In a longitudinal wave, the oscillations / vibrations are in the same direction in which the wave travels.
Displacement is the distance one part of a wave is moved away from its rest position.
In a longitudinal wave, forwards displacement is positive and backwards displacement is negative.
Wavelength of a longitudinal wave is the distance between one compression (or rarefaction) and the next.

Keywords

Longitudinal wave - each part of a longitudinal wave oscillates forward and backward in the same direction in which the wave moves.
Compression - part of a longitudinal wave in which parts of the wave are squashed closer together is called a compression.
Rarefaction - part of a longitudinal wave in which parts of the wave are spaced further apart is called a rarefaction.
Wavelength - the wavelength of a longitudinal wave is the distance along the wave between one compression and the next.
Amplitude - the amplitude of a longitudinal wave is the maximum distance each part of the wave moves forward or backward.

Common misconception

Particles in a longitudinal wave move forward and backward between one compression and the next.

Demonstrate the movement of a candle flame in front of a loudspeaker set to a very low frequency with a wavelength of about 30 m (at 10 Hz).

Use a slinky spring to demonstrate longitudinal waves. Tie a piece of string around one turn of coil to show how each part of the spring moves as the wave travels forwards.

Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.

How does a loud speaker move to make a sound wave that moves forwards?

up and down

side to side

Correct answer: forwards and backwards

Q2.

How do air particles move as a sound wave moves forwards through the air?

side to side

Correct answer: forwards and backwards

forwards with the wave

Q3.

A sound wave is an example of a wave.

Correct Answer: longitudinal, mechanical

Q4.

What is the wave medium of a longitudinal wave travelling along a spring stretched across a table?

the air

Correct answer: the spring

the table

Q5.

What is the maximum displacement of a particle in a wave called?

wavelength

period

frequency

Correct answer: amplitude

Q6.

What is the name given to the number of complete wave cycles passing a point each second?

period

Correct answer: frequency

displacement

amplitude

Exit quiz

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6 Questions

Q1.

How do particles in a soundwave move?

Correct answer: they oscillate backwards and forwards and do not move along the wave

they oscillate backwards and forwards as they move along the wave

they move along the wave without oscillating

Q2.

How far do the particles in a soundwave vibrate?

they vibrate between one end of the wave and the other

they vibrate between one compression and the next

Correct answer: they vibrate less than the distance between one compression and the next

Q3.

What is the amplitude of a longitudinal wave?

the distance between two compressions

Correct answer: the maximum displacement of the wave

the distance between a compression and a rarefaction

the wavelength of the wave

Q4.

What is the distance between two wave crests on a displacement-distance graph equal to?

the amplitude

the frequency

the period

Correct answer: the wavelength

Q5.

What does a positive displacement in a longitudinal wave indicate about that part of the wave?

it is behind its rest position

Correct answer: it is in front of its rest position

it is at its rest position

it is above its rest position

Q6.

What does a negative displacement in a longitudinal wave indicate about that part of the wave?

Correct answer: it is behind its rest position

it is in front of its rest position

it is above its rest position

it is below its rest position