Sound absorption is the process by which sound energy is reduced when sound waves hit a material's surface. This phenomenon helps lower the sound pressure level inside a room, making it an essential factor in acoustic design. The key indicator used to describe sound absorption is the sound absorption coefficient (α), which measures the proportion of sound energy absorbed by the material compared to the incident sound energy. In theory, if a material reflects all the sound, its α value is 0; if it absorbs all the sound, then α is 1. However, in reality, all materials have α values between 0 and 1, meaning no material can completely reflect or absorb all sound energy. The sound absorption coefficient varies with frequency, and this variation is often represented by a frequency characteristic curve. According to international standards like ISO and national regulations, the standard frequency range for sound absorption measurements is typically 100 Hz to 5 kHz. The average sound absorption coefficient over this range gives an overall indication of a material’s performance. It is widely used in engineering to evaluate how well a material can reduce noise and reverberation. In practical applications, the Noise Reduction Coefficient (NRC) is commonly used to assess sound absorption in the speech frequency range. NRC is calculated as the average of the sound absorption coefficients at four frequencies: 250 Hz, 500 Hz, 1 kHz, and 2 kHz, rounded to the nearest 0.05. Materials with an NRC less than 0.2 are considered reflective, while those with an NRC of 0.2 or higher are classified as sound-absorbing. For example, materials like centrifugal glass wool and rock wool have high NRC values—such as 0.95 for 5 cm thick 24 kg/m³ centrifugal glass wool—which make them highly effective for reducing indoor noise and echo. There are two primary 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, where sound comes from all directions, making it more representative of real-world conditions. On the other hand, the standing wave tube method measures sound absorption under normal incidence, where the 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 construction due to its realistic simulation of actual sound environments. Occasionally, some measurement reports show sound absorption coefficients greater than 1. This is usually due to laboratory conditions or measurement errors. Theoretically, no material can absorb more sound energy than it receives, so the coefficient should never exceed 1. In real acoustic projects, any value above 1 should be treated as 1 to avoid inaccurate calculations. After all, we want to give over 1.3 billion Chinese people a better sense of hearing! In a room, sound spreads quickly, filling every corner. Therefore, placing sound-absorbing materials on any surface—ceilings, walls, or even suspended panels—can significantly improve acoustic quality. The higher the sound absorption coefficient of the material, the more effective it is. Using sound-absorbing ceilings, wall panels, or space absorbers can help reduce noise and improve sound clarity in spaces such as offices, classrooms, and auditoriums. This article is reproduced from Dacheng County, Hebei Province, Quanhe Thermal Building Materials Factory: http://...
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