In the previous section we completed the discussion on Work, power and energy. In this section we will see the basic details about Sound.
We will see how sound is produced and how it is transmitted from it's source to our ears. Let us do two activities:
1. Take a tuning fork. Set it vibrating by striking it's prongs on a rubber pad. Bring it near the ear. A sound can be heard.
2. Take a rubber band. Stretch it and pluck it. It will vibrate. If it is brought near the ear, a sound can be heard.
■ We can produce sound by scratching, rubbing, blowing or shaking of objects.
• Humans produce sound by the vibration of 'vocal cords'. Animals and birds also have organs similar to vocal cords
♦ But the buzz of a bee is produced by the rapid to and fro movement of their wings. The movement of the wings causes air to vibrate, and we hear the buzzing sound
♦ The sound of a cricket is due to the scraping together of it's front wings.
• The sound of a drum is produced when it is struck with an object. When it is struck, it's membrane vibrates and produces sound
• When one or more of the strings of a violin or harp is plucked, the strings vibrate. This will cause some other parts of the violin or harp to vibrate. Thus sound is produced
• In the case of a flute or trumpet, the vibration of air inside them causes the sound.
■ Thus we see that, what ever method (scratching, rubbing, blowing or shaking of objects) we use for producing sound, vibration of objects is involved.
1. Consider a vibrating tuning fork. When the prongs move to the right, the air molecules near the right side of the prongs will get compressed. This is shown in fig.5.1(a) below:
• That means, a force is applied on those air molecules, and a 'compression zone' is produced .
2. These molecules will collide with the adjacent molecules further to the right. So the compressive force is transferred to the right. Thus another compression zone is produced.
3. The first compression zone will lose the energy and will become normal. The second compression zone will create a third compression zone further right.
4. When the third one is created, the second one will return to normal. Thus the compression zone moves outwards from the tuning fork.
5. But that is not all. After creating the first compression zone, the prongs will move towards the left. This is shown in fig.5.1(b) above.
• So a region of low pressure will be created on the right side.
♦ Rarefaction is the opposite of compression. The dictionary meaning of rarefaction is: Reduction in the density of something, especially air or a gas.
6. As the prongs are moving rapidly to and fro between left and right, 'zones of high pressure' and 'zones of low pressure' will be created alternately.
7. These alternate pressure zones are propagated outwards from the tuning fork. When they reach our ear, we receive these zones alternately. Thus we get the sensation of sound. This is shown in the fig.5.2 below:
Thus the energy from the prongs will reach our ears. In the fig.5.2,
• C indicates Compression. They are regions of high pressure
• R indicates Rarefaction. They are regions of low pressure
■ An important point should be noted at this stage:
• When the air molecules collide, there is only 'transfer of energy'. There is no 'material transfer'.
♦ That is., the air molecules do not get transferred from the prongs to the ears.
♦ Only the energy get transferred.
• Why do the molecules oscillate?
• That is., why do they first move to the right, then to the left and then again to the right and so on?
• That is., once they are given a push to the right by the prongs, why don't they continue travelling to the right?
■ The explanation can be written in steps:
1. The molecules first travel to the right due to the rightward push of the prongs.
2. But just after that, the prongs move leftwards creating a rarefaction. So the molecules move leftwards towards this rarefaction.
3. Again the prongs move rightwards, repeating the cycle. So the molecules also repeat the cycle, creating an oscillation.
• Such waves are called longitudinal waves. So we can write the definition of longitudinal waves:
■ Longitudinal waves are those waves in which particles oscillate parallel to the direction of propagation of the wave.
■ There is another type: Transverse waves. In this type, the particles oscillate in a direction perpendicular to the direction of propagation of the wave. We will learn about such waves in higher classes. The difference between the two types of propagation can be seen here. A video can be seen here.
We have seen the basic details about Longitudinal waves. In the next section, we will see some mathematical calculations associated with these waves.
We will see how sound is produced and how it is transmitted from it's source to our ears. Let us do two activities:
1. Take a tuning fork. Set it vibrating by striking it's prongs on a rubber pad. Bring it near the ear. A sound can be heard.
