12.5 Sound Waves

A free GCSE Physics lesson for AQA (8463) from OBA School: physics only. Loudness and pitch, oscilloscope traces, how the ear works, the range of human hearing and echo sounding. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Link the loudness of a sound to amplitude and its pitch to frequency, using oscilloscope traces
• Use an oscilloscope trace and time-base setting to find the period and frequency of a sound
• State the range of normal human hearing and describe how the ear detects sound
• (Higher tier) explain why the conversion of sound waves to vibrations of solids limits human hearing
• (Higher tier) use echo sounding to calculate the depth of water: depth = ½ × v × t

Key words

• Pitch: How high or low a note sounds. Higher frequency = higher pitch.
• Loudness: How loud a sound is. Larger amplitude = louder sound.
• Oscilloscope: A device that shows a changing voltage (e.g. from a microphone) as a trace on a screen, with time across the screen.
• Waveform: The shape of the trace. A pure note has a smooth, regular waveform; noise has no repeating pattern.
• Ear drum: A thin membrane in the ear that vibrates when sound waves reach it.
• Range of hearing: The frequencies a normal human ear can hear: about 20 Hz to 20 kHz (20 000 Hz).
• Echo sounding: Using the time for pulses of high-frequency sound to reflect from the sea bed (or a shoal of fish) to find its depth. (Higher tier)

Loudness, pitch and the oscilloscope

A microphone turns sound waves into an electrical signal. Connected to an oscilloscope, it shows how the air pressure at the microphone changes with time. The trace is a graph – sound is still a longitudinal wave in the air.

• Louder sound → bigger amplitude → taller trace (same spacing).
• Higher pitch → higher frequency → more waves on the screen, closer together (same height).
• A tuning fork or a signal generator with a loudspeaker gives a pure note: a smooth, regular waveform. Musical notes repeat regularly; noise varies in frequency with no pattern.

Key fact: Pitch ↔ frequency. Loudness ↔ amplitude. A quiet, high note has a short trace with waves close together.

Finding frequency from a trace

The time-base setting tells you the time represented by each square across the screen. Count the squares for one complete wave to find the period T, then use f = 1 ÷ T.

Worked example: more than one cycle

Time-base 0.5 ms/div. Three complete cycles fill 6 divisions.
T = (6 × 0.5 ms) ÷ 3 = 1.0 ms = 0.001 s; f = 1 ÷ 0.001

Answer: f = 1000 Hz (1 kHz)

Sound in solids, liquids and gases

Sound waves can make a solid vibrate, and a vibrating solid makes sound waves. You can hear a neighbour’s music through a wall: the sound makes the wall vibrate and the wall sets the air on your side vibrating. A tuning fork held on a table sounds louder because the large table top vibrates too and pushes on much more air.

When sound passes from one substance to another, its frequency does not change (it is set by the source), but its speed changes, so its wavelength changes too (λ = v ÷ f). That is why the note you hear does not change pitch as it passes from the air into the different parts of your ear.

Worked example: a 500 Hz note in air and in water

air: λ = v ÷ f = 340 ÷ 500 = 0.68 m
water: same frequency, speed 1500 m/s, so λ = 1500 ÷ 500 = 3.0 m

Answer: the wavelength in water is longer, but the pitch is the same

The ear and the range of hearing

The normal range of human hearing is about 20 Hz to 20 kHz. The upper limit falls as people get older and can be damaged by loud music. Other animals have different ranges: dogs and bats hear much higher frequencies than we can.

(Higher tier) Hearing depends on changing sound waves into vibrations of solids – the ear drum and the tiny bones. This conversion only works over a limited range of frequencies: the parts have mass and stiffness, so they cannot follow vibrations that are too fast, and very slow pressure changes do not make them respond in the right way. Outside this range no vibrations reach the cochlea, so no matter how intense the sound, we do not hear it.

Echo sounding (Higher tier)

Ships measure the depth of water using pulses of high-frequency sound. A transmitter on the hull sends a pulse down; it reflects from the sea bed and a receiver times how long it takes to come back. The pulse travels down and back, so the depth is half the total distance:

depth (m) = ½ × speed of sound in water v (m/s) × time for the echo t (s)

Worked example: depth of water under a ship

An echo returns 0.84 s after the pulse is sent. Speed of sound in sea water = 1500 m/s.
total distance = 1500 × 0.84 = 1260 m; depth = 1260 ÷ 2

Answer: depth = 630 m

Common mistakes

• Linking loudness to frequency. Loudness depends on amplitude; pitch depends on frequency.
• Reading the period from the number of squares and forgetting to multiply by the time-base setting (and to convert ms to s).
• Saying the frequency changes when sound enters water. The speed and wavelength change; the frequency stays the same.
• Forgetting to halve the distance in echo sounding.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the range of normal human hearing? (Choose from: 20 Hz to 20 kHz, 20 Hz to 2000 Hz, 2 kHz to 200 kHz, 0.2 Hz to 20 Hz.)

Answer: 20 Hz to 20 kHz

Working: Normal human hearing is about 20 Hz to 20 kHz (20 000 Hz). Above that is ultrasound.

Question 2. A 750 Hz sound travels through water at 1500 m/s. Calculate its wavelength in the water.

Answer: 2 m

Working: λ = 1500 ÷ 750 = 2.0 m.

Question 3. True or false? When sound passes from air into water, its frequency changes.

Answer: False

Working: False. The frequency is set by the source. The speed and wavelength change.

Related lessons

• Previous lesson: 12.4 More About Waves
• Next lesson: 12.6 The Uses of Ultrasound
• In this chapter: 12.1 The Nature of Waves
• In this chapter: 12.2 The Properties of Waves
• In this chapter: 12.3 Reflection and Refraction

Continue in the full course

This lesson is section 12.5 (AGCPH1205) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.