Sound

Sound is a form of energy which causes sensation of hearing. Sound is produced by vibrating bodies. When a bell is struck with a metallic rod, it produces sound. We can sense the vibrations of the bell while it is producing sound by touching it. We can even observe that the sound ceases when the bell stops vibrating.

Experiment to verify that vibrating bodies produce sound

 

Experiment to verify that vibrating bodies produce sound
Experimental arrangement to show that vibrating bodies produce sound 

 

 

Consideration a tuning fork fixed to a rigid support as shown in Fig. 2. A pith ball suspended from a support is held close to one of the prongs of the tuning fork. When the fork is excited with a rubber hammer and if the pith ball is in contact with the fork, we observe that the ball is flicked away by the fork, indicating that the tuning fork which is producing sound is vibrating and the pith ball is flicked away by the vibrations of the fork. From this we may conclude that sound is produced by vibrating bodies. When we speak, the vocal chords present in a cavity of our throat, called larynx, vibrate and produce sound. The vibrating strings of a guitar, veena, etc. , cause the production of sound in the sounding boxes’ of these instruments.

types of sound

2 types of sound

 

  1. Transmission sound
  2. Longitudinal sound

 

Transmission of sound 

Sound propagates in the form of mechanical waves. So, it requires a material medium to propagate from one place to another. Sound can propagate through solids, liquids and also through gases. When an earthquake occurs, the shock waves produced at the point where the earthquake occurred travel in all possible directions. They travel through layers of the Earth which consist of solid rock materials as well as liquid water bodies. On reaching the surface of the Earth, these sound waves travel through air and reach our ears and produce the sensation of hearing. Thus, sound waves travel through solids, liquids and gases too.

If a person scratches one end of a long rail, the sound produced would almost instantly be heard by another person holding his ear to the rail at the other end. The same sound traveling through air would take longer (nearly 14 times more time!) To be heard over the same distance. Furthermore, its intensity would also be much lower. Thus, we find that sound travels faster in solids than in gases or air.

Consider two divers under water, separated by a considerable distance. If one of them produces a sound, the other can hear it after a certain time. Separated by the same distance, if these two persons are above water, and if one produces a sound, the other would take a little more time to hear it than the time taken under water. This indicates that sound travels faster in liquids than in gases or air.

If the two divers under water are stationed at two ends of a sufficiently long metal rod, and one of them taps the rod at his end, the other diver would find that he is able to hear the sound earlier if he holds his ear to the rod. This shows that sound travels faster in solids than in liquids.

Thus, velocity of sound is the highest in solids, less in liquids and the least in gases or air.

Longitudinal nature of sound waves in air

consider sound emitted by a loudspeaker with high intensity. When a piece of paper is held in front of the speaker, we observe that the paper is set into vibration by the sound emitted by the speaker. Thus, the sound produced by a vibrating body sets the surrounding layers of air (just as the piece of paper considered above) to vibrate.  These layers of air vibrate in a direction parallel to the direction of wave motion; thus forming longitudinal waves. Thus, sound travels in air in the form of longitudinal waves.

Sound Requires a Medium for Propagation

As discussed earlier, sound requires a material medium for its propagation and does not travel through vacuum. Consider a glass jar with an outlet at the bottom and an electric bell suspended from its lid, made of cork, by means of strings as shown in the Fig. The electrical connections (not shown in the figure) are made to the bell and it is switched on (switch not shown in figure).

 

Experimental arrangement of a belljar experiment
FIGURE 3 Experimental arrangement of a bell jar experiment 

 

 

As the bell rings we can hear the sound produced by the bell. Now the outlet of the jar is connected to a vacuum pump, and the air in the jar is gradually removed. As the air in the jar is gradually removed, we observe a gradual decrease in the loudness of sound and finally we cannot bear the sound. As the air is removed, vacuum is created in the jar, because of which we are unable to hear the sound. This shows that sound cannot travel through vacuum and requires a material medium for its propagation.

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