P3.4P3. Waves

Sound — Combined Science 0653 - Physics Revision Notes & Topical Questions

5 min read 10 parts Lesson 8 of 9 1 quiz

📚 Combined Science 0653 - Physics Hub / Topical Revision

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Lesson summary

P3.4 Sound — COMBINED SCIENCE notes: Describe sound as a longitudinal pressure wave, measure its speed, relate pitch and loudness to frequency and amplitude, and describe echoes and ultrasound.Source — sound is produced by a vibrating source.Longitudinal wave — the particles of t…

Compression
Rarefaction
Medium needed
In different states
Wave equation
What the symbols mean

Part 1 of 8Sound waves

Source

Source sound is produced by a vibrating source.

Longitudinal wave

Longitudinal wave the particles of the medium vibrate back and forth parallel to the direction of travel.

Compression

Compression a region of higher density and pressure where particles are close together.

Rarefaction

Rarefaction a region of lower density and pressure where particles are spread apart.

Pressure wave

Pressure wave the changing pressure makes a solid surface vibrate in step with the sound, so a glass can be made to vibrate and even shatter.

Medium needed

Medium needed sound cannot travel through a vacuum because there are no particles.

Drawing

Drawing a longitudinal wave is drawn as lines close together for compressions and far apart for rarefactions.

Part 2 of 8Speed of sound

In air

In air about 330 to 350 m/s, and the speed is higher when the air is warmer.

In different states

In different states sound is slowest in gases, faster in liquids (about 1500 m/s) and fastest in solids (about 5000 m/s).

Wave equation

Wave equation v = f × λ,
- and v = d ÷ t for the distance travelled in a time.
  What the symbols mean: v = wave speed (m s⁻¹) · f = frequency (Hz) · λ = wavelength (m)

Part 3 of 8Measuring the speed of sound in air

Use a trundle wheel to measure a distance of about 100 m between two people.

The first person bangs two wooden blocks together above their head.

The second person starts a stopwatch when they see the blocks hit and stops it when they hear the sound.

Repeat several times and calculate the average time.

Repeat for other distances such as 120 m, 140 m, 160 m and 180 m.

Calculate the speed as distance ÷ average time, which should be about 340 m/s.

Reducing error
- a long distance makes the human timing error small compared with the time, and repeated readings are averaged.

Part 4 of 8Measuring the speed of sound with an oscilloscope

Connect two microphones to an oscilloscope and place them about 2 m apart.

Trigger the oscilloscope when the first microphone detects sound and set a suitable time base.

Make a loud clap with wooden blocks next to the first microphone.

Read the time difference between the signals reaching the two microphones.

Repeat for several distances and calculate speed = distance ÷ time.

Check
- take the scale of the time base into account, since the times are in milliseconds.

Part 5 of 8Pitch and loudness

Pitch

Pitch it depends on frequency, so a high-pitched sound has a high frequency and a short wavelength.

Loudness

Loudness it depends on amplitude, so a larger amplitude gives a louder sound.

Hearing range

Hearing range humans hear frequencies from about 20 Hz to 20 000 Hz, and the range shortens with age.

Part 6 of 8Echoes

Echo

Echo the reflection of a sound wave from a hard surface.

Distance to the surface

Distance to the surface distance = ½ × speed × time, where the time is the total time for the sound to travel to the surface and back.

Speed from an echo

Speed from an echo speed = 2 × distance to the wall ÷ time.

Part 7 of 8Ultrasound

Ultrasound

Ultrasound sound with a frequency higher than 20 kHz, above the range of human hearing.

Boundaries

Boundaries ultrasound is partly reflected at a boundary between two media and the rest is transmitted.

Depth of a boundary

Depth of a boundary depth = ½ × speed × time between emission and receipt of the pulse.

Material testing

Material testing pulses reflected early from a crack inside a metal object show the flaw on an oscilloscope.

Medical scanning

Medical scanning reflections from tissue boundaries build images of a foetus or organs, and it is non-invasive and believed to be harmless.

Treatment

Treatment ultrasound is also used to break up kidney stones.

Sonar

Sonar pulses from a ship are reflected from the sea bed, and the depth is half the distance travelled.

Part 8 of 8More from the specification

Key terms

Term	Meaning
Sound wave	A longitudinal pressure wave produced by a vibrating source.
Pitch	How high or low a sound is, determined by frequency.
Loudness	How loud a sound is, determined by amplitude.
Echo	The reflection of sound from a hard surface.
Ultrasound	Sound with a frequency above 20 kHz.
Sonar	Using reflected ultrasound pulses to find depth or detect objects.
Oscilloscope	A device that displays electrical signals such as sound waves.

Avoid these mistakes

Saying sound travels fastest in gases — it is fastest in solids.

Saying sound travels in a vacuum — it needs a medium.

Saying a louder sound has a higher frequency — loudness depends on amplitude and pitch on frequency.

Using the total echo time as the time to reach the surface — halve the distance because the sound travels there and back.

Saying ultrasound is audible — its frequency is above the human range.

Last updated: 15 October 2026