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Sound is a longitudinal pressure wave: a disturbance of matter transmitted outward from its source, made of compressions and rarefactions. Like all waves it obeys v = f·λ, where v is the wave speed, f the frequency and λ the wavelength. The human audible range is roughly 20 Hz to 20,000 Hz.

In air the speed of sound depends on temperature: v = 331 m/s × √(T / 273 K), with T in kelvin. At 20 °C (293 K) this gives about 343 m/s. In fresh water sound travels at about 1480 m/s and in steel at about 5960 m/s.

Sound needs a medium. Most of space is essentially a vacuum, so sound waves cannot travel there, although the hot gas inside galaxy clusters is dense enough to carry very low-frequency pressure waves.

Experiment: Measure the speed of sound with an echo

1. Stand at least 50 m from the wall. Measure the distance d to the wall.
2. Clap the blocks once and listen for the echo.
3. Now clap steadily so that each clap is made exactly when you hear the echo of the previous one. Practise the rhythm.
4. A partner times 20 claps (start counting at zero). Repeat three times.
5. Between two claps the sound travels to the wall and back, a distance of 2d. Time per clap t = (total time) ÷ 20.
6. Calculate v = 2d / t. Compare with 331 × √(T/273) for today's air temperature.

Run	Distance d (m)	Time for 20 claps (s)	t per clap (s)	v = 2d/t (m/s)
1
2
3

Questions

1. Write the wave equation and calculate the wavelength of a 20 Hz and a 20,000 Hz sound in air at 20 °C.

2. Calculate the speed of sound in air at 35 °C.

3. A noise rises from 60 dB to 80 dB. By what factor did the intensity increase?

4. Your echo result: v = ____ m/s. Name two sources of error and how you reduced them.

Sources for every fact on this sheet: echoesof.science/school/sound/sources/

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