medium difficulty5 min

Longitudinal vs Transverse — Sound as a Wave

Sound in air is a longitudinal mechanical wave because medium particles vibrate parallel to the direction in which the wave travels.

In one line: Longitudinal means along the line of travel.

Practice This Concept

Understand it

In a longitudinal wave, particles move back and forth in the same line as wave travel, producing compressions and rarefactions. In a transverse wave, particles move perpendicular to the direction of wave travel, producing crests and troughs. Sound needs a medium, so it is also called a mechanical wave.

Example

A slinky pushed and pulled along its length shows longitudinal motion similar to sound in air.

Rule to remember

Rule: Longitudinal wave has particle vibration parallel to wave direction; transverse wave has particle vibration perpendicular to wave direction.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1If a slinky is pushed forward and pulled back along its length, close and spread regions move along the slinky.
2This models sound because the particle motion is parallel to the wave direction.

Method you can reuse

1Check the direction of wave travel.
2Check the direction of particle vibration.
3If both are parallel, classify it as longitudinal.
4If perpendicular, classify it as transverse.

Visual tip

A comparison diagram may show compressions and rarefactions for longitudinal waves and crests and troughs for transverse waves.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is sound in air called a longitudinal wave?

Think first, then show answer

Sound in air is called a longitudinal wave because air particles vibrate parallel to the direction in which the sound wave travels.

Common wrong answer

Sound is transverse because wave diagrams look wavy.

Why it loses marks: The diagram is only a representation; actual air particle motion is longitudinal.

Don't mix this up

Students often draw crests and troughs for sound in air without explaining compressions and rarefactions.

Exam tip

When asked why sound is longitudinal, compare particle vibration direction with wave travel direction.