells us how much sound is blocked from going through a product.
The STC rating, or Sound Transmission Class rating, is useful when you are concerned with soundproofing, or how much sound enters and leaves a room. For example, apartment buildings usually have STC rating requirements for the structure for reducing the amount of sound that leaks out or in.
If you’re only trying to reduce echo and improve the sound in a room, you won’t have to worry much about STC ratings. However, if you are trying to improve the acoustics in a room, while also keeping that sound in, or outside sound out, you’ll likely be looking at different types of products that have either NRC or STC ratings.
The NRC rating of an acoustical product is a useful tool when looking for an absorber product. As a single number it can be used to compare different acoustical products to find the right one for the job. However, depending on the noise issue you are trying to solve, it may be necessary to look beyond the NRC rating.
https://www.acousticalsurfaces.com/blog/acoustics-education/nrc-rating-101/
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