Speed of Sound in Steel

There is no single speed of sound in a solid, and this is the constant people most often get wrong. A long thin bar carries extensional waves at √(E/ρ) = √(200e9/7850) = 5050 m/s. A bulk medium, where lateral contraction is prevented, is stiffer and carries longitudinal waves at about 5860 m/s. Shear (transverse) waves, which have no analogue in fluids, travel at √(G/ρ) ≈ 3140 m/s. Ultrasonic thickness gauges work on the bulk longitudinal value, and using the bar value instead would read about 14 % thin.

The practical uses follow from the mode. Bulk longitudinal speed does UT thickness and flaw detection; shear speed does angle-beam weld inspection; bar speed governs impact and vibration in slender members. Steel's high wave speed is also why structure-borne noise travels so far in a building: a pump vibration reaches a distant room through the piping and framing long before it arrives through the air, and why isolators and flexible connectors, not more air-side attenuation, are the fix.