Propagation of Sound- Physics Guide for Class 8

Propagation of Sound- Class 8 Science Guide

Information about Propagation of Sound


Propagation of Sound


Class 8


Class 8 Physics

Topics Covered

  • Propagation of Sound
  • Speed of Sound in Air, Liquid and Solids
  • Propagation of Light wave

Propagation of Sound

  1. Unlike the earth, the moon has no atmosphere. Even if a loud explosion were to take place at any point on the moon, it would not be heard even at nearby points. Why?
  2. The Earth's atmosphere extends to a height of nearby 200 km above its surface. There may be many 'explosions' and 'noises' taking place in the outer space but we do not hear them. Why?
It has now been established that sound needs a medium for its propagation. It cannot propagate through vacuum.
It can, however, propagate through all the three states of matter, i.e. gases, liquids and solids.

We can verify the fact - that a material medium is necessary for propagation of sound - by the following activity. 

Activity 1
  • Take a small bell jar having a rubber cork fitted in its neck.
  • Drill a small hole centrally through this rubber core.
  • Suspend a small bell inside the jar with the help of a glass rod.
  • Let the upper end of the rod project out from the hole in the rubber cork and fit this cork firmly in the neck of the bell jar.
  • Close the small opening around the glass rod with an adhesive solution so that the rubber cork closes the jar's neck in an 'air tight' way.
  • Shake the jar so that the bells starts vibrating.
You will be able to hear the sound of the bell quite clearly.
  • Now, connect the bell jar to a vacuum pump and start evacuating the air from it.
You will no longer hear the sound of the bell when the jar is shaken. You may see the bell vibrating but you do not hear its ringing sound. This is because of the absence of air in the jar.

This shows that sound needs a medium for its propagation. The above activity shows that we could hear the sound of the bell when the air was there. We can, therefore, conclude that sound can travel through gases.
What about liquids and solids?
  • As in the case of gases, sound can propagate through both liquids and solids.
  • In fact, it propagates faster through liquids than it does in the case of gases.
  • It propagates even faster through solids. 

Activity 2
  1. Take two small stone pieces and strike them together. You will hear a sound.
  2. Now, take a tub full of water. If you press your ear against the tub wall and strike the stone pieces under water, you get a sound again.
  3. This sound reaches your ear by propagating through water. 

Speed of sound depends on the:
  1. elasticity
  2. the density of the medium through which it propagates.
In case of air, it also depends upon the temperature of air. 

Activity 3
  1. Take a long (about two metres) solid smooth aluminium rod with its ends well-rounded off and smoothened.
  2. Let one friend keep one end of this rod near her/his ear and let the other friend strike some other metal object lightly near the other end of the rod.
  3. The first friend will then hear a sound but it may not be heard by another friend standing near her/him. 
  4. This sound must have then propagated through the solid aluminium rod.

Scientists are now able to measure the speed of sound through air (as well as other gases) and through water (as well as other liquids).
  • The speed of sound through air is about 330 m/s, whereas its speed through water is close to 1,500 m/s. 
  • Sound, thus, propagates much faster through liquids as compared to gases. 
  • People, living near railway tracks know well in advance about an approaching train. 
  • The sound, produced by the wheels of an approaching train, propagates much faster through the (solid) track and can, therefore, be heard and detected much before it is heard through the air. 
  • Precise measurements show that the speed of sound in solid is more than that in liquids. It is least for the gases. Thus, speed of sound in solid is greater than its speed in liquids. The latter, in turn, is greater than its speed in gases. 

Light Propagates faster than Sound

All of us have, at some time, been shaken and frightened by the roaring and thundering sounds that follow flashes of lightning. The thundering sounds are always heard a little after seeing the flash of lightning. Why?
  • In early stages, people thought that we hear the thunder, a little after seeing the lightning, simply because the thunder (perhaps) occurred a little later. We now know that this is not the case.
  • Both lightening and thunder really occur together. The reason, for the delay in hearing the thunder, is due to the difference in the speeds of sound and light.
  • Measurements, of the speed of light and sound, tell us that light propagates very fast as compared to sound. The speed of sound in air is nearly 330 m/s (or 0.33 km per second); the speed of light (in vacuum) is as high as (nearly) 3 lakh kilometres per second.
  • Light, thus, propagates nearly a million times faster than the sound. Hence, when lighting and thunder occur together, the flash of lighting (i.e. light) reaches us in practically zero time.
  • The sound of the thunder, however, takes a few seconds to reach us. We, therefore, hear the thunder a little later after seeing the flash of light. 

The human ear has a special feature. This is its property of persistence of hearing. When a given sound reaches the human ear and affects the ear drum, its effect 'stays on' in the ear drum for nearly 1/10th, i.e. 0.1 of a second. This helps us to get a 'feeling of continuity' while hearing one sound after another. 
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