Sound Needs a Medium to Propagate

Introduction

Sound is produced by vibrating objects. After production, sound travels from the source to the listener through a material medium.

The material through which sound travels is called a medium. Sound can travel through solids, liquids, and gases, but it cannot travel through vacuum.

Sound needs a material medium to propagate.

What is a Medium?

A medium is the material through which sound travels from one place to another.

The medium may be:

• A solid, such as wood, metal, or glass
• A liquid, such as water
• A gas, such as air

Medium = Material through which sound propagates

Why Does Sound Need a Medium?

Sound travels by the vibration of particles of a medium. When a source of sound vibrates, it makes nearby particles of the medium vibrate.

These vibrating particles transfer the disturbance to neighbouring particles. In this way, sound travels from the source to the listener.

If there are no particles, the sound disturbance cannot be transferred. Therefore, sound cannot travel without a material medium.

What is Vacuum?

A space where there is no matter or material medium is called vacuum.

In vacuum, there are no particles to carry sound vibrations. Therefore, sound cannot propagate in vacuum.

Sound cannot travel through vacuum.

Vacuum Bell Jar Experiment

The vacuum bell jar experiment is a common experiment used to show that sound needs a medium to travel.

In this experiment, an electric bell is placed inside a glass bell jar. The bell is switched on and sound is heard. Then air is gradually removed from the jar using a vacuum pump.

As air is removed, the sound becomes fainter. When almost all air is removed, almost no sound is heard, although the bell can still be seen ringing.

Figure

Explanation of the Figure

The figure shows an electric bell placed inside a bell jar. Initially, air is present inside the jar. When the bell rings, sound travels through air and reaches our ears.

When air is slowly removed from the bell jar using a vacuum pump, the number of air particles inside the jar decreases. As a result, the sound becomes fainter.

When a near vacuum is created, almost no sound is heard, even though the bell is still ringing. This shows that sound needs a medium to propagate.

Observation

• When air is present inside the bell jar, sound is heard clearly.
• When air is gradually removed, the sound becomes weaker.
• When almost all air is removed, almost no sound is heard.
• When air is allowed back into the jar, the sound becomes clear again.

Conclusion from the Bell Jar Experiment

The vacuum bell jar experiment shows that sound cannot travel in vacuum.

Sound needs a material medium such as solid, liquid, or gas for propagation.

Without medium → No sound propagation

Why Can Sound Not Travel in Vacuum?

Sound travels through the vibration of particles. In vacuum, there are no particles.

Since there are no particles to vibrate and pass the disturbance forward, sound cannot travel through vacuum.

Sound in Outer Space

Outer space is almost a vacuum. Therefore, sound cannot travel directly through outer space.

Astronauts in space cannot directly hear each other speaking during a spacewalk. They also cannot directly hear the sound of metal objects clanking together.

Astronauts communicate with each other using special communication devices fitted in their spacesuits.

Can Sound Travel Through Air?

Yes. Air is a gas and contains particles. These particles vibrate and transfer sound from the source to the listener.

Most sounds that we hear in daily life reach our ears through air.

Can Sound Travel Through Solids and Liquids?

Yes. Sound can also travel through solids and liquids because they also contain particles.

• Sound travels through solids such as wood, metal, and glass.
• Sound travels through liquids such as water.
• Sound travels through gases such as air.

Examples from Daily Life

• We hear a bell because sound travels through air.
• We hear a teacher’s voice because sound travels through air.
• We can hear knocking through a door because sound travels through a solid.
• We can hear sounds underwater because sound travels through water.
• Astronauts cannot directly hear sounds in space because space has no proper medium.

Important Terms

1. Medium

The material through which sound propagates is called a medium.

2. Vacuum

A space where there is no material medium is called vacuum.

3. Propagation of Sound

The travelling of sound from its source to the listener is called propagation of sound.

4. Vacuum Bell Jar Experiment

An experiment used to show that sound cannot travel without a material medium.

Important Points

• Sound needs a material medium to propagate.
• Sound can travel through solids, liquids, and gases.
• Sound cannot travel through vacuum.
• A medium contains particles that transfer sound vibrations.
• Vacuum is a space without matter or material medium.
• The vacuum bell jar experiment proves that sound needs a medium.
• As air is removed from the bell jar, the sound becomes fainter.
• In near vacuum, almost no sound is heard.
• Outer space is nearly vacuum, so sound cannot propagate there.
• Astronauts communicate in space using special devices.

Conclusion

Sound needs a material medium to propagate. It travels through the vibration of particles of the medium. Since solids, liquids, and gases contain particles, sound can travel through them. However, vacuum has no particles, so sound cannot travel through it. The vacuum bell jar experiment clearly shows that sound cannot propagate without a medium.