The sound absorption coefficient of a material is a quantity that describes how much of the intensity of the incident sound is not reflected by the material. The sound absorption coefficient is used to evaluate the sound absorption efficiency of materials. This value is the ratio of the absorbed energy to the incident energy and is denoted by “α”. If the acoustic energy can be completely absorbed, the “α” value is 1.
The sound absorption coefficient of materials is related to frequency and varies at different frequencies. The sound absorption coefficient frequency characteristic curves can be used to fully demonstrate the sound absorption characteristics of different frequencies. Since it is inconvenient to compare and specify, the average sound absorption coefficient, which is the average of the absorption coefficients of an acoustic material at a certain set of frequencies, is used for simplification.
The sound absorption capacity of a particular building element is defined by the sound absorption coefficient, which is equal to the product of the sound absorbing surface area.
The parameter that best describes the sound absorption of materials is the absorption coefficient “α”. It is defined as the ratio of the sound energy absorbed by a surface to the sound energy incident on that surface and takes values between 0 and 1. The sound absorption coefficient depends on the incident angle. However, when there is a diffuse sound field spreading over a surface, it is assumed that sound waves have an equal probability of hitting the surface from all directions. This parameter also depends on the frequency of the sound wave and therefore values are presented in the literature for standard frequency bands. Porous and fibrous absorbers are most efficient at higher frequencies, but also perform well at medium and low frequencies if they are thick enough or supported by an air gap.
The sound absorption of fibrous materials can be measured in two ways: in a reverberation chamber or in an impedance tube. The reverberation time method closely simulates the conditions encountered in buildings. The standard “ISO 354 Acoustics - Measurement of sound absorption in a reverberation chamber” developed by the International Organization for Standardization (ISO) describes a test method for measuring the sound absorption coefficient of acoustic materials used as wall or ceiling applications in a reverberation chamber or the equivalent sound absorption area of objects such as furniture, persons or space absorbers. This method is not intended to be used to measure the absorption characteristics of poorly damped resonators. The results obtained may be used for comparison purposes and for design calculations in terms of room acoustics and noise control.
According to this standard, the test shall be carried out in a reverberation chamber with a volume of 200 m3 to create a sufficiently diffuse distortion sound field. The area of the test specimen to be placed in the chamber shall be between 10 m2 and 12 m2. The mounting of the test specimen depends on the type of material or its application and a number of mountings are standardized in this standard. Since the reverberation time is inversely proportional to the absorption, the sound absorption coefficient is calculated from the reverberation time measurements. The reverberation time is measured with and without the test specimen to enable the absorption provided by the specimen to be calculated. The absorption is divided by the area of the specimen to obtain the absorption coefficient “α” per unit area.
For the purposes of this standard, the following terms and definitions apply:
When testing, the average reverberation time in the reverberation chamber is measured with and without the test sample installed. From these reverberation times, the equivalent sound absorption area of the test sample is calculated using the Sabine equation.
In the case of a test sample uniformly covering a surface (a flat absorber or a specific array of test objects), the sound absorption coefficient is obtained by dividing the sound absorption area by the treated surface area.
If the test sample consists of several identical objects, the equivalent sound absorption area of a single object is found by dividing the sound absorption area by the number of objects.
Measurements are made in one-third octave bands with center frequencies in hertz specified in the ISO 266 standard. Additional measurements can be made in one-third octave bands with center frequencies outside this range specified by the ISO 266 standard. Especially at low frequencies (below 100 Hz), it can be very difficult to obtain accurate measurement results due to the low modal density of the reverberation chamber.
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