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Topic 3 · 3.4

Sound

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe how sound is produced by vibrating sources and its longitudinal nature; describe compressions and rarefactions EXTENDED.
  • State the human audible range (20 Hz to 20 000 Hz); know that sound needs a medium and travels at about 330–350 m/s in air.
  • Describe a method to measure the speed of sound in air; know that sound is faster in liquids and solids EXTENDED.
  • Relate loudness to amplitude and pitch to frequency; describe echoes and define ultrasound; describe its uses and calculate distances EXTENDED.

📚The physics

What sound is

Sound is produced by vibrating sources (a loudspeaker cone, a guitar string, vocal cords). It travels as a longitudinal wave: the air particles vibrate backwards and forwards along the direction the sound travels. A medium is needed — sound cannot cross a vacuum.

A loudspeaker cone vibrating backwards and forwards sends out a sound wave: bands of air particles crowded together, compressions, alternate with bands where they are spread out, rarefactions. The wavelength is the distance from one compression to the next.
EXTENDED Compressions are regions of higher pressure; rarefactions of lower pressure.
  • Humans hear frequencies from about 20 Hz to 20 000 Hz.
  • The speed of sound in air is about 330–350 m/s. EXTENDED In general, sound travels faster in solids than in liquids, and faster in liquids than in gases.
  • An echo is the reflection of sound waves.
One student claps two blocks together 80 m from a large wall; another times the interval until the echo is heard. The echo returns after 0.48 s; the sound travelled 2 times 80 m, 160 m, so its speed is 160 divided by 0.48, about 330 metres per second.
Measure a distance and a time; repeat several times and take a mean.

Loudness and pitch

Four oscilloscope traces. A quiet sound has a small amplitude; a loud sound of the same pitch has a large amplitude. A low-pitched sound has few waves on the screen (low frequency); a high-pitched sound of the same loudness has many (high frequency).
Larger amplitude: louder. Higher frequency: higher pitch.

Ultrasound

Ultrasound is sound with a frequency higher than 20 kHz — above the range of human hearing. EXTENDED Its short pulses reflect from boundaries, so the time for an echo gives a distance:

\[ \text{distance to the reflector} = \frac{\text{speed} \times \text{time for the echo}}{2} \]
  • EXTENDED Non-destructive testing: echoes from a crack inside a metal casting or weld show the flaw.
  • EXTENDED Medical scanning of soft tissue (e.g. a fetus), without the dangers of X-rays.
  • EXTENDED Sonar: finding the depth of the sea or locating shoals of fish.
A ship sends an ultrasound pulse down to the seabed; the echo returns after 0.60 s. At 1500 metres per second the sound travels 900 m in that time, so the depth is 450 m.
EXTENDED The pulse goes down and back, so halve the distance.

✏️Worked example

(Modelled on 0625/42 June 2026 Q7.) (a) Complete the explanation of how a loudspeaker produces sound: an alternating current in the coil produces a changing magnetic field; … [3] (b) The loudspeaker plays a note of frequency 512 Hz. The speed of sound is 330 m/s. Calculate the wavelength, with its unit. [2] (c) State what is meant by ultrasound. [1]

(a) This field interacts with the field of the permanent magnet, so there is a force on the coil that changes direction as the current alternates; the coil and cone vibrate, making the air particles vibrate, which carries energy to the listener as sound.

(b) \( \lambda = \dfrac{v}{f} = \dfrac{330}{512} = \) 0.64 m.

(c) Sound with a frequency above 20 kHz (above the upper limit of human hearing).

Check it. Audible sounds have wavelengths from about 17 m (20 Hz) down to 1.7 cm (20 kHz); 0.64 m sits comfortably in that range.
Multiplying instead of dividing. v = fλ, so λ = v/f. 330 × 512 gives an absurd wavelength of 169 km.

📝Practise

In the style of the multiple-choice, theory and practical papers. EXTENDED marks Supplement content.

1. (Multiple choice.) A loudspeaker plays a note more loudly without changing its pitch. Which property of the wave increases? A: amplitude. B: frequency. C: speed. D: wavelength.
A.
2. (Multiple choice.) EXTENDED In still air the pressure is p. In a compression of a sound wave the pressure is P. Which is correct? A: P < p and the particles are further apart. B: P > p and the particles are closer together. C: P > p and the particles are further apart. D: P = p.
B.
3. (Theory.) Hearing ranges: cat 48–85 000 Hz; elephant 14–12 000 Hz; goldfish 20–3000 Hz; rat 200–76 000 Hz. Which animals can hear ultrasound, and which can hear frequencies too low for humans? [2]
Ultrasound (above 20 000 Hz): cat and rat. Below 20 Hz: elephant only.
4. (Practical.) Describe how to measure the speed of sound in air outdoors. [4]
Measure a large distance (e.g. 200 m) with a trundle wheel or tape. One person makes a sharp sound with a visible action (clapping boards, starting pistol); the other starts a stop-watch on seeing the action and stops it on hearing the sound. Repeat several times and average; speed = distance ÷ time. (Or time an echo from a wall and use twice the distance.)
5. (Theory.) EXTENDED An ultrasound pulse in steel returns from a crack after 12 μs. The speed of sound in steel is 6000 m/s. How deep is the crack? [2]
Distance travelled = 6000 × 12 × 10−6 = 0.072 m; depth = 0.072 / 2 = 0.036 m (3.6 cm).
6. (Theory.) Explain why astronauts on the Moon must use radios to talk to each other, even when standing close together. [2]
There is no air (a vacuum) on the Moon, and sound needs a medium to travel through; radio waves are electromagnetic and can cross a vacuum.
7. (Theory.) EXTENDED State the order of the speed of sound in air, water and steel, from slowest to fastest. [1]
Air, water, steel.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • PhET “Sound Waves” — compressions, rarefactions, frequency and amplitude
  • Academo — online tone generator and oscilloscope for pitch and loudness