What Do NRC and STC Mean?

Technical terms like Noise Reduction Coefficient (NRC) and Sound Transmission Class (STC) can be somewhat confusing.  While one measures the build-up of noise within a space, the other measures the sound transmission between spaces.  It is important to understand the difference of these two ratings. (See demonstration of sound control for Cellulose vs. Fiberglass Insulation.)

The Noise Reduction Coefficient (NRC) measures the build-up of noise within a space.  A single number index rating is used to measure the sound absorption of a material.  Fiberlite’s Cellulose Insulation products have an NRC rating from .75 to .82 depending on wall design, materials and applied density of the product.  Simply put, FTI’s Cellulose Insulation products will absorb 75% to 82% of the sound that it comes in contact with and will reflect 18% to 25% of the sound back into the space.  However, NRC does not address a material’s barrier effect.  Nor, does it give information as to how absorptive a material is in the low and high frequencies.  NRC is only the average of the mid-frequency sound absorption coefficients (250, 500, 1000 and 2,000 hertz) rounded to the nearest 5%. 

The Sound Transmission Class (STC) measures the sound transmission between spaces.  A single number rating is used to measure the assembly’s barrier effect.  A higher STC rating blocks more noise from transmitting through a partition.  Loud speech can be understood through an STC 30 wall but should not be audible through an STC 60 wall.  Fiberlite’s Cellulose Insulation products have a STC rating of 44 to 68 depending on the wall construction.  STC ratings do not assess the low frequency sound transfer.  It is based on performance with frequencies from 125 to 4,000 hertz (speech frequencies). The STC rating is a lab test that does not take into consideration weak points, penetrations, or flanking paths.

Acoustical wall treatments with a high NRC can stop sound from reflecting back into the space and lowering the noise level within the space.  Improving the STC rating of a wall (wall cavity insulation, etc.) will reduce the noise transfer to the adjacent space.

More than likely you will have a builder or architect that requires this information.  The NRC and STC ratings information may be found on our submittal data sheets.



What Do NRC and STC Mean? — 3 Comments

  1. Kindly suggest a suitable treatment for under deck/ ceiling sound insulation designed to prevent noise percolation from the apartment above.

    • You can dense pack the subfloor with FTI cellulose, which will reduce the Sound Transmission Coefficient (STC). This can reduce it 45%-68% with the sound absorption being higher as you add more insulation. Keep in mind that it would be tough to completely reduce all noise transfer from a floor above the ceiling. Such activities as very loud yelling from upstairs could still bleed through. Similarly, it would be tough to reduce all jumping up and down and the subsequent vibration noises, which is NRC. But in most cases, a dense pack applied cellulose insulation will be the best option for reducing noise between floors in this type situation. For optimum results hire an insulation contractor experienced with this kind of work.

  2. Pingback: The Ultimate Guide to the Best Soundproofing Materials

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