Sound crawls through air but races through steel at almost 6 km a second
A sound wave is just a travelling squeeze in some material, and the material decides its pace. In warm air it covers about 343 metres a second; in steel, nearly 6,000; in solid atomic hydrogen, around 36,000. In a vacuum it cannot move at all, because there is nothing to squeeze.
Physicists treat sound as a mechanical wave of pressure moving through an elastic medium, whether solid, liquid, gas or plasma. A loudspeaker cone pushes on the air beside it, and the disturbance spreads outward. The air molecules themselves do not ride along with the wave; they jostle back and forth while the energy moves on. In fluids the wave is purely longitudinal, a train of compressions and rarefactions, but solids can also carry transverse waves of shear, which unlike the longitudinal kind can be polarised.
Getting the speed right took two great minds. Isaac Newton made the first serious attempt, proposing that speed equals the square root of pressure divided by density. Laplace showed that was wrong: the compressions happen too fast for heat to escape, so the process is adiabatic, not isothermal, and he added a correction factor. The result, the Newton-Laplace equation, ties speed to the medium's stiffness over its density. Temperature matters too, which is why the 343 m/s figure applies to air at 20 degrees Celsius. Wind adds to or subtracts from the speed depending on direction, while fresh water carries sound at roughly 1,482 m/s.
Human hearing spans roughly 20 Hz to 20 kHz and an enormous range of loudness, so levels are given on a logarithmic decibel scale measured against a reference, normally 20 micropascals in air. Even a fairly loud 94 dB sound only makes air pressure wobble between about 101323.6 and 101326.4 pascals. A-weighting adjusts readings to match how the ear responds to noise.
The word has two meanings, and a US acoustics standard defines both: a physical oscillation and the sensation it causes. That split is why the old puzzle about a tree falling unheard in a forest gets different answers. Theory even suggests sound waves carry a minuscule effective gravitational mass, which in ordinary materials comes out negative.
Source: Sound
