Sound Travels Faster in Steel Because of Stiffness, Not Density

Temperature matters, pressure does not

Air at 0 °C carries sound at 331 m/s and at 35 °C at 352 — a six per cent spread across the range a day can produce. Pressure, in contrast, does essentially nothing: squeezing air raises its stiffness and its density in the same proportion, and the two cancel inside the square root. That is why the speed of sound at altitude depends on how cold it is up there rather than on how thin the air is, and why an aircraft's Mach number changes with outside temperature at constant airspeed. For thunder the practical version is simpler: sound covers a kilometre in 2.9 seconds and a mile in 4.7, so counting three seconds per kilometre or five per mile is close enough for a storm.

What breathing helium actually changes

Not the pitch, which is the common misunderstanding. Vocal folds vibrate at the same rate in helium as in air, so the fundamental note of a voice is unchanged; what shifts is the set of resonances of the throat and mouth above it, and those depend directly on the speed of sound in whatever fills the cavity. At three times the speed the resonances land at roughly three times the frequency, which recolours the voice into something thin and duck-like without moving the note at all. The same principle runs in reverse with a dense gas, and it is why a singer and a saxophone are affected differently: one has a fixed source with a variable resonator, the other has both in the same tube.

Frequently asked questions

Does the loudness or the pitch change the speed?

No, and it is fortunate. If high notes travelled faster than low ones, an orchestra heard from the back of a hall would arrive scrambled, with the piccolos ahead of the double basses by a growing margin. Air is very nearly non-dispersive over the audible range, so everything arrives together and music survives distance. Loudness is a different matter only at extremes: a shock wave from an explosion does travel faster than ordinary sound, which is why the sharp crack of a nearby blast is not simply a louder version of a distant one.

Why does sound carry so far on a cold clear night?

Not because cold air carries it better — it carries it slightly slower. What matters is that on a still clear night the ground cools faster than the air above it, so for once the air gets warmer with height instead of colder. Sound travelling upward into warmer, faster air is bent back towards the ground rather than escaping, and a listener a long way off receives what would normally have gone over their head. The same bending explains why sound is harder to hear across a sunlit field in the afternoon, when the usual arrangement refracts it upward and away.