ASTM C423: Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method

Standard: ASTM C423 - Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method

Certification Required: Acoustical certification is currently not required by building codes, however the NRC rating is required by acoustical consultants that are designing the acoustical environment in auditoriums, classrooms, offices and other types of building spaces.

Scope: This test method covers the measurement of the sound absorptive properties of a product in a reverberation room. The amount of sound absorption provided by the product is determined by measuring the decay rate of the broadband noise when it switched on and off in the room.

Applicable Products: The sound absorption test is applicable to wall panels, ceiling panels, office screens, draperies, sound baffles, bass traps, catenary panels, theater chairs, furniture, highway barriers and many other types of products. The ASTM test method recommends a sample size of 8’ wide by 9’ long for products that cover an extended plane surface. The ASTM E795 standard practice provides the mounting technique that is applicable for all sound absorptive products.

Test Procedure: There are two parts to the sound absorption test. The sound absorption of the reverberation room, without a specimen installed, has to be determined. This test is accomplished by generating a high level of broadband noise into the room, turning it off, and measuring the time it takes for the sound level to drop 25 decibels. This is done simultaneously for frequencies ranging from 80 to 5000 hertz. The sound decay rate is measured 10 times at each of the five microphone locations and decays are averaged to determine the “Empty Room” absorption. The test specimen will then be installed in the room and another 50 decays will be measured. This measurement, with the sample installed, will be referred to as the “Full Room” absorption. The absorption coefficients (at each 1/3 octave frequency) will then be determined by subtracting the “Empty Room” absorption from the “Full Room” absorption and dividing by the area of the test specimen or by the number of sound absorbing units.

End Result: The NRC (Noise Reduction Coefficient) rating is the average of the absorption coefficients at 250, 500, 1000 and 2000 hertz, rounded to the nearest multiple .05. The SAA (Sound Absorption Average) rating is the average of the absorption coefficients at the 1/3 octave band frequencies from 200 to 2500 hertz, rounded to the nearest multiple .01. The NRC and SAA will be reported for products that have extended plane surfaces. The sound absorption of individual objects, such as space absorbers, theater chairs or ceiling baffles will be reported in square meters per unit at the 1/3 octave band frequencies. The sound absorption test can also be conducted in accordance with ISO 354 but a larger, 10 m2 (107.6 ft2) specimen must be tested.

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