Explain why sound needs a material medium
Use the evacuated-bell model carefully and distinguish reduced coupling from a quantitative proof of perfect vacuum.
Exam scope and assessment notes
Core and Extended · Physics 0972, 2026–2028. Follow the selected spectrum or sound section. Diagrams illustrate stated ideal models and supplied data. Written practice is self-checked. Human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Describe longitudinal sound in air.
- Explain local particle interaction.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What supplies the sound transmission mechanism in air?
What you will learn
- State that sound needs a medium.
- Interpret reduced air transmission without ignoring solid paths or source changes.
Supply particles for the disturbance
Sound is transmitted by mechanical disturbances in a material such as air, water or a solid. Neighbouring particles interact as the wave passes. In a true vacuum there is no material along the path to carry this sound disturbance.
A visible vibrating bell inside a chamber can become harder to hear as air is removed. Light from the bell still reaches an observer because electromagnetic radiation can cross vacuum. The two propagation requirements are different.
Interpret the apparatus honestly
An evacuated-bell demonstration supports the need for a medium when the source continues operating while the air-transmitted sound weakens. Supports, wires and chamber walls can still transmit some vibration. Pump noise or a changing source may also affect observations.
A quiet result does not measure a perfect vacuum or prove the source stopped vibrating. A good description compares the source's operation, the air condition and the received sound while acknowledging other paths. The model is explanatory, not a claim of practical competence from reading it.
Pause and explain
Why can a bell remain visible while its air-transmitted sound weakens during evacuation?
Worked example
A vibrating bell remains visible but becomes much quieter as air is removed. Explain and state one limit.
Show the worked solution
- Less air reduces the air sound-transmission path.
- Light can still reach the observer through vacuum.
- Supports or residual air can retain other sound paths.
Answer Sound needs a medium; remaining support paths or residual air limit the demonstration.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Name three media that can transmit sound.
Give me a hint
Use a gas, a liquid and a solid example.
Compare my reasoning
- Air is a gas medium.
- Water is a liquid medium.
- A metal bar is a solid medium.
Air, water and a metal bar.
Look for these in your work
- Include material examples.
- Exclude vacuum.
Explain the quieter evacuated-bell observation with a still-vibrating source.
Give me a hint
Follow the altered air path while retaining source motion.
Compare my reasoning
- The source continues vibrating.
- Removing air reduces that mechanical path.
- Less air-transmitted sound reaches the receiver.
Reduced air transmission weakens the received sound.
Look for these in your work
- Keep the source operating.
- Link the change to the medium.
Some vibration is still detected through a chamber support after air removal. Does that refute the need for a medium?
Give me a hint
Identify whether a material path remains.
Compare my reasoning
- The support is a solid.
- It can transmit mechanical vibration.
- A remaining solid path is still a medium.
No; the solid support supplies a remaining material path.
Look for these in your work
- Identify the solid route.
- Avoid claiming a perfect vacuum from silence.
Common mistakes
- Quiet does not prove no source vibration.
- Supports can transmit mechanical disturbances.
- Seeing the bell uses light, not sound.
What can you explain now?
Can an air sound wave cross a truly empty path?
Compare with the explanation
No.
A mechanical sound disturbance needs a material medium along its path.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain medium-dependent sound and interpret an evacuated-source demonstration with its apparatus limits.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.