Acoustic materials can be categorized into absorbent and insulating materials based on their functional uses. Absorbent materials primarily address the noise caused by sound reflections, as they can attenuate the reflected energy from the incident sound source, thus achieving fidelity to the original sound source. Insulating materials, on the other hand, focus on reducing the transmission of sound, mitigating the sense of noise within a space. They can also attenuate the transmitted energy from the incident sound source, leading to a quieter environment.
    

Environmental sound-absorbing cotton is a porous sound-absorbing material. Its sound-absorbing mechanism lies in the presence of numerous tiny interconnected pores within the material. Sound waves can penetrate deeply into the material along these pores, where they interact with the material, converting sound energy into thermal energy. The sound-absorbing characteristic of porous materials is that the sound-absorption coefficient gradually increases with frequency, indicating that low-frequency absorption is not as effective as high-frequency absorption. The necessary conditions for sound absorption in porous materials are: the material has a large number of voids, the voids are interconnected, and the pores extend deeply into the material.
    

One common misconception is that rough-surfaced materials have sound-absorbing properties, which is not the case. For instance, materials like textured cement and uneven stone surfaces generally do not possess sound-absorbing capabilities. Another misconception is that materials with a lot of internal holes, such as polystyrene, polyethylene, and closed-cell polyurethane, have good sound-absorbing properties. In reality, these materials have very low sound-absorption coefficients because their internal holes are not interconnected, preventing sound waves from penetrating deeply into the material for vibration and friction.