Sound Waves Need A Medium For Their Propagation: Explanation and Experiment

In our daily lives, we get to hear a variety of sounds, be it pleasant, unpleasant, soft, loud, heavy, or shrill. Sound is something without which our life is impossible. In fact, everything around us is associated with sound waves- The chirping of birds, the beeping of an alarm clock, drilling of machines, playing the guitar, and so on, everything produces sound. But we hardly wonder about how do we hear these sounds? What really is sound? We are able to perceive all these sounds from our environment because we have a highly specialized sense organ- the Ears that collects the sound waves from around us and converts them into the vibrations of the Eardrum which is then carried to the inner ear from where the impulse is produced and transmitted to the brain. Let’s understand how sound waves need a medium for their propagation. 

Key Takeaways: Sound waves, Propagation of sound, Bell jar, speed of sound is different in different media.

Also check: Potential Energy

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How Does Sound Reach Our Ears?

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We know that a sound is a form of energy. This energy travels in the form of waves, which are called sound waves. A sound wave consists of vibrations of particles of the medium through which it travels. If there is no medium, the vibrations from an object will not travel through it. The source of sound always produces some kind of vibrations. These sound waves travel in the air as sound waves. It is because of the propagation of sound waves due to which the sound reaches our ears. Sound always requires a material medium for its propagation.

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Can We Hear Any Sound On The Moon?

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The answer is no. We cannot hear any sound on the moon because on the surface of the moon there is no availability of atmosphere through which sound can travel. Sound always requires a medium for propagation. Astronauts who land on the moon talk to each other using wireless headsets using radio waves.

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We can demonstrate the same with the help of a simple Bell jar experiment.

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Bell Jar Experiment

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Bell jar experiment is used to demonstrate that sound needs a medium to travel.

1. Apparatus- Airtight Bell jar, electric Bell, and a vacuum pump.

2. Procedure-

• Take an Airtight Bell jar and fit an Electric Bell inside it.
• Switch on the electric connection and hear the sound of the Electric Bell.
• Now, pump out all the air inside the Bell jar using the connected vacuum pump.

3. Observation-

As the air is being extracted out by the vacuum pump, the sound of the Bell begins diminishing. When all the air is extracted from the Bell jar the sound of the Bell cannot be heard but the vibrations of the Bell can be seen through the Bell jar.

4. Conclusion-

This experiment proves that sound requires a medium for propagation. As long as there was air inside the Bell jar, the sound of the Electric Bell could be heard but when a vacuum is created inside it, (no medium) the sound becomes inaudible.

The air inside the Bell jar provided a medium for sound from the Electric Bell to reach the listener.

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Propagation Of Sound In Different Media

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Sound can travel in all mediums, solids, liquids, and gases.

• Put your ears on a bench and ask anyone of your friends to tap or scratch on the surface of the bench. You will be able to hear a clear and loud sound. This shows that sound travels in solids.

• Place a bell inside a tub of water. Now put your ear just on the surface of the water and make the bell ring. You will hear the clear sound of the bell. This proves that liquids propagate sound waves.

• While we are talking to someone the sound of the person speaking travels through the air and then reaches the ear. For the same reason, we are able to hear our own voices. Thus, sound travels in gases as well.

But the speed of sound is different in different media. Sounds travel the fastest in solids. The speed of sound in liquids is lesser than in solids but greater than in gases.

Read also: Speed of sound

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Examples Showing Speed Of Sound Is Different In Different Media

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Some common phenomena proving that sound travels the fastest in solids than in air are:

• A person living near the railway track often presses his ear against the steel rail to guess whether a train is coming or not. The reason is that the vibrations produced by the moving wheels of the train travel much faster through the steel rail than through air. So the sound is heard through track much before it is heard through air. Thus the person by hearing sound through the steel rail gets an indication of the coming of train well before its sound is heard.
• If the sound is produced at one end of a very long-steel bar, two sounds are heard at the other end. One which reaches first is propagated through steel and the other which is heard later is through the air.

Read also: Transverse and Longitudinal Waves

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Things to Remember

• Sound travels in the form of sound waves produced by the vibrations of the source.
• Sound always needs a material medium for its propagation.
• Sound cannot travel in a vacuum.
• Sound travels in all mediums- solids, liquids, and gases.
• The speed of sound is fastest in solids. In liquids, the speed of sound is greater than in gases but lesser than in solids. The speed of sound is least in gases.

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