Sound in air travels at roughly — about a million times slower than light, which is why you see lightning before you hear thunder. That speed is not a constant, though. It depends on the temperature of the air, and your formula sheet gives the relationship.
Warmer air means faster-moving molecules, which pass the compression on more quickly. Every degree Celsius adds about . At the speed is ; at a comfortable it is .
| Air temperature | Speed of sound | Where you would meet it |
|---|---|---|
| An Ontario winter | ||
| Freezing point | ||
| Room temperature | ||
| A hot summer day |
Echoes and echolocation
An echo is sound reflected from a surface and returning to the source. The trick with every echo problem is that the sound makes a round trip — out to the surface and back again — so the distance it travels is twice the distance to the object.
- Sonar — ships time a pulse to the seabed and back. In water at , a round trip means a depth of about .
- Ultrasound imaging — the same principle at megahertz frequencies, using tissue boundaries as the reflecting surfaces.
- Bats and dolphins — echolocation, with the delay giving distance and the change in the returning pitch giving relative motion.
- Thunder — roughly three seconds of delay per kilometre, which is the familiar rule of thumb turned into physics.
Mach number and supersonic flight
| Mach number | Name | Meaning |
|---|---|---|
| Less than 1 | Subsonic | Slower than sound — all ordinary airliners |
| About 1 | Transonic | Near the speed of sound; wavefronts bunch at the nose |
| Greater than 1 | Supersonic | Faster than sound — Concorde cruised near Mach 2 |
| Greater than 5 | Hypersonic | Re-entry vehicles and experimental craft |
Because Mach number is a ratio against the local speed of sound, a given Mach number is a different ground speed at different altitudes. High in the atmosphere the air is much colder, so sound travels slower, so Mach 1 is a lower speed there than at sea level.
The Doppler effect, qualitatively
When a sound source moves toward you, each successive wavefront is emitted from a little closer, so the wavefronts bunch up: the wavelength you receive is shorter and the pitch sounds higher. As the source passes and recedes, the wavefronts spread out and the pitch drops. That sudden fall as an ambulance goes by is the Doppler effect.