What does NRC and SAA stand for?
Sound absorption properties of acoustic materials can be measured in lab tests. Specifications for materials used in sound absorption commonly include an NRC (Noise Reduction Coefficient) for simplicity, in addition to more detailed frequency versus amplitude charts.
The Noise Reduction Coefficient (NRC) and Sound Absorption Average (SAA) values are both single number ratings that indicate the level of sound absorption provided by the product being tested
NRC Noise Reduction Coefficients
The NRC is a single-number index determined in a lab test and used for rating how absorptive a particular material is. It is a scalar representation of the amount of sound energy absorbed upon striking a particular surface.
For example, an NRC of 0 indicates perfect reflection; an NRC of 1 indicates perfect absorption. It is the average of four sound absorption coefficients of the particular surface at frequencies of 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz rounded to the nearest 5%. These frequencies encompass the fundamental frequencies and first few overtones of typical human speech, and, therefore, the NRC provides a decent and simple quantification of how well the particular surface will absorb the human voice.
SAA Sound Absorption Average
The single number rating obtained from ASTM C423 is the Sound Absorption Average (SAA). This is the average of the absorption coefficients for the twelve one-third octave bands from 200 to 2500 Hz. Â The SAA supersedes the Noise Reduction Coefficient (NRC), which is the average of the sound absorption coefficients of a test specimen for 250, 500, 1000 and 2000 Hz rounded to the nearest multiple of 0.05. The higher the SAA or the NRC value, the better the material absorbs sound.
NRC and SAA values are both single number ratings that indicate the level of sound absorption provided by the product being tested. The NRC value is the average of the sound absorption coefficients at four 1/3 octave frequencies (250, 500, 1000 and 2000 hertz). The SAA value is the average of the sound absorption coefficients at twelve 1/3 octave frequencies ranging from 200 to 2500 hertz. The NRC value is rounded off the nearest 0.05 increment. The SAA value is rounded off the nearest 0.01 increment. The NRC and SAA values normally range from 0.00 to 1.00, with 1.00 indicating 100% sound absorption per square foot of material. These values can exceed 1.00 when thick specimens or specimens with large air spaces are being tested.
Limitations of NRC number
Â· Same material can show different NRC values depending on the method it was measured by. The method by which sound absorption is ultimately obtained can be: the Reverberation Room Method (ASTM C423) or the Impedance Tube Method (ASTM C384). The Reverberation Room Method is the more popular of the two in terms of tests conducted on acoustical room treatments. The method of mounting used for the test specimen in the reverberation chamber can affect the numbers. Test specimen can be laid directly on the chamber floor (A-mount) or hang like a curtain ( G-75 mount). Other mounting methods also available depending on how the material is intended to be used and test requirements. Depending on the type of the mount the exact same sample can produce very different rating.
Â· The range of frequencies used for NRC testing encompasses the fundamental frequencies and first few overtones of typical human speech and show how well the particular surface will absorb the human voice. A more broad frequency range should be considered for applications such as music or controlling mechanical noise.
NRC provides very simplistic and average indication of absorptive properties of the material, because it is simply the average of the mid-frequency sound absorption coefficients (250, 500, 1000 and 2000 Hertz). For example two materials may have identical NRCs, but do not perform identically in individual bands and may be direct opposites in the spectrum of frequencies they absorb.
Noise Reduction Coefficients (NRC) for some common Materials:
Brick, unpainted .00 – .05
Carpet, indoor-outdoor .15 – .20
Cork, wall tiles (1″ thick) .30 – .70
Drapery, light weight (10oz.) .05 – .15
Fiberglass, 3-1/2″ batt .90 – .95
Fiberglass, 1″ Semi-rigid .50 – .75
Plywood .10 – .15
Moving Blankets 0.3 – 0.5
While NRC is widely used and accepted, it can also be abused or misunderstood. Make sure the mounting procedure used in the tests is consistent with your intended installation if you expect the same results.