Sound Absorbing Properties of Selected Green Material—A Review

Noise pollution is often overlooked and invisible, but it significantly impacts the quality of human life. One of the most straightforward solutions to mitigate noise pollution is by using sound-absorbing materials. Recently, research trends to develop sound absorbing green materials, typically derived from agricultural by-products, have witnessed an uptick. This paper summarizes the sound-absorbing properties of various green materials found in the literature, including coconut fiber, kenaf fiber, rice bran, rice husk, rice straw, Hanji (a traditional Korean paper), tea-leaf fiber, mandarin peel, pineapple-leaf fiber, corn husk, peanut shell, sugar palm trunk, yucca gloriosa fiber, fruit stones, wood barks, flax fiber, and nettle fiber. Natural fibers can be made by compressing the raw material or manufacturing them into fibrous materials or composites. The key variables that determine sound absorption performance are the thickness and density of the green material, as well as the presence of an air back cavity. Generally, thicker materials exhibit better sound absorption performance in the low- and mid-frequency range. Moreover, higher density is associated with better sound absorption performance at the same thickness. Additionally, increasing the distance between the sound-absorbing material and the air back cavity enhances sound absorption performance at low frequencies. Thus, these physical variables, rather than the specific materials used, primarily influence sound absorption capabilities. Therefore, various green materials, such as fibers, granules, and porous materials, can be effective sound absorbers if their thickness, density, and air back cavity are properly controlled.