Describe sound compressions and rarefactions
Read pressure and particle-density regions while keeping local vibration separate from the moving pattern.
Extended only (Supplement); Physics 0625, 2026–2028. Follow the selected spectrum or sound section. Diagrams illustrate stated ideal models and supplied data. Written practice is self-checked.
Before you begin
- Explain longitudinal particle oscillation.
- Distinguish a position snapshot from a time trace.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which region has greater particle separation than the undisturbed medium?
What you will learn
- Describe compression and rarefaction in longitudinal sound.
- Identify wavelength using matching successive regions.
Name two alternating regions
A compression is a region where particles are closer together, giving higher pressure and density than the undisturbed medium in the model. A rarefaction has particles farther apart, giving lower pressure and density. These regions travel with the disturbance while particles oscillate locally.
A rarefaction is not an empty vacuum gap. It is a lower-density region of the same medium. Pressure and density vary about their undisturbed values rather than requiring negative absolute pressure.
Measure a full spatial repeat
Wavelength is the distance between successive matching compressions or successive matching rarefactions at one instant. From a compression centre to the next rarefaction centre is half a wavelength in the sinusoidal model. Count intervals between centres rather than the number of marked centres.
A pressure-time trace at one position instead gives the period between successive matching peaks. Its horizontal time interval is not a wavelength until combined with the propagation speed. The physical local particle motion stays parallel to the travelling pattern.
Pause and explain
Four successive compression centres span 0.90 m. How many wavelengths are between the first and fourth?
Worked example
Four successive compression centres occupy 0.90 m from first to fourth. Find wavelength.
Show the worked solution
- Four centres define three matching intervals.
- Each interval is one wavelength.
- λ = 0.90/3 = 0.30 m.
Answer 0.30 m.
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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.
Describe a compression and a rarefaction.
Give me a hint
Compare particle spacing, pressure and density with the undisturbed medium.
Compare my reasoning
- Compression has closer particles and higher pressure/density.
- Rarefaction has wider spacing and lower pressure/density.
- Both are regions of the same material disturbance.
Compression: closer/higher; rarefaction: farther/lower, relative to the undisturbed medium.
Look for these in your work
- State both regions.
- Retain material in a rarefaction.
Three successive rarefaction centres span 0.80 m. Find wavelength.
Give me a hint
Count the two complete intervals.
Compare my reasoning
- Three centres give two intervals.
- 0.80 m = 2λ.
- λ = 0.40 m.
0.40 m.
Look for these in your work
- Count intervals.
- Use a spatial distance.
A pressure-time trace has successive peaks 0.0020 s apart. Is that directly a wavelength?
Give me a hint
Read the horizontal axis before naming the repetition.
Compare my reasoning
- The axis measures time.
- The interval is the period at one position.
- A wavelength needs spatial information or the wave speed.
No; 0.0020 s is a period, not a length.
Look for these in your work
- Identify the period.
- State what additional information is needed.
Common mistakes
- Rarefaction is not a vacuum.
- Count intervals rather than centre positions.
- A time interval is not a spatial wavelength.
What can you explain now?
What changes in a compression relative to undisturbed air?
Compare with the explanation
Pressure and particle density are higher.
Particles are more closely spaced in the compression region.
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Make it stick
Explain compression and rarefaction and distinguish full spatial repeats from temporal periods.
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.