Lesson overview
Echoes and ultrasound: How can a ship measure the depth of the sea?
Time echoes from a wall 100 m away and use 20 intervals to find the speed of sound. Then send ultrasound pulses from a ship and map a reef from the echo times.
Suggested sequence
- Lesson 1 · Echoes and the speed of sound. An echo is reflected sound; measuring the speed of sound with echoes and with a flash and a bang. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Ultrasound and sonar. Ultrasound above 20 kHz; Supplement: sonar depths, medical scanning and testing for cracks. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 5 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 5 questions, 15 min, lesson closed.
Learning goals
Part 1: Echoes and the speed of sound
- Describe an echo as the reflection of sound waves (3.4.8)
- Describe a method involving a measurement of distance and time for determining the speed of sound in air (3.4.6)
Part 2: Ultrasound and sonar
- Define ultrasound as sound with a frequency higher than 20 kHz (3.4.9)
- Supplement: Describe the uses of ultrasound in non-destructive testing of materials, medical scanning of soft tissue and sonar including calculation of depth or distance from time and wave speed (3.4.12)
Misconceptions to listen for
- “The wall starts to vibrate and makes a new sound of its own.” — The wall is not a new source here: it sends back the sound that reached it. That reflected sound is the echo.
- “The sound waves return from the far side of the wall.” — A hard, thick wall reflects most of the sound at the surface facing the pupil. The echo comes from that side.
- “The second sound is the ear still vibrating.” — The second sound comes after a clear gap, and the gap depends on how far away the wall is. So it came from outside the ear.
- “The distance to the wall is speed × echo time.” — Speed × echo time gives the whole path, out and back. Halve it to find the distance to the wall.
- “Any surface gives a clear echo if it is far enough away.” — Soft materials such as curtains absorb much of the sound, so their echo is weak even from far away.
- “A nearer wall gives a clearer echo.” — A nearer wall reflects well, but its echo returns within about 0.1 s and merges with the clap instead of being heard on its own.
- “Row D works because curtains are close.” — Row D fails both checks: curtains absorb sound, and at 10 m any reflection merges with the clap.
- “Time = 119 ÷ 340.” — That is the time for the sound to reach the building only. The echo has to come back too, so use 238 m.
- “There is no echo because the tiles absorb the sound.” — Hard tiles reflect sound well, and the ringing is those reflections. They return too soon to be heard as a separate echo.
- “A wall so close cannot reflect sound.” — Distance does not stop reflection. A near wall reflects too; its reflection simply arrives too soon to be heard on its own.
Scope
Cambridge IGCSE Physics 0625 (2026–2028): 3.4.6, 3.4.8, 3.4.9; Supplement 3.4.12. The speed of sound in air, water and steel (3.4.5) is SOUND01’s.