Sound Absorption Coefficient and Noise Reduction Coefficient

- Oct 09, 2018-

Sound absorption is the phenomenon of energy loss after sound waves hit the surface of the material. The indicator describing sound absorption is the sound absorption coefficient a, which represents the ratio of the acoustic energy absorbed by the material to the incident sound energy. In theory, if a material completely reflects the sound, then a = 0; if a material absorbs all of the incident sound energy, then a = 1. In fact, a of all materials is between 0 and 1, that is, it is impossible to totally reflect, and it is impossible to absorb them all.

There will be different sound absorption coefficients at different frequencies. The sound absorption coefficient frequency characteristic curve is used to describe the sound absorption performance of the material at different frequencies. According to the ISO standard and the national standard, the frequency range of the sound absorption coefficient in the sound absorption test report is 100-5KHz. The average value of the sound absorption coefficient of 100-5KHz is the average sound absorption coefficient, and the average sound absorption coefficient reflects the sound absorption performance of the material as a whole. In the engineering, the noise reduction coefficient NRC is often used to roughly evaluate the sound absorption performance in the language frequency range. This value is the arithmetic mean of the sound absorption coefficients of the materials at the four frequencies of 250, 500, 1K, 2K, rounded to the nearest 0.05. It is generally considered that a material having an NRC of less than 0.2 is a reflective material, and a material having an NRC greater than or equal to 0.2 is considered to be a sound absorbing material. When it is necessary to absorb a large amount of sound energy to reduce indoor reverberation and noise, it is often necessary to use a material with a high sound absorption coefficient. For example, centrifugal glass wool, rock wool and the like are high NRC sound absorbing materials, and the NRC of 5cm thick 24kg/m3 centrifugal glass wool can reach 0.95.

There are two methods for measuring the sound absorption coefficient of a material, one is the reverberation chamber method, and the other is the standing wave tube method. The reverberation chamber method measures the sound absorption coefficient when the sound is randomly incident, that is, the ratio of the energy loss when the sound is injected into the material from all directions, and the standing wave tube method measures the sound absorption coefficient when the sound is normally incident, and the sound incident angle is only 90 degrees. . The sound absorption coefficients measured by the two methods are different. The most commonly used in engineering is the sound absorption coefficient measured by the reverberation chamber method, because the sound incidence is random in the actual application of the building. In some measurement reports, the sound absorption coefficient is greater than 1, which is caused by the laboratory conditions of the measurement. In theory, the sound energy absorbed by any material cannot be greater than the incident sound energy, and the sound absorption coefficient is always less than 1. Any measured sound absorption coefficient value greater than 1 cannot be used greater than 1 in actual acoustic engineering calculation, and may be calculated at a maximum of 1.

In the room, the sound fills the corners quickly, so placing the sound absorbing material on any surface of the room has a sound absorbing effect. The louder the sound absorption coefficient of the sound absorbing material is, the more the sound absorption area, the more obvious the sound absorption effect. Acoustic noise reduction can be performed by using sound absorbing ceiling, sound absorbing wall panel, space sound absorbing body, and the like.

 

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