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AQA GCSE Physics · Topic 4.6 Waves · Physics only
Sound and Ultrasound: Hearing, Echoes and Finding Distances
Physics only Combined Science students don't need this page. Ultrasound is Higher tier.
In this lesson you'll learn to: describe sound as a longitudinal wave, give the range of human hearing, explain how we hear and why that range is limited, describe the echo method for the speed of sound, define ultrasound, and use s = v × t ÷ 2 to find a depth or the distance to a crack.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
In this lesson you'll learn to: describe sound as a longitudinal wave, give the range of human hearing, explain how we hear and why that range is limited, describe the echo method for the speed of sound, define ultrasound, and use s = v × t ÷ 2 to find a depth or the distance to a crack.
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Start here — have a play
Don't read anything yet. On tab 1, slide the frequency and press Play: where does your hearing stop? Tab 2 follows a sound into the ear. On tab 3, clap three times and work out the speed of sound. Tab 4 uses ultrasound echoes to find a shoal of fish and a hidden crack.
Frequency
Sound
How we hear a sound
- The compressions and rarefactions of the sound wave reach the ear.
- The compressions exert a force on the eardrum, so it vibrates at the same frequency.
- The vibrations pass through the ossicles: small bones, which are solids.
- The inner ear (cochlea) turns the vibrations into signals sent to the brain.
Distance to the wall
Results
Use
Fish at
Physics with Kate · physicswithkate.com
▲ Four tabs: sound as a longitudinal wave, how we hear, the echo method, ultrasound echoes
Worked out the speed of sound? Now learn to write it up.
The notes below give the four steps for how we hear, the echo method, the ultrasound definition and the "÷ 2" calculation that catches people out.
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Revision notes
Sound is a longitudinal wave
⭐ Sound
The particles vibrate parallel to the direction of energy transfer. Sound is a series of compressions (particles bunched up) and rarefactions (particles spread out). It needs a medium: sound can't travel through a vacuum.
The range of human hearing
Humans can hear from 20 Hz to 20 000 Hz (20 kHz). Below 20 Hz is infrasound; above 20 kHz is ultrasound.
The range of human hearing.
How we hear
- The compressions and rarefactions of the sound wave reach the ear.
- The compressions exert a force on the eardrum, so it vibrates at the same frequency.
- The vibrations pass through the ossicles: small bones, which are solids.
- The inner ear turns the vibrations into signals sent to the brain.
Sound wave → eardrum → ossicles → cochlea → brain.
⭐ Why the range is limited
Hearing works by turning sound into vibrations of solids (the eardrum and the bones). These only work over a limited range of frequencies: above 20 000 Hz they can't vibrate fast enough, so the sound isn't passed on.
Measuring the speed of sound in air: the echo method
- Measure the distance from the person to a large wall with a trundle wheel.
- One person makes a loud sound, e.g. a clap. It echoes off the wall and returns.
- Another person times from the clap to the echo with a stopwatch. Repeat and find the mean.
- speed = (distance × 2) ÷ time. Why × 2? The sound goes to the wall and back.
The echo method.
Ultrasound Higher
⭐ What is ultrasound?
Sound with a frequency higher than the upper limit of human hearing: above 20 000 Hz (20 kHz).
How an ultrasound device finds a distance (3 marks):
- A pulse of ultrasound is emitted.
- It is partially reflected at a boundary between two materials with different densities.
- The time for the pulse to go out and return is measured. The speed of ultrasound in the material is known.
⭐ Distance to the boundary
distance = speed × time ÷ 2 · s = v × t ÷ 2
Why ÷ 2? The time is for the pulse to go there and back, so speed × time is twice the distance to the boundary.
| Use | The boundary that reflects |
|---|---|
| Sonar: finding shoals of fish or the depth of the sea | water and fish, or water and the seabed |
| Finding cracks in metal | metal and the air in the crack |
| Medical scans, e.g. of an unborn baby | different tissues in the body |
Watch the units. Change ms to s (÷ 1000) and µs to s (× 10⁻⁶) before you calculate.
