Lesson focus: Sound begins with a vibrating source and travels as a longitudinal disturbance through particles in a mediumThe material that a wave travels through, such as air or water..
Explain sound transfer through solids, liquids and gases and why sound cannot travel through a vacuumA completely empty space with no particles at all, like most of outer space..
Build on earlier learning
WavesThe wave model distinguished local vibration from energy transfer. Sound now provides a particle-level longitudinal example.
You will be successful when you can…
- Identify a vibrating source.
- Describe compressions and rarefactions in a sound wave.
- Explain why sound needs a material medium.
- Use particle interactions cautiously to compare sound speed in different media.
Words that carry the science
- sound wave
- A longitudinal mechanical wave produced by a vibrating source.
- medium
- The material through which a mechanical wave travels.
- compression
- A region in a longitudinal wave where particles are closer together than average.
- rarefaction
- A region where particles are farther apart than average.
- vacuum
- A region containing no matter, so there are no particles to carry sound.
- oscillation
- Repeated movement back and forth around an equilibrium position.
Follow sound energy from source to detector
01A vibrating source creates a pressure disturbance
A loudspeaker cone, tuning fork or vocal cord produces sound by vibrateTo move quickly backwards and forwards on the spot.. It pushes and pulls on nearby particles, forming moving compressionA part of a sound wave where the particles are squashed closer together. and rarefactionA part of a sound wave where the particles are spread further apart..
Each particle oscillates around its equilibrium position and interacts with neighbouring particles. The disturbance and its energy travel through the mediumThe material that a wave travels through, such as air or water.; the particles do not stream from source to listener.
02Sound requires matter
Sound is a mechanical waveA wave that needs a material, such as air or water, to travel through, like sound., so it needs particles. In a vacuumA completely empty space with no particles at all, like most of outer space. there are no particles to interact and no sound waveA vibration that travels through a material, like air, carrying sound. can propagate.
Light can cross a vacuum because it is an electromagnetic wave. This is why a distant event can be seen before it is heard and why space scenes cannot transmit sound through empty space.
03Speed depends on the medium and its conditions
Sound travels at about 343 m/s in air at 20 °C, but speed changes with temperature and material. It is generally faster in liquids and many solids because their particle interactions transmit the disturbance efficiently.
“Particles are closer” is a useful starting model, but spacing alone is incomplete. Stiffness, density and temperature also affect speed, so measured data should decide comparisons.
Sound waves and the oscilloscope
A loudspeaker makes air particles vibrate backwards and forwards, in the same direction the sound travels. A microphone turns the vibrations into a trace on an oscilloscope.
Controls
Taller trace: louder. More waves on the screen: higher pitch. Speed of sound in air ≈ 340 m/s.
Explain the vacuum-bell observation
A ringing electric bell becomes quieter as air is removed from a sealed jar, although the hammer can still be seen moving.
- Identify the hammer and bell as the vibrating source.
- Sound transfer requires particles between the bell and jar wall.
- Removing air reduces the number of particle interactions available to carry the disturbance.
- The bell mechanism still vibrates, but progressively less sound reaches the outside.
How to make the reasoning visible: The result supports the model that sound needs a medium; it does not show that the source stopped vibrating.
Compare sound transmission evidence
The speeds are approximate and depend on conditions. Use the data rather than a single particle-spacing rule.
| Medium | State | Approximate speed / m/s | Interpretation |
|---|---|---|---|
| Air at 20 °C | Gas | 343 | Slowest measured medium here |
| Fresh water | Liquid | 1480 | Faster than air |
| Glass | Solid | about 5000 | Fast in a stiff solid |
| Vacuum | No matter | Not applicable | No sound transmission |
Use data to compare water and air.
Why should “sound always travels fastest in solids” be treated as a broad pattern rather than a definition?
Check your understanding
Answer from memory, then check against the suggested answer.
What produces a sound wave?
Why can sound not travel through a vacuum?
Do air particles travel from a loudspeaker to the ear?
Astronauts outside a spacecraft can see an impact but cannot hear it directly through space. Explain.
Explain why the statement “particles are closer in solids, so sound is always fastest in every solid” is incomplete.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A loudspeaker plays a sound that is picked up by a microphone across the room.
Explain how the sound travels from the loudspeaker to the microphone.
(3)Explain why sound cannot travel through a vacuum.
(1)Total for this question: 4 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., write a causal chain from source vibration to particle interactions to detector response.