Sound Wave Phenomena

Sound waves are disturbances that travel through an elastic medium, whether gaseous, liquid, or solid. A person receives sound vibrations through the eardrum within the frequency range of human hearing. These vibrations arise from variations in air pressure produced by a sound source and propagate into the surrounding medium, a region known as the acoustic field. When sound encounters a boundary of the medium through which it is traveling, the energy carried by the sound waves may be transmitted, absorbed, or reflected by that boundary. All three phenomena generally occur to varying degrees, depending on the nature of the boundary. The behavior of sound waves in terms of absorption, reflection, and transmission is illustrated in the figure.

Sound Absorption

Sound absorption refers to the conversion of sound energy into heat energy. A portion of the acoustic energy that enters a material is absorbed by its mass, which then transforms the acoustic energy into heat—this process is known as sound absorption. The ability of a material to absorb sound is typically measured by its sound absorption coefficient.

Several factors influence sound absorption, including material density, modulus of elasticity, moisture content, temperature, sound intensity and frequency, as well as surface conditions of the material. Materials with low density and low modulus of elasticity, combined with high moisture content and high temperature, tend to absorb more sound. The quality of a sound-damping material is indicated by its α value (sound absorption coefficient); the larger the α, the better the material performs as a sound absorber. The α value ranges from 0 to 1. If α = 0, no sound is absorbed. If α = 1, 100% of the incident sound is absorbed.

The amount of sound energy that is reflected, absorbed, or transmitted depends on the type and nature of the material. In general, porous materials absorb more sound energy than other material types. The presence of pores allows sound waves to penetrate into the material, where the absorbed sound energy is converted into other forms of energy—most commonly heat.

Absorber materials, reflective materials, and diffuser materials are the three main classes of acoustic materials. Common questions include: How do these materials respond when subjected to sound waves? How can a soundproof room be constructed? How can speech in a classroom be heard clearly?

To answer these questions, one must understand the behavior of a material when exposed to sound waves. For further details, refer to the following figure.

 

As illustrated in the figure, when a sound wave strikes a material, part of the wave is reflected, part is absorbed by the material, and part is transmitted through it. Acoustic materials are classified based on their dominant ability: reflecting sound waves, absorbing sound waves, or transmitting sound waves. A sound-absorbing material is one whose primary capability is to absorb sound waves. A soundproof material (or sound barrier) is one whose main function is to block the transmission of sound waves. Meanwhile, a sound diffuser material serves to spread the incoming sound waves in various directions; this typically requires a specific arrangement or modification to achieve the desired diffusion.

The ability of a sound-absorbing material to absorb sound waves is expressed by the sound absorption coefficient, which ranges from 0 to 1. A value of 0 means the material does not absorb any incoming sound energy, while a value of 1 means all incoming sound energy is absorbed. However, these extreme values are never found in actual acoustic absorbent materials.

The ability of a material to block sound energy from entering or leaving a room is expressed by the transmission loss (TL). This ability can also be represented by the Sound Transmission Class (STC). Selection of soundproofing materials is generally based on the STC value: the higher the STC value, the better the material is at blocking sound energy from entering or exiting a room.

A material's ability to absorb or block sound waves depends on the frequency of the incoming waves. Both absorbent materials and soundproofing materials perform well only at certain frequencies.

Thank you, and may this information be useful.