Sound absorption is a process where sound energy is lost when sound waves hit the surface of a material. This phenomenon helps reduce the sound pressure level inside a room. The key indicator used to describe sound absorption is the sound absorption coefficient, denoted as α. It represents the proportion of acoustic energy that is absorbed by the material compared to the incident sound energy. In theory, if a material reflects all sound, its α is 0; if it absorbs all the sound, its α is 1. However, in reality, no material has an α exactly equal to 0 or 1—most fall between these values, meaning some sound is always reflected and some is always absorbed. The sound absorption coefficient varies with frequency. To illustrate this, engineers use a frequency characteristic curve that shows how a material performs at different sound frequencies. According to international standards like ISO and national regulations, the typical frequency range for measuring sound absorption is from 100 Hz to 5 kHz. The average value of the sound absorption coefficient within this range gives an overall measure of the material's sound-absorbing capability. In practical engineering, the Noise Reduction Coefficient (NRC) is often used to estimate a material’s performance in the speech frequency range. NRC is calculated as the average of the sound absorption coefficients at 250 Hz, 500 Hz, 1 kHz, and 2 kHz, rounded to the nearest 0.05. Materials with an NRC below 0.2 are considered reflective, while those with an NRC of 0.2 or higher are classified as sound-absorbing. For applications requiring significant noise reduction, such as in rooms with high reverberation, materials with high NRC values are preferred. Examples include fiberglass and rock wool, which can achieve NRC values as high as 0.95 when used in thicknesses like 5 cm and densities around 24 kg/m³. There are two main methods for measuring the sound absorption coefficient: the reverberation chamber method and the standing wave tube method. The reverberation chamber method simulates random sound incidence, meaning sound comes from all directions, making it more representative of real-world conditions. The standing wave tube method, on the other hand, measures sound absorption under normal incidence, where sound hits the material at a 90-degree angle. These two methods yield different results, and the reverberation chamber method is more commonly used in building acoustics because it better reflects actual usage scenarios. Sometimes, measurement reports show sound absorption coefficients greater than 1. This occurs due to specific laboratory conditions, but in reality, no material can absorb more sound energy than is incident upon it. Therefore, any value above 1 should not be used in real-world acoustic calculations and should be capped at 1. After all, it's important to ensure accurate and safe design for the over 1.3 billion people in China who rely on good hearing and sound environments. In a room, sound spreads quickly and fills every corner. Placing sound-absorbing materials on any surface can help reduce echoes and noise. The higher the sound absorption coefficient of the material, the more effective it is in reducing sound. Common solutions include sound-absorbing ceilings, wall panels, and suspended acoustic elements. These materials are essential in creating comfortable and quiet indoor spaces. This article is reproduced from Quanhe Thermal Building Materials Factory in Dacheng County, Hebei Province: http://...
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