The standard "ASTM C423 Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Chamber Method," developed by the American Society for Testing and Materials (ASTM), describes a test method for measuring the decay rate of sound absorption in a reverberation chamber. This standard includes procedures for determining the absorption of a room, the absorption of an object such as an office screen, and the sound absorption coefficients of a sound-absorbing material such as an acoustic ceiling tile.
Although this test method involves laboratory measurements, it can also be used to make field measurements of room absorption. Another method for measuring room absorption in the field is included in ASTM E2235.
This standard has annexes (Annex A1, Annex A2, and Annex A3) that contain information on laboratory accreditation, asymmetric displays, and echo chamber qualification.
Measuring the sound absorption of a room is part of the procedure for other acoustic measurements, such as determining the sound power level of a noise source or the sound transmission loss of a partition. It is also used in certain calculations, such as estimating the sound pressure level in a room when the sound power level of a noise source is known.
The sound absorption coefficient of a surface is a property of the material comprising the surface. Ideally, it is defined as the fraction of incident sound power that is absorbed by the surface, but in this test method, it is defined operationally. The relationship between the theoretically defined and operationally measured coefficients is constantly being studied.
Diffraction effects often cause the apparent area of a sample to be greater than its geometric area, thus increasing the coefficients measured by this test method.
Coefficients measured with this test method should be used with caution because not only are the fields encountered in practical use often larger than the test sample, but the sound field is also rarely diffuse. Laboratory measurements should be made under reproducible conditions, but the conditions that determine effective absorption in practical use are often unpredictable. Regardless of the variations and the need for judgment, coefficients measured with this test method have been successfully used by architects and consultants in the acoustic design of architectural spaces.
When sound absorption measurements are made in a building where the size and shape of the room are not under the operator's control, it is unlikely that a diffuse sound field will be even remotely close. This should be taken into account when assessing the accuracy of field measurements.
For the purposes of this standard, in addition to the terms and definitions given in ASTM C634, the following definitions apply:
When measuring the sound absorption of a room, a random noise band is used as the test signal and is turned on long enough for the sound pressure level to reach steady state (the time required for a 20 dB decrease in the test band with the smallest rate of decrease). When the signal is turned off, the sound pressure level decreases, and the rate of decrease in each frequency band can be determined by measuring the slope of a straight line fitting the average decay curve to the sound pressure level. The absorption of the room and its occupants is calculated using the Sabine formula, assuming that the incident sound field is dispersive before and during decay and that no additional energy enters the room during decay. These conditions must be met for the measurement to be meaningful. In general, sound absorption is a function of frequency, and measurements are made across a range of frequency bands.
When measuring the sound absorption coefficient, the absorption of a reverberation chamber is measured before and after a sample of the material to be tested is placed in the chamber. The increase in absorption divided by the area of the test sample is the dimensionless sound absorption coefficient.
When measuring the sound absorption of an object such as an office screen, theater seating, or space absorber, the absorption of a reverberation room is measured before and after one or more identical objects are placed in the room. Dividing the increase in absorption by the number of objects gives the absorption per object in square meters or the absorption per object in Sabins.
In short, this standard provides reliable acoustic data. It provides consistent, repeatable results on how well materials absorb sound across a range of frequencies (typically 100 Hz to 5000 Hz). These results are important when comparing different materials. Architects and designers also use this data to select materials that control reverberation and echo in spaces such as classrooms, offices, concert halls, and theaters. Manufacturers of acoustic panels, ceiling tiles, carpets, and other sound-absorbing materials are required to substantiate their performance claims.
Our organization, which has been supporting businesses across all sectors for years through a wide range of testing, measurement, analysis, and evaluation activities, boasts a strong team of employees who closely follow global developments in science and technology and are constantly improving themselves. In this context, we also provide testing services in accordance with the ASTM C423 Standard Test Method for Sound Absorption and Sound Absorption Coefficients Using the Echo Chamber Method.
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