No one likes noise. It can cause issues with focus and concentration, and if we are exposed to high levels of noise for long periods of time, it can negatively affect both our physical and mental health. For these reasons and more, it is important to plan a reduction in the amount of noise we are exposed to in our homes and workplaces.
NRC ratings, or Noise Reduction Coefficients, are integral tools for creating a plan that works. They represent a key metric for measuring acoustic performance, designed to quantify the overall sound absorption capabilities of materials in any given space. Specifically, they give us an idea of how well materials absorb or reflect sound waves across a range of frequencies.
When we understand NRC ratings, we are better able to design spaces that keep us happier and healthier through noise reduction. This article is designed to provide you with that understanding, and to help you understand some of the ways we can use NRC to treat your indoor spaces effectively.
What is an NRC Rating?
An NRC rating (Noise Reduction Coefficient) is a numerical value that measures how much sound a material absorbs.
NRC values range from 0.0 to 1.0, in theory. A value of 0.0 denotes materials that absorb no sound and a value of 1.0 denotes materials that absorb all sound. Due to anomalies within the testing process, it is possible for NRC values to exceed 1.0, but there is no statistical difference between a value of 1.0 and a value above 1.0.
Nearly all materials on Earth fall somewhere between the two endpoints on the scale. Even highly reflective materials like glass and concrete will typically not reflect all sound, leaving them with an NRC rating near zero, but with some wiggle room. Similarly, heavily absorbent materials like thick acoustic foam or specialized acoustic panels will absorb most sounds but not all, leaving most of them slightly shy of the 1.0 mark.
How NRC Ratings are Measured
NRC values are determined through standardized acoustic performance testing that evaluates the ability of each material to absorb sound at various frequencies. Specifically, those frequencies are 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz, which are incremental frequencies in the middle to high end of the audible spectrum, representative of the typical human speech range.
The most common standards for testing NRC are the ISO 354 and ASTM C423 standards. ASTM C423 is used for North American standards, and ISO 354 is used for international standards. For North American testing, the process is fairly straightforward.
Materials are exposed to broadband sound (covering a wide range of frequencies), and a sound absorption coefficient for each of the aforementioned frequencies is determined. This represents the fraction of sound energy absorbed by the material at that frequency. The average of the four sound absorption coefficient values represents the final NRC value.
Values are reported to the nearest 0.05 increment, meaning that if a material’s calculated NRC is 0.38, the value may be rounded to 0.4 in the final report. This number gives sound professionals an idea of the amount of noise pollution reduction that material provides.
Factors Affecting NRC Ratings
There are several testing factors that can influence a material’s final NRC rating. Factors like the material thickness, material density, and frequency range all affect the material’s ability to absorb or reflect sound.
Material Thickness
The thickness of the material can have a significant impact on the NRC rating. Thicker porous materials may very well absorb far more sound than thinner test subjects. When dealing with echo control materials like noise absorption panels, this is how the NRC values between different thicknesses of the same material ends up being different from others in the same product line.
Material Density
The density of a material can also affect the NRC rating, although it is usually affected by other factors as well. For instance, high density materials like concrete or metal may be more reflective and absorb less sound than low density materials like foam and fabric, but this is also due to porosity. High density materials that are highly porous can absorb more sound than lower density, porous materials.
Frequency Range
Materials may absorb different amounts of sound, depending on the frequency range of the broadband sound used in testing. For example, many materials are far better at absorbing sounds in the higher end of the spectrum than they are at the lower end.
Interpreting NRC Ratings
While NRC ratings are pretty exact numbers that represent how well the material performed in standardized testing, the exact number is not as important to your noise reduction plan than the range in which it falls. Quite simply, you can use the following ranges to give you a good indication of how well each material will perform when placed in your space.
NRC = 0.00
This would be a perfect reflector, meaning all sound is reflected. This result occurs very infrequently, but smooth concrete, glass, and metal are the types of materials that would come closest. They are great materials for spaces like theaters and lecture halls, where a substantial amount of sound reflection may be desired.
NRC = 0.10-0.35
This range represents the most common building materials in your average rooms. They reflect a great deal of sound but do provide a minimal amount of absorption. Bare walls, windows, and hard floors would fall into this category.
NRC = 0.40-0.65
Materials that fall into this range provide moderate absorption and are effective for providing simple sound control in spaces that already display minimal acoustical issues. Thin carpets, window curtains, and light foam panels are helpful in these spaces, but not effective when more aggressive acoustical treatments are necessary.
NRC = 0.70-0.95
This range represents materials that would be considered effective sound absorbers. Things like dense acoustic panels, acoustic tiles, and acoustical curtains absorb sound reflections very well for effective echo and reverberation control.
NRC = 1.00 and Up
A perfect absorber would be assigned this value range. Again, NRC ratings may exceed 1.00, due to the testing standard, but everything in this range would not reflect any sound whatsoever. Very few materials outside a very heavy acoustic foam or extremely thick, porous materials would fall into this category. These materials would be used when total absorption is needed in your wall treatment for sound.
Typical NRC Values for Common Materials
| Material | NRC Range |
| Fiberglass Panels | 0.80 – 1.00 |
| Acoustic Foam | 0.70 – 0.95 |
| PET felt | 0.40 – 0.90 |
| Carpet | 0.20 – 0.40 |
| Drywall | 0.05 – 0.15 |
Choosing the Right NRC for Your Space
Higher NRC ratings are not necessarily better than lower ones. It all depends on your needs. For an overly quiet space, it may be helpful to include materials with lower NRC ratings in your acoustic design. This will prevent the room from sounding “dead” or devoid of any reverberation whatsoever.
