Noise Reduction Coefficient Calculator
The Noise Reduction Coefficient (NRC) is the standard single-number rating used by architects and acoustic engineers to compare the sound absorption performance of materials. It is the average of the absorption coefficients at 250, 500, 1,000, and 2,000 Hz, rounded to the nearest 0.05. Enter the measured or published absorption coefficients at each octave band frequency to calculate NRC and Sound Absorption Average (SAA).
NRC calculation formula (ASTM C423)
NRC = (alpha250 + alpha500 + alpha1000 + alpha2000) / 4
NRC = round to nearest 0.05
SAA = (alpha250 + alpha500 + alpha1000 + alpha2000) / 4 (rounded to 0.01)
ASTM C423 defines the NRC as the average of the absorption coefficients at 250, 500, 1,000, and 2,000 Hz, rounded to the nearest 0.05. The SAA is rounded to the nearest 0.01. The 125 Hz and 4,000 Hz values are reported for information but are not included in the NRC calculation.
Typical NRC values for common materials
- Bare concrete or brick: NRC 0.02 to 0.05
- Carpet on concrete: NRC 0.20 to 0.35
- Acoustic ceiling tile (standard): NRC 0.50 to 0.75
- High-performance acoustic ceiling tile: NRC 0.80 to 0.95
- Acoustic foam panels (50 mm): NRC 0.75 to 0.95
- Glass or plaster: NRC 0.02 to 0.05
Noise reduction coefficient calculator: frequently asked questions
What is the Noise Reduction Coefficient (NRC)?
The NRC is the arithmetic average of the measured sound absorption coefficients at 250 Hz, 500 Hz, 1,000 Hz, and 2,000 Hz, rounded to the nearest 0.05. It ranges from 0.0 (perfect reflector) to 1.0 (perfect absorber). It is defined in ASTM C423 and used to compare acoustic performance of building materials.
What is the Sound Absorption Average (SAA)?
The SAA is the arithmetic average of the sound absorption coefficients in the twelve one-third octave bands from 200 Hz to 2,500 Hz, rounded to the nearest 0.01. It was introduced in ASTM C423-09 as a more detailed single-number rating. SAA is approximately equal to NRC but may differ slightly because it includes one-third octave data.
What do the octave-band absorption coefficients mean?
The absorption coefficient alpha at each frequency is the fraction of incident sound energy absorbed by the material (range 0 to 1.0, though values slightly above 1.0 can occur in laboratory tests due to edge diffraction effects). A material with alpha = 0.90 at 1,000 Hz absorbs 90% and reflects 10% of the sound energy at that frequency.
What NRC values should I look for in acoustic materials?
For room acoustics control in offices and classrooms, ASTM E1477 and ISO 11654 classify materials: Class A (excellent absorbers): NRC greater than 0.80. Class B (good): NRC 0.60 to 0.80. Class C (moderate): NRC 0.40 to 0.60. Class D (low): NRC 0.20 to 0.40. Class E (negligible): NRC less than 0.20. Bare concrete has NRC approximately 0.05; acoustic ceiling tiles typically 0.70 to 0.90.
How is sound attenuation (dB reduction) calculated from NRC?
NRC is a material property; the actual noise reduction in a room depends on the total area of absorbing material (in sabins: S = A * alpha). Noise reduction NR = 10 * log10(R2/R1) where R is the room constant R = S * alpha / (1 - alpha_avg). A simplified relationship: doubling the total absorption reduces reverberant noise by approximately 3 dB.
Official sources
- ASTM C423 Standard Test Method for Sound Absorption and Sound Absorption Coefficients: ASTM C423.
- NIST Engineering Laboratory Acoustics: nist.gov/el.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.