speed of sound
Press your ear to a long iron railing while a friend taps the far end: you hear the knock surprisingly soon, well before its echo reaches you through the air. That brisk arrival is the speed of sound in the metal — how fast a vibration races from atom to atom through the solid.
When one atom is nudged, the springy bonds yank its neighbours along, and the disturbance hands itself down the line as a travelling wave. Two things set its pace. Stiffer bonds snap their neighbours into motion faster, raising the speed; heavier atoms are more sluggish to get moving, lowering it. So the speed of sound goes up with stiffness and down with mass — which is why sound flies through stiff, light steel several times faster than through air, and faster still through diamond.
The speed of sound matters because it sets the slope of the acoustic branch near long wavelengths and feeds straight into how a solid conducts heat. A common surprise: sound is far faster in solids and liquids than in gases, the opposite of what many guess. Tightly bonded atoms pass the message along far more promptly than the loose, sparse molecules of a gas.
In air sound ambles along at about 340 metres per second; in steel it tears along at around 5,000. That is why, in old films, a character presses an ear to the train rail to hear an approaching engine long before the sound arrives through the air.
Sound through steel is roughly fifteen times faster than through air.
A solid actually has more than one speed of sound: a faster one for pushing waves along the direction of travel, and a slower one for sideways shearing waves. The single 'speed of sound' you usually hear quoted is a convenient average.