Lesson overview
Sound waves: Why can't sound travel through space?
Change frequency and amplitude and compare microphone traces. Remove air from a bell jar to find why sound needs a medium.
Suggested sequence
- Lesson 1 · Making sound. Vibrating sources, longitudinal sound, frequency and pitch, amplitude and loudness; Supplement: compression and rarefaction. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Sound needs a medium. Sound needs a medium; the bell jar and sound speed; Supplement: air, water and steel. 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. 4 questions, 15 min, lesson closed.
Learning goals
Part 1: Making sound
- Describe the production of sound by vibrating sources (3.4.1)
- Describe the longitudinal nature of sound waves (3.4.2)
- State the approximate range of frequencies audible to humans as 20 Hz to 20 000 Hz (3.4.3)
- Describe how changes in amplitude and frequency affect the loudness and pitch of sound waves (3.4.7)
- Supplement: Describe compression and rarefaction (3.4.10)
Part 2: Sound needs a medium
- Know that a medium is needed to transmit sound waves (3.4.4)
- Know that the speed of sound in air is approximately 330–350 m / s (3.4.5)
- Supplement: Know that, in general, sound travels faster in solids than in liquids and faster in liquids than in gases (3.4.11)
Misconceptions to listen for
- “The finger must keep moving to make the continuing sound.” — The finger starts the vibration; the string continues to vibrate afterwards.
- “Air particles in sound always vibrate left and right, as in a drawing of a speaker facing sideways.” — The vibration follows the direction of travel, whichever way that is. Here the sound goes up, so left and right would be at right angles to it: that is a transverse vibration.
- “The particle moves upwards only, all the way to the ceiling.” — The disturbance travels to the ceiling, but each particle moves up and back down about its own place. Only the vibration is passed on.
- “A rarefaction is an empty gap.” — J and L still have 12 and 14 particles; they are more widely spaced, not absent.
- “The box with the most particles has the lowest pressure, because the particles have no room to move.” — Crowding more particles into the same space raises the pressure. K, the most crowded box, has the highest pressure.
- “The range is 20 Hz to 20 Hz.” — The upper end is 20 kHz, and 1 kHz is 1000 Hz, so it is 20 000 Hz.
- “W is quieter than U because it has fewer cycles.” — Fewer cycles in the same time means a lower frequency and a lower pitch. W and U have the same height, so the same loudness.
- “V has a lower pitch than U because its trace is shorter.” — Pitch comes from the number of cycles in the same time, and U and V both have 5. The height tells you about loudness.
- “The 4 kHz call must be louder because its number is larger.” — A frequency compares pitch, not loudness. Loudness depends on amplitude, which is not given.
- “The 800 Hz call has the higher pitch, because 800 is more than 4.” — The k in kHz means one thousand: 4 kHz = 4000 Hz, which is five times 800 Hz.
Scope
Cambridge IGCSE Physics 0625 (2026–2028): 3.4.1–3.4.5, 3.4.7; Supplement 3.4.10–3.4.11. Echoes, ultrasound and measuring sound speed belong to the next module.