The process by which the intensity of sound is diminished by the conversion of the energy of the sound wave into heat. The absorption of sound is an important case of sound attenuation. Regardless of the material through which sound passes, its intensity, measured by the average flow of energy in the wave per unit time per unit area perpendicular to the direction of propagation, decreases with distance from the source. This decrease is called attenuation. In the simple case of a point source of sound radiating into an ideal medium (having no boundaries, turbulent fluctuations, and the like), the intensity decreases inversely as the square of the distance from the source. This relationship exists because the spherical area through which the energy propagates per unit time increases as the square of the propagation distance. This attenuation or loss may be called geometrical attenuation.
In addition to this attenuation due to spreading, there is effective attenuation caused by scattering within the medium. Sound can be reflected and refracted when incident on media of different physical properties, and can be diffracted and scattered as it bends around obstacles. These processes lead to effective attenuation, for example, in a turbulent atmosphere; this is easily observed in practice and can be measured, but is difficult to calculate theoretically with precision. See Diffraction, Reflection of sound, Refraction of waves
In actual material media, geometrical attenuation and effective attenuation are supplemented by absorption due to the interaction between the sound wave and the physical properties of the propagation medium itself. This interaction dissipates the sound energy by transforming it into heat and hence decreases the intensity of the wave. In all practical cases, the attenuation due to such absorption is exponential in character. See Sound intensity
The four classical mechanisms of sound absorption in material media are shear viscosity, heat conduction, heat radiation, and diffusion. These attenuation mechanisms are generally grouped together and referred to as classical attenuation or thermoviscous attenuation. See Conduction (heat), Diffusion, Heat radiation, Viscosity
Sound absorption in fluids can be measured in a variety of ways, referred to as mechanical, optical, electrical, and thermal methods. All these methods reduce essentially to a measurement of sound intensity as a function of distance from the source.
The amount of sound that air absorbs increases with audio frequency and decreases with air density, but also depends on temperature and humidity. Sound absorption in air depends heavily on relative humidity. The reason for the strong dependence on relative humidity is molecular relaxation. One can note the presence of two transition regimes in most of the actual absorption curves, representing the relaxation effects of nitrogen and oxygen, the dominant constituents of the atmosphere. See Atmospheric acoustics
Sound absorption in water is generally much less than in air. It also rises with frequency, and it strongly depends on the amount of dissolved materials (in particular, salts in seawater), due to chemical relaxation. See Underwater sound
The theory of sound attenuation in solids is complicated because of the presence of many mechanisms responsible for it. These include heat conductivity, scattering due to anisotropic material properties, scattering due to grain boundaries, magnetic domain losses in ferromagnetic materials, interstitial diffusion of atoms, and dislocation relaxation processes in metals. In addition, in metals at very low temperature the interaction between the lattice vibrations (phonons) due to sound propagation and the valence of electrons plays an important role, particularly in the superconducting domain. See Crystal defects, Diffusion, Ferromagnetism, Superconductivity