Alternately, for an overly loud or reverberant room, you will want to introduce materials with higher NRC ratings to absorb some of the reflections to prevent it from becoming too loud. This helps significantly with things like speech intelligibility and our ability to focus.
Applications of NRC Ratings
There are a number of ways acoustical professionals use NRC ratings to improve the sound quality in any space. Most often, we are utilizing noise reduction solutions to reduce the amount of noise in a room, but in some circumstances, it is necessary to incorporate more reflective materials to make the space sound more lively.
Commercial Spaces
In commercial settings like office and industrial spaces, we are typically trying to reduce the volume of the room noise. That’s because overexposure to loud sounds can contribute to a reduced ability to focus and can even raise stress levels and lead to serious physical and mental problems if not addressed. Improving office acoustics with absorbent panels and foam can have a significant impact on the health and happiness of everyone inside.
Residential Spaces
Residential settings encounter a similar set of problems. Excess noise entering the home through doors, windows, and walls present a noise issue, but the sound created within the home can be just as problematic.
If the space is left untreated, this noise can distract from the enjoyment of interacting with friends and family, negatively impact the sound quality of our favorite media, and make it difficult to sleep. Introducing materials with higher NRC ratings can tame all that noise and make our residential spaces more serene.
Specialized Settings
Not all situations require noise reduction measures. In some circumstances, it is preferable to increase the reverberation in order to deliver a more natural sound. In spaces with too many absorptive materials, the “deadness” of the sound can feel unnatural and jarring and may take away from the overall effectiveness of the space in its intended purpose.
Recording Studios
Many people think that they should cover all the walls in a recording studio with acoustical treatments. It’s an understandable thought process. The use of acoustic panels and acoustic foam benefits the mix by isolating vocal and instrumental elements, but in a complete absence of sound reflections, the final mix can come out sounding lifeless and bland.
Theatrical Productions
Many theaters are specifically designed to allow for an appropriate amount of reverberation. This can help create an appropriate atmosphere for productions, make it easier to hear actor dialogue and accentuate emotional moments.
Lecture Halls
In any space where there are a lot of people who must be able to hear a speaker correctly, it is important that sound reflections are harnessed through room acoustics optimization in a way that projects the speaker’s voice evenly throughout the room. This ensures the content of the speech is accessible to everyone from the front row to the back wall.
NRC Ratings vs Other Acoustic Metrics
NRC ratings are just one of a number of sound blocking and sound insulation ratings designed to help you identify issues and determine the appropriate response. NRC measures material reflectivity and absorptivity, but others may measure room reverberation time, clarity of speech and music, or the amount of sound that passes through walls. No one metric provides a final determination on sound quality, and they are best used in combination to evaluate a room’s acoustic performance from multiple angles.
How to Improve Acoustic Performance Using NRC Ratings
Many people use NRC ratings to improve the acoustic performance in their spaces through very scientific methods, but you can still use NRC ratings to improve sound without the need for specialized or expensive equipment.
As a general rule of thumb, if the space sounds too loud or reverberant, you will want to incorporate materials with higher NRC ratings, meaning they absorb more sound. A simple acoustic panel installation can result in drastic improvements.
If the space sounds too quiet, you will want to incorporate an appropriate number of materials with lower NRC ratings. This will ensure the space sounds lively but not overly loud.
Common Myths About NRC Ratings
There are situations where NRC ratings are very helpful, and there are situations where a different kind of information is required to make informed decisions. Here are a couple of common myths about NRC ratings that will help you use this information to its fullest potential.
Higher NRC Products Always Provide the Best Soundproofing
NRC ratings refer to a material’s ability to absorb sound, not block its passage through solid materials like walls, ceilings, and floors. While noise reduction materials are found near the top of any effective soundproofing tips list, the two concepts are not entirely the same. If you want to evaluate different soundproofing materials, STC (Sound Transmission Class) ratings are a far more relevant measure to consider.
NRC Ratings are the Same Across All Frequencies
Since the final NRC rating of a material is based on an average of how it performs across a range of frequencies, there will often be some frequencies that the material absorbs less efficiently. Additionally, since only a set range of frequencies is used in testing, some frequencies on the low end of the spectrum fall outside the testing limits and may not be absorbed as expected.
All Acoustic Materials Have a Published NRC Rating
While it is true that all materials have an NRC rating in theory, only materials that have been subjected to standardized testing may be labeled with a verified NRC rating. If there is no NRC rating listed on products you find through your search, any performance metrics are strictly subjective.
A Material with High NRC Will Totally Eliminate Echo or Reverberation
As mentioned previously, very few materials on Earth are completely absorptive. They may come close, but most materials will allow at least a small amount of sound reflection. Understanding the way a material performs and using it in a well-devised plan is the best way to achieve the echo and reverberation control you’re after.
Conclusion: Leveraging NRC Ratings for Better Sound Control
Understanding NRC ratings can help you create better sounding spaces, no matter what kind of acoustical issues you may be dealing with. It allows you to incorporate materials with the appropriate reflection to absorption ratio to dial in your acoustics more precisely. The result is a lively sound that makes communication effortless and reduces the presence of unwanted noise.
If you need help determining the right materials and treatment plan based on sound acoustic design principles for your space, reach out to Material LogIQ to learn more about how to effectively treat your space for a well-balanced mix.
Sources:
- Acoustical Surfaces. (Sept, 23, 2020). NRC Rating 101. Retrieved from https://www.acousticalsurfaces.com/blog/acoustics-education/nrc-rating-101/
