The propagation speed of sound is lowest in which biological tissue?

• A. Fat
• B. Blood
• C. Muscle
• D. Bone

Answer: (A)

Sound travels through tissue most slowly when the tissue is very soft and easily compressible, because that lowers the tissue’s stiffness relative to its density. The speed of a compressional wave depends on the ratio of stiffness to density, so softer tissues with low elastic moduli transmit the wave more slowly. Among common tissues, fat is the softest and least stiff, giving the smallest stiffness-to-density ratio and the lowest propagation speed (around 1450 m/s). Blood behaves as a fluid and has a higher bulk modulus than fat, so the wave moves faster. Muscle is stiffer than fat, increasing the speed, and bone is very stiff and dense, making it transmit sound the fastest. So the slowest propagation occurs in fat.