Sound absorption

Sound absorption is the conversion of acoustic energy into heat or other forms of energy when sound interacts with a material. It reduces reflected sound and depends on frequency, material properties, and installation.

Viscous damping: Energy loss caused by internal friction as fluid layers or material components move relative to one another. Air moving through porous materials experiences friction that turns acoustic motion into heat.
Sound wave: A traveling pressure disturbance that carries mechanical energy through a material medium. Absorption acts on the energy carried by these waves.
Reverberation room: A highly reflective test room used to measure acoustic properties, including sound absorption. Standard reverberation-room methods estimate absorption from changes in decay time.
Sound insulation: The reduction of sound transmitted from one space or side of a partition to another. Insulation limits transmission; absorption dissipates energy where sound meets a material.
Sabine equation: An equation relating a room's reverberation time to its volume and total equivalent sound absorption area. It predicts how added absorption changes reverberation, assuming a diffuse sound field.
Thermal damping: Acoustic energy loss caused by heat exchange between compressed and expanded regions of a sound wave. Temperature fluctuations in pore air dissipate energy as sound penetrates absorbers.
Frequency: The number of cycles of a repeating signal per unit time, measured in hertz. Absorption often changes sharply with frequency, so a single value cannot describe every sound.
Impedance tube: A laboratory tube that measures sound absorption and acoustic impedance on small material samples. It measures absorption under controlled, typically normal-incidence conditions.
Sound diffusion: The scattering of reflected sound energy across directions rather than its concentration in a specular reflection. Diffusion redistributes reflected energy instead of necessarily converting it into heat.
Norris–Eyring equation: A room-acoustics formula that relates reverberation time to room volume and average boundary absorption. It adjusts reverberation estimates when average absorption is too high for Sabine's assumptions.

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