Noise Reduction Coefficient

Imagine standing in the middle of a room with all concrete surfaces and one open window, and you blow a loud whistle.    The sound that hits the open window passes right through.    The sound that hits the concrete surfaces in the room begin to reflect back.    The NRC of the open window would be 1.00 which is the absence of sound reflection.    The NRC of concrete is .03, indicating that 97% of the noise that hits the concrete surfaces will reflect back off the surface.   The NRC value of a surface simply denotes its ability to absorb vs reflect sound.    The higher the NRC value, the stronger the sound absorption and the lower the sound reflection in the room.

As we outlined in our course on the Sabin here at the Academy, all surface exposed materials in a room are rated on their ability to absorb sound reflections.    This results in an absorption coefficient being assigned to all building materials measured at varying frequency levels.    A sample of these figures are linked below on NetWell’s Absorption Coefficients Chart.   A value of “0” indicates perfect reflection (the absence of absorption), while a value of “1.0” indicates perfect absorption (the absence of reflection).   The stronger the NRC value, the more absorptive the material is, and the weaker the sound reflection will be in the room.

Independent laboratories assign sound absorption values to all surface exposed building materials as well as sound control products.    The standard test is called the ASTM C-423 test.   The test results reveal how much sound energy a surface is able to capture and convert, delivering less reflection back into a room.   These results can then be used to measure the existing absorption in a room prior to a NetWell soundproofing treatment.   The results can also be used to help rank the performance of various sound control products and target the appropriate product for the treatment.

The testing procedure starts with a minimum 72 square feet of a product placed inside a concrete sound chamber.   A sound signal is triggered, producing sound reflections inside the chamber.   The time is then measured between the end of the sound signal and the end of the resulting sound reflection.    This is called Reverberation Time.   The difference in Reverberation Time between the room with the material and without will determine the absorption coefficient of the material being tested.   The test is performed six times, once each at the intervals of 125, 250, 500, 1000, 2000 and 4000 Hertz.    This helps us understand how the material performs against high, medium and low pitched sound sources.    The NRC value equals the average of the middle four numbers.

Each interval rating will reveal a product’s ability to absorb and convert sound energy at that corresponding frequency.   To illustrate the value of the NRC, note the corresponding values between our 1″ and 2″ thick Fabric Panels:

	125	250	500	1000	2000	4000	NRC
1″ Thick Fabric Panels	.11	.30	.77	1.05	1.0	1.0	.80
2″ Thick Fabric Panels	.46	1.0	1.07	1.05	1.1	1.18	1.05

 

The NRC value is represented as a decimal figure.   In our example above, the NRC value of our 1″ thick Fabric Panel is .80.   This means that 80% of the sound wave entering into the material will be captured and converted, while 20% will reflect back out of the material into the room.   The figure also tells us that for every ten square feet of this material added into a room, that 8.0 additional Sabins of absorption will be added.   As we outline in our course on the Sabin, this figure can then be used to determine square foot coverage required to trigger a collapse in unwanted Reverberation Time down to under 2.0 seconds.

The NRC value can also be used to guide product selection.   Note in our example above, that the 2″ thick material absorbs 4x the amount of sound reflection at 125 Hertz than the 1″ thick material does.    When a church first calls to NetWell seeking sound control in their sanctuary, our first question is whether the worship services are traditional or contemporary.    The NRC tells us that contemporary services with bass guitars and drums require the 2″ thick panel for low bass absorption, while the traditional services filled with human voice, organ and choir will do just fine with the 1″ thick material.   Note that at the mid range tones of 1000 Hertz, the two products perform evenly.

To recap, it is important to note that every surface in your existing room carries corresponding noise reduction coefficient values prior to our soundproofing treatment.    The problem is that these values just aren’t strong enough.   On their own, they are not able to absorb enough of the sound reflection going on inside your room.    This builds unwanted levels of background noise that interfere with new original sounds in your room.    This can range from nuisance noise in a fellowship hall, to poor classroom learning environments, to unsafe industrial exposure to noise.   Our course on Sound Absorption will teach you how to better control this background noise through the use of the NRC.

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