Wave features and echoes

Waves can be described with a few simple measurements. Echoes let ships, bats and doctors 'see' with sound.

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Describing a wave

Amplitude: the height of a wave from the middle (rest position) to the top of a peak. Bigger amplitude, louder sound.
Wavelength: the distance from one peak to the next peak.
Frequency: how many waves pass each second, in hertz (Hz). Higher frequency, higher pitch.
On an oscilloscope screen, a louder sound shows taller waves, and a higher sound shows more waves squeezed in.
middle line wavelength amplitude trough crest
Wavelength is peak to peak; amplitude is from the middle to the top of a peak.

Echoes

An echo is a reflected sound. The sound travels to the surface and back again.
distance = speed × time gives the total distance there and back, so halve it to find how far away the surface is.
Speed of sound: about 340 m/s in air and about 1500 m/s in water.

Hearing and ultrasound

Humans hear about 20 Hz to 20 000 Hz. The upper limit falls as people get older.
Some animals hear sounds we cannot: dogs and bats hear higher frequencies, and elephants can detect very low sounds.
Bats and dolphins use echolocation: they send out high-pitched sounds and listen for echoes to find food. Ships use sonar to find the depth of the sea in the same way.
Worked example

A bat makes a squeak and hears the echo from a moth 0.02 s later. Sound travels at 340 m/s. How far away is the moth?

  1. Total distance = 340 × 0.02 = 6.8 m (there and back)
  2. Halve it: 6.8 ÷ 2

Answer: 3.4 m

Key idea

Amplitude: loudness. Frequency (Hz): pitch. Wavelength: peak to peak. For an echo, distance to the surface = speed × time ÷ 2, because the sound goes there and back.

Key words
Amplitude
The size of a vibration or the height of a wave. A bigger amplitude means a louder sound.
Frequency
The number of vibrations or waves each second, measured in hertz (Hz). Higher frequency means higher pitch.
Speed
How far something travels each second: speed = distance ÷ time, often in m/s.
Water
Pure water melts at 0 °C and boils at 100 °C. Its density is 1 g/cm³. Ice is less dense than water, so it floats.

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  1. A guitar string vibrates 1000 times in 4 seconds. What is its frequency?

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    Answer: 250 Hz

    frequency = vibrations ÷ time = 1000 ÷ 4 = 250 Hz

  2. A ship's sonar pulse returns from the sea bed after 3 s. Sound travels at 1500 m/s in water. How deep is the sea?

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    Answer: 2250 m

    1500 × 3 = 4500 m there and back. Halve it: 2250 m.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.