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Theoretically, if a material completely reflects sound, then its a = 0; if a material absorbs all incident sound energy, then its a = 1. In fact, a for all materials is between 0 and 1, which means that it is not possible to reflect all, and it is not possible to absorb all of it. There will be different sound absorption coefficients at different frequencies. 
People use the sound absorption coefficient frequency characteristic curve to describe the sound absorption performance of the material at different frequencies. According to the ISO standards and national standards, the frequency range of the sound absorption coefficient in the sound absorption test report is 100-5 KHz. The value obtained by averaging the sound absorption coefficient of 100-5 KHz is the average sound absorption coefficient, and the average sound absorption coefficient reflects the overall sound absorption performance of the material.
The noise reduction coefficient NRC is often used in the project to roughly evaluate the sound absorption performance in the language frequency range. This value is the arithmetic mean value of the sound absorption coefficient of the material at 250, 500, 1K, and 2K, rounded to the nearest 0.05. It is generally considered that materials with an NRC less than 0.2 are reflective materials, and materials with NRC greater than 0.4 are considered as sound-absorbing materials. When it is necessary to absorb a large amount of sound energy to reduce indoor reverberation and noise, it is often recommended to use materials with a high sound absorption coefficient. Centrifugal glass wool is a high NRC sound absorbing material, and the NRC of 5cm thick 24kg/m3 centrifugal glass wool can reach 0.90.
Porous sound-absorbing materials, such as centrifugal glass wool, rock wool, mineral wool, plant fiber spraying, etc., the sound absorption mechanism is that there are a lot of tiny pores inside the material. Acoustic waves can penetrate deep into the material along these pores, and the materials will rub against the sound. Can be converted to heat energy. The sound-absorbing properties of porous sound-absorbing materials are as the frequency increases and the sound-absorbing coefficient gradually increases, which means that low-frequency absorption is not as good as high-frequency absorption.
There is a perforated plate with air layer on the wall or ceiling, even if the material itself has poor sound absorption performance, this structure also has sound-absorbing properties, such as perforated gypsum board, wood board, metal plate, even slit brick, etc. The sound absorption mechanism is Helmholtz resonance. Similar to a thermos bottle, the external space and the internal space are connected by a narrow bottleneck. When the sound wave is incident, resonance occurs between the resonance frequency and the neck air and internal space. Sound energy. Helmholtz resonance absorption is characterized by a large sound absorption coefficient only at certain frequencies.
Films or thin plates can also absorb sound when forming cavities with other structures, such as wood boards, metal plates, etc. The mechanism of sound absorption of such structures is the resonance of the thin plate. At the resonance frequency, sound energy is largely absorbed due to the violent vibration of the thin plate. Thin plate resonance absorption mostly has good sound absorption performance at low frequencies.
Sound Absorption Board Sound Absorption Principle and Application
Sound absorption is the phenomenon of energy loss after a sound wave impinges on the surface of a material. Sound absorption can reduce the sound pressure level in a room. The index describing the sound absorption is the sound absorption coefficient a, which represents the ratio of the sound energy absorbed to the incident sound energy.