Describe sound as a longitudinal disturbance
Compare local particle motion with the outward sound direction and distinguish a time graph from the path of air.
Core and Extended; 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
- Describe a vibrating sound source.
- Distinguish transverse and longitudinal directions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which relationship defines a longitudinal wave?
What you will learn
- Describe sound waves as longitudinal.
- Explain local oscillation parallel to propagation.
Compare two directions
In a sound wave in air, air particles oscillate back and forth parallel to the direction in which the disturbance travels. That parallel relationship makes the wave longitudinal. The sound pattern and energy can move outward while each particle stays near a local equilibrium position.
Successive regions become more closely spaced or more widely spaced as the disturbance passes. This is different from a transverse rope model in which particles move across the propagation direction. A particle arrow and a propagation arrow must therefore be labelled separately.
Do not turn the graph into a literal path
A sinusoidal sound-pressure or displacement graph can represent a longitudinal wave. Its drawn height is the plotted quantity, not a claim that air particles trace an up-and-down snake through the room. Read whether the horizontal axis is position or time.
A position snapshot shows different locations at one instant; a time trace follows a fixed point at successive times. Wavelength comes from a spatial repetition and frequency from temporal repetition. The wave's classification depends on particle motion relative to travel, not the pen's curve shape.
Pause and explain
Sound propagates horizontally in air. Which particle motion fits the longitudinal model?
Worked example
Sound travels to the right. Describe air-particle motion and explain the longitudinal classification.
Show the worked solution
- Air particles move locally back and forth.
- That motion is parallel to the rightward propagation direction.
- The parallel relationship defines a longitudinal wave.
Answer Local left–right oscillation parallel to the travelling disturbance.
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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 particle motion for rightward sound propagation.
Give me a hint
State both local motion and its relation to wave travel.
Compare my reasoning
- The disturbance travels rightward.
- Particles oscillate locally left and right.
- Their oscillation is parallel to propagation.
Local left–right vibration parallel to travel.
Look for these in your work
- Use local oscillation.
- Name the parallel relationship.
Why is an air-pressure sine graph not evidence of transverse air motion?
Give me a hint
Read what the vertical axis represents.
Compare my reasoning
- The height represents pressure variation.
- It is not the particle's vertical coordinate.
- Longitudinal sound can have a sinusoidal pressure trace.
The plotted height is pressure, not a transverse particle path.
Look for these in your work
- Identify the axis meaning.
- Retain longitudinal motion.
A sound pulse travels 30 m through still air. Must an individual air particle travel 30 m?
Give me a hint
Separate the moving pattern from local particle positions.
Compare my reasoning
- The disturbance and energy propagate.
- Particles oscillate near their local positions.
- No 30 m bulk particle transport is required.
No; the pulse propagates while individual particles oscillate locally.
Look for these in your work
- Distinguish pattern and material.
- Avoid replacing sound with an air current.
Common mistakes
- Particles do not flow with the wave to the receiver.
- Graph height is a plotted quantity.
- Label vibration and propagation separately.
What can you explain now?
Does a sine-shaped graph prove sound is transverse?
Compare with the explanation
No.
The graph can plot pressure or longitudinal displacement rather than the particles' physical path.
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Make it stick
Explain longitudinal sound and correctly interpret a pressure or displacement graph.
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.