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How to write it in the exam
Every echo calculation, the same three steps
Change the time to seconds →
s = v × t →
÷ 2 for the distance to the boundary (or × 2 the distance to find a speed).
ms or µs → sthe equationthere and back
Workbook page 9 · the echo method results
A student stands 85 m from a wall. The echo times are 490 ms, 515 ms and 495 ms. Calculate the speed of sound.
- Mean time = (490 + 515 + 495) ÷ 3 = 500 ms = 0.500 s
- Distance there and back = 85 × 2 = 170 m
- Speed of sound = 170 ÷ 0.500 = 340 m/s
Exam-style question · page 9 · 9 marks
(a) Describe how two students could use an echo to measure the speed of sound. Include the measurements and the equipment. (4)
- Measure the distance to the building with a trundle wheel or tape measure.
- One student makes a loud sound, e.g. a clap.
- The other measures the time until the echo is heard with a stopwatch.
- Speed = 2 × distance ÷ time; repeat and take a mean.
(b) They stand 99 m from the building. The echo returns 600 ms after the clap. Calculate the speed of sound. (3)
- 600 ms = 0.600 s; distance there and back = 2 × 99 = 198 m
- v = s ÷ t = 198 ÷ 0.600 = 330 m/s (165 m/s forgets the × 2)
(c) Explain why humans cannot hear sounds with a frequency above 20 000 Hz. (2)
- The ear turns sound into vibrations of solids (the eardrum and bones).
- The bones in the ear can't vibrate above 20 000 Hz, so the sound isn't passed on.
Workbook page 10 · how deep is the shoal of fish? Higher
Pulse down, echo back up.
A boat's ultrasound pulse reflects off a shoal of fish and is detected 400 ms later. Ultrasound travels at 1500 m/s in water. How deep are the fish?
- Time = 400 ms = 0.40 s
- s = v × t ÷ 2 = 1500 × 0.40 ÷ 2 = 300 m
Common mistakes: not changing ms to s; 1500 × 0.40 = 600 m is there and back, so halve it.
Exam-style question · finding a crack in a metal block · page 10 · Higher 9 marks
The block and the transmitter.
The oscilloscope trace: 1 division = 10 µs.
(a) Identify each pulse P, A and B. Give a reason for each. (3)
- P is the transmitted pulse: it comes first and has the largest amplitude (most energy).
- A is reflected from the crack: it returns first, so the boundary is nearer.
- B is reflected from the bottom of the block: it returns last, so it travelled furthest.
(b) Explain how the pulses are used to find the distance to the crack. (3)
- A pulse of ultrasound is emitted.
- It is partially reflected at the boundary between materials of different density (the crack).
- The time between emitting and detecting is measured; distance = ½ × speed × time.
(c) Ultrasound travels at 5000 m/s in the metal. Calculate the distance from the top of the block to the crack. (3)
- P to A = 4 div × 10 µs = 40 µs = 4.0 × 10⁻⁵ s
- s = v × t ÷ 2 = 5000 × 4.0 × 10⁻⁵ ÷ 2 = 0.10 m (10 cm)
A and B are smaller than P: only part of the pulse is reflected, and some energy is absorbed by the metal. 0.20 m forgets the ÷ 2; 40 s forgets the µ.
📘 Now do it in your workbook
Page 9: sound as a longitudinal wave, the range of hearing, how we hear, the echo method with real results, and a 9-mark exam question. Page 10: ultrasound, how a device finds a distance, the shoal of fish, and a 9-mark exam question on finding a crack in metal.
AQA GCSE Physics Workbook · WAVES · pages 9 and 10The full workbook covers the whole of Topic 4.6 Waves, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for these spec points.
Don't forget to hit “Mark complete” at the bottom of the lesson.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
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Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Pages 9 and 10 of the workbook have hearing, the echo method, ultrasound and two 9-mark exam questions, with answers at the back.
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