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Propagation of Sound

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

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

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

  2. 2

    Then show how to use it

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

  3. 3

    Add one concrete example

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

  4. 4

    Avoid this incomplete answer

    Wrong answer: Sound travels because air particles fly all the way to the ear. This is wrong because particles vibrate about their mean positions.

Definition

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

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.

Memory hook

Particles vibrate; sound travels.

Examples and method

Worked example

In a bell-jar experiment, the bell is heard clearly when air is present. As air is pumped out, the sound becomes weaker. This shows that sound needs a medium for propagation.

Method to apply

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

Diagram support

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

How CBSE asks it

Exams may ask why sound is not heard in space or ask students to label compression and rarefaction in a wave diagram.

Avoid common mistakes

Common confusion

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

Common wrong answer

Wrong answer: Sound travels because air particles fly all the way to the ear. This is wrong because particles vibrate about their mean positions.

Exam tip

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

Quick check

Why cannot sound travel through a vacuum?

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

Study the propagation of sound diagram carefully

Use the labelled diagram to keep propagation of sound clear in short answers and revision.

What this diagram makes clear

This diagram keeps the labels and direction of propagation of sound in the right order.

Where this helps in exams

Use this for labelled diagram work and short exam answers on propagation of sound.

Revision cue

Revise propagation of sound through the labels before writing the answer.

Answer writing and exam use

1-mark answer

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

2-mark answer

Propagation of sound is the travelling of sound energy through a material medium by successive compressions and rarefactions. Rule: Sound needs a material medium such as solid, liquid, or gas. It travels by transfer of disturbance from one particle to the next. A ringing bell inside a jar becomes faint when air is removed because fewer air particles are available to carry the sound.

3-mark answer

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. Rule: Sound needs a material medium such as solid, liquid, or gas. It travels by transfer of disturbance from one particle to the next. In a bell-jar experiment, the bell is heard clearly when air is present. As air is pumped out, the sound becomes weaker. This shows that sound needs a medium for propagation. Exams may ask why sound is not heard in space or ask students to label compression and rarefaction in a wave diagram. Wrong answer: Sound travels because air particles fly all the way to the ear. This is wrong because particles vibrate about their mean positions.
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