Stokes's law of sound attenuation

Stokes's law gives the classical attenuation of sound in a viscous fluid, predicting that absorption per unit distance grows with the square of frequency.

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Acoustic attenuation: The reduction of a sound wave's amplitude or intensity as it propagates through a medium. Stokes's law is a specific prediction for attenuation caused by fluid viscosity.
Frequency: The number of cycles of a periodic phenomenon occurring per unit time. The predicted attenuation per distance scales with frequency squared.
Kirchhoff's theory of sound attenuation: A theory of sound absorption in gases that includes both viscosity and thermal conduction. It extends the classical viscous account by adding heat flow across the sound wave.
Acoustics: The science of sound, including its production, propagation, and effects. Stokes's law is a foundational propagation-loss result within acoustics.
Viscosity: A fluid's resistance to deformation and flow under shear stress. Shear viscosity dissipates the oscillatory motion of fluid particles in a sound wave.
Wavelength: The distance between corresponding points on successive cycles of a wave. It links frequency to propagation distance and helps express losses per wavelength.
Molecular relaxation: Delayed molecular energy exchange that absorbs sound at characteristic frequencies. Relaxation can produce frequency-dependent absorption not captured by Stokes's simple law.
Ultrasound: Sound waves with frequencies above the upper limit of human hearing. Its high frequencies make absorption and the law's frequency scaling practically relevant.
Bulk viscosity: A measure of a fluid's resistance to uniform compression or expansion. Compression and expansion in sound can dissipate energy through this distinct viscosity.
Shear modulus: A measure of a solid's resistance to shear deformation. Unlike a solid's shear modulus, fluid viscosity describes dissipative resistance rather than elastic storage.