Acoustics terms



  • Avatar
    Michael Wardlaw

    NRC is talking about materials in a room. Putting fabric ceiling clouds in a large open office to reduce sound. 

    STC is sound between rooms, typically solved with wall assemblies. Hotel rooms have to have higher STC, a staggered stud wall w/ resilient channels. 


    I am not familiar with TL. The only other sound type I've seen referenced is IIC, Impact Isolation Class. This deals with sound traveling though the structure itself. Think high heeled shoes on the floor above you. 


    Comment actions Permalink
  • Avatar
    Jennifer Schuler


    I know your not trying to purchase anything new however if you already have either the Ballast ARE 4.0 Review Manual, Amber or Ballast ARE 5.0 review materials, these all talk about the STC, TL, NRC and NC in detail. 

    Copied below are a few of my own notes I made while using the materials above to help me remember the differences.

    Hope these help. Good luck!  


    NRC = About absorptivity of materials in a space. More absorptivity means sound goes away more quickly 

    NC = Is about background noise 

    STC = higher number means a better wall assembly 

    IIC = Is about football. 60-70 is typically high, 20-30 is low. Using materials like carpet or a sub floor elevated from the structure can help reduce transfer to the space below. Typically a problem in apartment/housing. 

    According to IBC = STC & IIC rating of 50 typically required between multifamily housing 

    A space which is to dry = the space does not have enough reverberation, sound goes away to quickly 

    Things that affect the performance of a wall assembly = no acoustic caulking at the top and bottom of wall assembly, how bumpy or smooth a surface is, how dense the wall is concrete vs. wood stud, etc.  

    Transmission Loss = Is about the decrease of sound that is reduced by a wall at a given frequency. Higher number rating for TL = more sound will be 'lost' through the wall assembly. Transmission loss at low frequency for a heavy mass like concrete is better than wood studs

    To remember frequency I try to 'exaggerate' the term, high frequency = high pitched annoying sounds. Low frequency = someone talking, okay to listen to




    Comment actions Permalink
  • Avatar
    Michael Ermann

    Those acoustics terms are somewhat confusing

    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 wind speed 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.

    Both NRC and STC are single-number measurements that combine low-frequency (bass content: mechanical noise, transportation noise, bump’n bass from amplified music, vowels) and high-frequency (high-pitched: hissing pipes, truck backup beepers, consonants). Because humans are less sensitive to low-frequency sound, NRC and STC give varying weights to the performance of the assembly across the frequency spectrum, bass to treble. These single-number values make for easy comparisons: acoustical ceiling tile A has an NRC of 0.75, acoustical ceiling tile B has an NRC of 0.95 so B absorbs more sound. . . floor-ceiling assembly A has an STC of 35, floor-ceiling assembly B has an STC of 55 so B is more robust at keeping the room upstairs quiet while the room downstairs has the TV blasting. But, while these single-number ratings make it easy, they gloss over the importance of knowing how a material will behave at a given frequency. This is especially problematic in the presence of low-frequency sound like an orchestra (NRC doesn’t account for how much low-frequency tuba sound is being absorbed. . . just a single number for all sound), and it doesn’t account for low frequency noise (STC is unable to tell you if you will be able to hear the roar of an air handling unit in the adjacent room, because it is just a single number). Many, but not all, materials absorb high-frequency sound much better than low-frequency sound, and many, but not all, assemblies prevent high frequency sound from passing through (speech) but don’t do well at mitigating low-frequency sound (a bus passing outside).

    So what is an architect, or acoustician, to do? If they want more detailed frequency-level information, the absorption coefficient (denoted by a lower-case alpha) measures the absorption at each octave band so we can know if the tuba’s energy will be absorbed in the concert hall when we specify a certain weight of velour curtain. And if we want to know if the window will sufficiently block the low-frequency roaring sound of the passing bus accelerating outside, looking at transmission loss (TL) will give us data at the low-frequencies and high-frequencies and everything in between.

    And while NRC measures room absorption as a convenient single number, and absorption coefficient measures it at each octave band for more detailed analysis. . . and while STC measures airborne sound attenuation between rooms, and TL measures it at each octave band for more detailed analysis. . .  impact noise measures a floor-ceiling assembly’s response to structure-borne noise. Specifically IIC measures how well the floor-ceiling mitigates impacts from footfall on the surface above. If you took no measures to address impact noise, your assembly might earn an IIC rating of 35, and most reasonably-minded downstairs occupants would judge that as unacceptable.  And if you took exhaustive steps and achieved an assembly with an IIC-65 rating, most reasonably-minded downstairs occupants would judge that as acceptable. If IIC was a person in your office, you wouldn’t like him very much. Not only is he piss-poor at rating wood frame floor-ceiling assemblies, but the code requires IIC of at least 50 for multi-family housing, and many reasonably-minded downstairs occupants would find that unacceptable.

    An assembly could have a high IIC and a low STC, meaning you can’t hear the person walking upstairs, but you can hear their stereo. And an assembly could have a high STC and a low IIC, meaning you can’t hear their stereo, but you can hear them puttering around.

    Hope that helps. –Michael Ermann, Amber Book creator and author of Architectural Acoustics Illustrated , Wiley 2015. (like a Ching book for architectural acoustics. . . That illustration above was one of hundreds like it from this book)



    Comment actions Permalink

Please sign in to leave a comment.

Powered by Zendesk