Welcome! Bats, ships and hospitals all “see” using sound. In this lesson you’ll learn how sound waves travel and reflect, then use echoes to guide a ship safely into harbour as a sound investigator. It takes about one hour. (Sound: Combined and Separate. Ultrasound and the range of human hearing: Separate only.)
Before you start
- You need: your exercise book or paper, a pen and a calculator.
- Write the title and date: Sound and ultrasound – Sound Investigators.
- The golden rule: write your answer first, then tap Show answer. Correct mistakes in a different colour.
- Look out for the blue 📝 boxes. Copy each one into your book neatly – these are your revision notes.
📝 Copy into your book
By the end of this lesson I will be able to:
1. describe sound as a longitudinal wave
2. use v = fλ and echo timings to calculate distances
3. explain how ultrasound is used for scans and depth finding.
Step 1: Starter (5 minutes)
In space, astronauts can’t talk to each other without radios – even if their helmets are touching nothing but empty space in between. Why can’t sound travel through space? Write your explanation.
Show answer
Sound is a vibration passed from particle to particle. Space is a vacuum – there are no particles to vibrate – so sound can’t travel through it. Light can, because it’s an electromagnetic wave and doesn’t need particles.
📝 Copy into your book
Sound is a LONGITUDINAL wave: the particles vibrate parallel to the direction the wave travels, making compressions and rarefactions. Sound needs a medium – it cannot travel through a vacuum.
Step 2: Read and write (8 minutes)
First, copy these three questions into your book. Then read the text below carefully and answer them in full sentences.
- What is an echo?
- What is ultrasound?
- Why do you have to halve the time when you use an echo to measure distance?
📖 Read: Seeing with sound
Sound waves travel at about 340 m/s in air and faster in liquids and solids – about 1500 m/s in water. When sound hits a hard surface, it is reflected. A reflected sound is called an echo. By timing how long an echo takes to return, we can work out how far away the surface is. The sound travels there and back, so the distance to the surface is half the total distance travelled.
Humans can hear sounds with frequencies from about 20 Hz to 20 000 Hz. Sound with a frequency higher than 20 000 Hz is called ultrasound – we can’t hear it. Ultrasound is partly reflected at the boundary between two different materials, such as between fluid and a baby’s body in the womb. Timing these reflections lets a computer build up an image. Ultrasound is used for pre-natal scans because, unlike X-rays, it is not ionising, so it is safe for the baby. Ships use the same idea, called echo sounding, to measure the depth of the water.
Show answers
(a) A reflected sound wave.
(b) Sound with a frequency above 20 000 Hz, too high for humans to hear.
(c) The sound travels to the surface and back again, so it covers twice the distance.
📝 Copy into your book
wave speed = frequency × wavelength v = f λ
distance = speed × time s = v t
ECHOES: distance to the surface = (speed × total time) ÷ 2
Step 3: Calculations (10 minutes)
Q1 You shout at a cliff and hear the echo 2.0 s later. The speed of sound in air is 340 m/s. How far away is the cliff?
Show answer
Total distance = 340 × 2.0 = 680 m
Distance to cliff = 680 ÷ 2 = 340 m
Q2 A sound wave in air has a frequency of 170 Hz. Calculate its wavelength.
Show answer
v = f λ, so λ = 340 ÷ 170 = 2.0 m
Q3 During a scan, an ultrasound pulse returns from a baby’s head after 0.000 04 s. Ultrasound travels at 1500 m/s in the body. How deep is the baby’s head below the skin?
Show answer
Total distance = 1500 × 0.000 04 = 0.06 m
Depth = 0.06 ÷ 2 = 0.03 m (3 cm)
✅ Score out of 3. Did you remember to halve the echo distances?
Step 4: Main task – Guide the Ship (14 minutes)
You’re on the bridge of a cargo ship heading into a harbour at night. The ship’s hull reaches 20 m below the surface, so the water must be deeper than 20 m everywhere it goes. The echo sounder sends ultrasound pulses down to the seabed. Speed of sound in seawater = 1500 m/s.
| Position | Echo time (s) | Depth of water (m) | Safe? |
|---|---|---|---|
| A – open sea | 0.20 | ? | ? |
| B – harbour entrance | 0.08 | ? | ? |
| C – left channel | 0.02 | ? | ? |
| D – right channel | 0.04 | ? | ? |
Your task: copy and complete the table. Then write a message to the captain: which channel should the ship take into harbour, and why? Finally, explain why echo sounding uses ultrasound rather than sound we can hear.
Show answers
A: 1500 × 0.20 ÷ 2 = 150 m – safe
B: 1500 × 0.08 ÷ 2 = 60 m – safe
C: 1500 × 0.02 ÷ 2 = 15 m – NOT safe (shallower than 20 m)
D: 1500 × 0.04 ÷ 2 = 30 m – safe
Message to the captain: “Take the right channel (D). It’s 30 m deep, so there’s 10 m of water under the hull. The left channel is only 15 m deep – the ship would run aground.”
Why ultrasound? It can’t be heard, so it doesn’t disturb the crew. It is also easier to send in a narrow beam, so the echoes give a more precise depth directly under the ship.
✅ Check: did you show your working and give the captain a clear reason?
Step 5: Exam practice (13 minutes)
Look at the marks – they tell you how many points to make. Answer in full sentences, then mark yourself.
Q1 (2 marks) Describe the difference between a longitudinal wave and a transverse wave.
Show model answer
In a longitudinal wave, the vibrations are parallel to the direction of energy transfer (1). In a transverse wave, the vibrations are at right angles (perpendicular) to the direction of energy transfer (1).
Q2 (3 marks) A ship sends an ultrasound pulse to the seabed. The echo returns after 0.60 s. Sound travels at 1500 m/s in seawater. Calculate the depth of the sea.
Show model answer
s = v t = 1500 × 0.60 (1) = 900 m (1)
Depth = 900 ÷ 2 = 450 m (1)
Q3 (2 marks) Explain why ultrasound, rather than X-rays, is used to scan unborn babies.
Show model answer
X-rays are ionising and could damage the baby’s cells / cause mutations (1). Ultrasound is not ionising, so it is safe; it is also partly reflected at boundaries between soft tissues, so it can produce an image (1).
Q4 (3 marks) A dog whistle produces sound with a frequency of 25 000 Hz. The speed of sound in air is 340 m/s. Calculate the wavelength, and explain why the dog’s owner can’t hear it.
Show model answer
λ = v ÷ f = 340 ÷ 25 000 (1) = 0.0136 m (1.36 cm) (1). The frequency is above 20 000 Hz, the upper limit of human hearing, so it is ultrasound (1).
✅ Add up your marks out of 10 and write the score in your book.
Step 6: Exit ticket (10 minutes)
Try it! Stand about 50 m from a large wall, clap, and listen for the echo. Is it easy to hear? Then finish:
- Sound can’t travel through space because…
- When using an echo to find a distance, I must remember to…
- One thing I’m still not sure about is…
🎉 Well done – lesson complete! Your 📝 boxes are your revision notes for this topic. Want more? Try the Waves and magnetism mixed equation practice.
