Secondary 3 Physics | Sound Waves: Frequency, Speed and Echoes — Tutor Lesson Prep Guide | v = f lambda, ultrasound and sonar ranging

2026-11-11

Tutor lesson-prep guides · Secondary Physics

Students merge loud with high because both are described as a big sound. Separating amplitude from frequency is what makes the wavelength work and echo ranging stand up afterwards.

Objectives — By the end students can: (1) explain that sound travels through vibrating particles of a medium and cannot cross a vacuum, and separate amplitude (loudness) from frequency (pitch); (2) use v equals f times lambda to get a 0.773 metre wavelength and a 2.27 millisecond period for 440 Hz in air; (3) work out that a wall 170 metres away returns an echo after 1.0 second and that a 255 metre sonar reading takes 1.5 seconds, explaining why the distance doubles.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Division and units	What is 340 divided by 440, roughly?	0.773, in metres
Milliseconds	How many milliseconds are in a second?	1000 milliseconds
Particle model	Are air particles ever still?	No, they vibrate constantly while staying roughly in place

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Diagnose the guess	Ask for the difference between shouting and screaming, put both answers in two columns, judge nothing	Say what separates loud from high
5-13 min	Particle diagram	Draw a row of air particles, show compression and rarefaction and the direction of vibration	Mark the compressions and rarefactions on the diagram
13-29 min	Three scripts	Teach vibration and transmission, v equals f times lambda, echoes and sonar; pause for recall after each	Say back that the echo travels both ways
29-36 min	Four question types	Model wavelength, period, echo time, sonar depth; one each	Do one of each and write the unit
36-42 min	Error contrast	Write the four common mistakes up and have students correct them	Correct each sentence and justify the factor of two
42-45 min	Exit check	Collect the three answers and sort the errors	Complete three questions alone

03 Teaching script (read it out)

Scene 1: Script 1: the particles vibrate, the energy travels

“When you speak, air does not fly out of your mouth into someone’s ear. The particles vibrate back and forth and pass the energy along, layer by layer. No particles means no sound, which is why a explosion in space is silent. Keep two things apart as well: a large amplitude means the particles swing further and you hear something loud; a high frequency means more vibrations per second and you hear a high pitch. Shouting is amplitude, screaming is frequency, and both can happen at once or separately.”

Can sound travel through a vacuum, and why not?

Scene 2: Script 2: v equals f times lambda, any two give the third

“Wave speed is frequency times wavelength. Sound travels at 340 metres per second in air, so the A at 440 Hz has a wavelength of 340 divided by 440, which is 0.773 metres. Period is the reciprocal of frequency: 1 divided by 440 is 0.00227 seconds, so 2.27 milliseconds. Note that the speed is set by the medium: in water sound moves at about 1500 metres per second, the frequency of the same note does not change, and the wavelength becomes several times longer.”

When the same note goes from air into water, which one changes, frequency or wavelength?

Scene 3: Script 3: the echo travels both ways, so multiply or divide by two

“An echo covers the trip there and the trip back. A wall 170 metres away means 340 metres of travel, divided by 340 metres per second, which is exactly 1.0 second. Sonar measuring a 255 metre depth sends 510 metres of travel and takes 1.5 seconds. So when a question gives time and asks distance, halve the result; when it gives distance and asks time, remember the second leg. Skip that and the answer is out by a factor of two.”

Does the echo time cover one journey or both?

04 Question types and worked steps

Type	Example	Steps	Answer
Wavelength	What is the wavelength of a 440 Hz sound wave in air?	(1) Write v equals f times lambda. (2) lambda is 340 over 440. (3) Unit metres, three decimals.	0.773 metres
Period	What is the period of a 440 Hz vibration in milliseconds?	(1) Period is 1 over frequency. (2) That gives 0.00227 seconds. (3) Multiply by 1000 for milliseconds.	2.27 milliseconds
Echo time	You shout at a wall 170 metres away. When do you hear the echo?	(1) Double the distance. (2) Divide 340 metres by 340 metres per second. (3) Unit seconds.	1.0 seconds
Sonar depth	Sonar receives a return after 1.5 seconds. How deep is the sea?	(1) Total distance is 340 times 1.5. (2) That is 510 metres. (3) Halve it for the depth.	255 metres

Sanity check (teach this): Sanity check: a wavelength must be far smaller than the 340 metre per second speed, so 340 times 440 means both the formula and the units went wrong. A period must be below one second, so 2.27 milliseconds is right. A 170 metre echo must take about 1 second, not 0.5, which would drop the return leg. A depth must be half the 510 metre round trip, not the whole of it.

05 Independent practice (3 questions)

Question	Answer	What to watch
What is the wavelength of a 340 Hz sound wave?	1.0 metre	Whether they divide 340 by 340 rather than multiplying
Sonar returns after 3.0 seconds. How deep is the water?	510 metres	Whether 1020 metres appears as the round trip before halving
What is the period of a 20 Hz vibration in milliseconds?	50 milliseconds	Whether they compute 1000 divided by 20

06 Common mistakes and what to say

Mistake	Student writes	Say this
Thinking sound carries air from source to ear	“Sound is air flying out of my mouth”	“What travels is energy, not air. The particles only vibrate, they stay where they are.”
Treating loud as high	“Shouting is the same as a high note”	“Shouting is a big amplitude, which is loudness. High pitch is a high frequency. Keep the two words separate.”
Leaving out the return trip in echo sums	“A 170 metre wall gives an echo at 0.5 seconds”	“The sound goes there and comes back, 340 metres in total, so 1.0 second.”
Assuming frequency changes with the medium	“In water the frequency rises, so the wavelength grows”	“The source sets the frequency and the medium does not change it. The speed changes, and the wavelength follows the speed.”

07 Homework (paper, section and question numbers)

Physics chapter index: the Waves chapter, redo two wavelength and two period questions — Waves chapter, questions 4 and 6
Physics chapter index: the Sound chapter, preview ultrasound and noise applications — Sound chapter, questions 2 and 7

08 Exit check (last 5 minutes)

Question	Answer	Decision
Can sound travel through a vacuum?	No, there is no medium to vibrate	Wrong: reteach vibration and transmission next lesson
What is the period of 440 Hz in milliseconds?	2.27 milliseconds	Wrong: reteach the reciprocal step from script 2
A wall 170 metres away: when is the echo heard?	1.0 second	Wrong: reteach the two-way journey from script 3

09 Tutor FAQ

Do students need to memorise 340 metres per second?

Yes. Most Secondary 3 questions use 340 metres per second unless another value is given; for water they only need to know it is about 1500 and faster than air.

Can echoes and sonar be taught together?

Yes, both use the same there-and-back structure. Do the wall first, then the sonar depth, and students will supply the halving themselves.

Is there practice material?

Yes. The homework rows link to the Waves and Sound chapters in the physics index, with question numbers and the same question types as this lesson.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.