2. Take a rubber band. Stretch it and pluck it. It will vibrate. If it is brought near the ear, a sound can be heard.
■ We can produce sound by scratching, rubbing, blowing or shaking of objects.
• Humans produce sound by the vibration of 'vocal cords'. Animals and birds also have organs similar to vocal cords
♦ But the buzz of a bee is produced by the rapid to and fro movement of their wings. The movement of the wings causes air to vibrate, and we hear the buzzing sound
♦ The sound of a cricket is due to the scraping together of it's front wings.
• The sound of a drum is produced when it is struck with an object. When it is struck, it's membrane vibrates and produces sound
• When one or more of the strings of a violin or harp is plucked, the strings vibrate. This will cause some other parts of the violin or harp to vibrate. Thus sound is produced
• In the case of a flute or trumpet, the vibration of air inside them causes the sound.
■ Thus we see that, what ever method (scratching, rubbing, blowing or shaking of objects) we use for producing sound, vibration of objects is involved.
Propagation of sound
We have seen how sound is produced. Now we want to know how the sound reaches our ears from the point of origin.1. Consider a vibrating tuning fork. When the prongs move to the right, the air molecules near the right side of the prongs will get compressed. This is shown in fig.5.1(a) below:
 |
| Fig.5.1 |
2. These molecules will collide with the adjacent molecules further to the right. So the compressive force is transferred to the right. Thus another compression zone is produced.
3. The first compression zone will lose the energy and will become normal. The second compression zone will create a third compression zone further right.
4. When the third one is created, the second one will return to normal. Thus the compression zone moves outwards from the tuning fork.
5. But that is not all. After creating the first compression zone, the prongs will move towards the left. This is shown in fig.5.1(b) above.
• So a region of low pressure will be created on the right side.
♦ Rarefaction is the opposite of compression. The dictionary meaning of rarefaction is: Reduction in the density of something, especially air or a gas.
6. As the prongs are moving rapidly to and fro between left and right, 'zones of high pressure' and 'zones of low pressure' will be created alternately.
7. These alternate pressure zones are propagated outwards from the tuning fork. When they reach our ear, we receive these zones alternately. Thus we get the sensation of sound. This is shown in the fig.5.2 below:
 |
| Fig.5.2 |
• C indicates Compression. They are regions of high pressure
• R indicates Rarefaction. They are regions of low pressure
■ An important point should be noted at this stage:
• When the air molecules collide, there is only 'transfer of energy'. There is no 'material transfer'.
♦ That is., the air molecules do not get transferred from the prongs to the ears.
♦ Only the energy get transferred.
Wave nature of sound.
■ Though the molecules do not travel from the source of sound to the ears, they do travel with in a small distance. This travel, is in fact, an oscillation. That is., a 'to and fro' motion like in a pendulum.• Why do the molecules oscillate?
• That is., why do they first move to the right, then to the left and then again to the right and so on?
• That is., once they are given a push to the right by the prongs, why don't they continue travelling to the right?
■ The explanation can be written in steps:
1. The molecules first travel to the right due to the rightward push of the prongs.
2. But just after that, the prongs move leftwards creating a rarefaction. So the molecules move leftwards towards this rarefaction.
3. Again the prongs move rightwards, repeating the cycle. So the molecules also repeat the cycle, creating an oscillation.
• So the sound is propagated through air, by the oscillation of air molecules. We can call it a special type of wave propagation.
• In this type of waves, particles oscillate parallel to the direction of propagation of the wave motion.• Such waves are called longitudinal waves. So we can write the definition of longitudinal waves:
■ Longitudinal waves are those waves in which particles oscillate parallel to the direction of propagation of the wave.
■ There is another type: Transverse waves. In this type, the particles oscillate in a direction perpendicular to the direction of propagation of the wave. We will learn about such waves in higher classes. The difference between the two types of propagation can be seen here. A video can be seen here.
We have seen the basic details about Longitudinal waves. In the next section, we will see some mathematical calculations associated with these waves.
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