medium difficulty7 min

Propagation of Sound

Propagation of sound is the travelling of sound energy through a material medium by successive compressions and rarefactions.

In one line: Particles vibrate; sound travels.

Practice This Concept

Understand it

Sound cannot travel in vacuum because it needs particles of a medium to pass the disturbance. In air, vibrating particles push nearby particles closer to form compressions, then move back to form rarefactions. The particles only vibrate about their positions, while the sound disturbance travels forward.

Example

A ringing bell inside a jar becomes faint when air is removed because fewer air particles are available to carry the sound.

Rule to remember

Rule: Sound needs a material medium such as solid, liquid, or gas.

It travels by transfer of disturbance from one particle to the next.

See it worked out

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

Worked example

In a bell-jar experiment, the bell is heard clearly when air is present.

1As air is pumped out, the sound becomes weaker.
2This shows that sound needs a medium for propagation.

Method you can reuse

1State that sound needs a medium.
2Explain that source vibration creates compression.
3Then rarefaction follows.
4These regions travel through the medium to carry sound energy.

Sketch it in your answer

Draw alternate crowded regions labelled compression and spread-out regions labelled rarefaction along the direction of wave travel.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why cannot sound travel through a vacuum?

Think first, then show answer

Sound cannot travel through a vacuum because there are no material particles to vibrate and carry the sound disturbance forward.

Common wrong answer

Sound travels because air particles fly all the way to the ear.

Why it loses marks: Particles vibrate about their mean positions.

Don't mix this up

Students often say air particles travel from the bell to the ear. Actually, particles vibrate locally while energy travels.

Exam tip

For propagation answers, include three words: medium, compression, rarefaction.