8th Class Science Sound Propagation of Sound

Propagation of Sound

Category : 8th Class

*       Propagation of Sound


Sound is propagated in the medium due to vibration produced in the medium. When an object starts vibrating in the medium, the air molecules in close contact with object also starts vibrating back and froth with the same frequency. These vibrating air molecules pass on their motion to the next layer of the air due to which they also starts vibrating. This process goes on. In this way the vibration reaches our ears and we hear the sound.

In human, sound is produced by the voice box present in their throat, which is also known as the larynx. The larynx contains two ligaments, known as vocal chord. The sound is produced by the vibration of the larynx. When we speak or sing, the frequency of the sound changes continuously. This happens due to the action of the muscles attached with the vocal chord. If the muscles attached to the vocal chord contracts and stretched, the vocal chord becomes tight and thin and the sound produced is of high frequency. On the other hand, if the muscles relax, the vocal chord becomes loose and the sound produced is of low frequency. The vocal chord of man is 20 mm and that of women is 15 mm. Thus, the voice of woman is softer than man. Hence, due to the difference in the length of the vocal chord, the voice of man, woman, and children are of different frequencies.


*         Sound Needs Medium for Propagation

The matter through which sound travel is known as medium. It can be solid, liquid orgas. The sound needs medium to travel. It cannot travel in vacuum, as vacuum do not have any molecules to vibrate. Thus, for propagation of sound material medium is necessary. If we go to the outer space, we cannot talk as we talk on the Earth. This is because there is no medium in the outer space. That is why the astronauts use wireless, i.e. radio waves to talk with each other in the outer space. The speed of the sound is different in the different medium. This is because, the density of different medium is different. It is fastest in solid and slowest in gas;The speed of sound is 340 m/s in air, 1500 m/sec in water and 5000 m/sec in air. But if we compare the speed of light with that of sound, we find that light travels faster than sound. For example, during rainy days, with thunder storm and lightning, we see the light first and hear the sound of thunder little later.





       Sound travels with different speed in different medium. The speed of sound is different due to the fact that, different medium has different density. In which one of the following medium sounds will travel fastest?

(a) Air

(b) Water

(c) Wood

(d) Steel

(e) None of these


Answer: (d)


The sound travels fastest in the medium having highest density. As steel has highest density, sound travels fastest in steel.



        Different persons produce different types of sound. This is possible due to the fact that each individual has vocal chord, which vibrates differently. What is the length of the vocal chord of man?

(a) 10mm

(b) 15mm

(c) 20mm

(d) 25mm

(e) None of these


Answer: (c)


The vocal chord of men and women are of different length. The length of vocal chord of men is 20 mm.

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