Echo and the speed of sound

In short: Sound travels in air at about 343 metres per second (at 20 °C). An echo arises when sound is reflected by a wall and arrives at the ear with a delay.

In more detail: In water sound is about four times, in steel more than fifteen times as fast as in air. The speed depends on the material and the temperature.

In detail

• Estimating a thunderstorm: Count the seconds between lightning and thunder. Divided by three it gives the distance in kilometres (donner-und-blitz).
• Minimum echo distance: For a separately audible echo the wall needs to be about 17 metres away.
• Reverberation: In rooms with nearby walls the reflections merge into reverb.
• Sonar and ultrasound: Bats, ships and medicine measure distances by the echo’s travel time.
• Supersonic: From about 1,235 km/h in air one speaks of Mach 1.