Infographics / Ultrasound Physics

Speed of sound in tissue and its determinants

Horizontal bar comparison of the speed of sound: air about 330 metres per second, fat about 1450, water about 1480, soft tissue average about 1540 (the value ultrasound scanners assume), muscle about 1580, and cortical bone about 3000 to 4000. Speed follows c equals the square root of stiffness (bulk modulus K) divided by density rho: stiffer media are faster, denser media are slower.

Speed of sound in different body media Horizontal bar comparison of the speed of sound: air about 330 metres per second, fat about 1450, water about 1480, soft tissue average about 1540 (the value ultrasound scanners assume), muscle about 1580, and cortical bone about 3000 to 4000. Speed follows c equals the square root of stiffness (bulk modulus K) divided by density rho: stiffer media are faster, denser media are slower. Speed of sound in different body media c = √(K ∕ ρ) — speed rises with stiffness (bulk modulus K), falls with density (ρ) medium m/s scanner assumes 1540 m/s Air ~330 highly compressible → slow Fat ~1450 Water ~1480 Soft tissue (avg) ~1540 Muscle ~1580 Cortical bone ~3000–4000 soft-tissue speeds vary only modestly (~1450–1580) 0 1000 2000 3000 4000 m/s Scanner assumes 1540 m/s for depth = c × t ∕ 2; fat and water are slower (lower stiffness). Bone is far stiffer than soft tissue, so it is much faster despite its higher density.
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