NRC relation to STC

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    Michael Ermann

    Ivan, that is somewhat confusing, isn’t it?

    Noise reduction coefficient (NRC) measures sound absorption inside a room and goes a long way to determining room acoustics. If a racquetball court sounds excessively reverberant, it is because it has a low NRC, so very little of the sound energy made in the room is absorbed or transmitted, and more than 99% of the sound energy impinging on room surfaces is reflected (for an NRC of 0.01). If you are reading this in your plush, velvet-covered smoking room, then most of the sound impinging on room surfaces is absorbed or transmitted by the room’s surfaces and very little is reflected. Maybe only 15% of the sound energy impinging on the surfaces is reflected (for an NRC of 0.85). It’s just the way you would think: thick, squishy, fuzzy materials with interconnected air pockets absorb more sound; massive, hard, dense, smooth materials reflect more sound.

    By contrast, sound transmission class (STC) measures sound transmission between rooms and goes a long way to determining sound isolation. If you live in an apartment and can easily hear your neighbor snoring, then you might have a wall STC of only 25. If you can’t hear her even when she’s hosting party and has the stereo cranked, you may have a wall STC of 65. Massive, airtight, and structurally discontinuous (like double-walled) barriers perform best at maintaining sound isolation.

    While both NRC and STC are measures of acoustics, they measure different things, just like temperature and windspeed are both measures of weather, but they measure different things. Generally, materials like glass fiber with high NRC values have interconnected small cells where air can pass through. The friction inherent when sound energy passes through that kind of material structure reduces the strength of the sound before it is reflected back into the room it came from. But if you separated two different rooms with only a fiberglass batt blanket for a barrier. . . you can see how those interconnected cells would allow you to hear your neighbor and your life experience tells you that a blanket isn’t a good barrier to sound transmission.

    Of course, you could design an assembly with both a high NRC and a high STC, but that would not likely be a homogeneous single-material assembly. Picture a wall with glass fiber panels mounted over 8 inches of concrete. Sound created inside the room that “sees” the absorptive panels would die quickly and be less likely to excessively reverberate. But the concrete part of the wall assembly would prevent your neighbor from hearing the argument you had with your roommate last night.--Michael Ermann, Amber Book creator

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    Ivan Laguna

    Hi Michael,

    Thank you very much for the detailed explanation! It's now clear that NRC measures sound absorption inside a room while STC measures transmission between rooms. The distinction is much clearer. Thanks again! 

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