Maharashtra Board Class 8 Science Chapter 15 Sound PDF Download

15. Sound

How is sound produced?

Production of Sound

We have learnt that sound can be generated from a vibrating object. With an example of tuning fork, we will now learn about how a sound is produced due to such vibrations. A photograph of tuning fork is shown in Figure 15.1.

A tuning fork is made of two prongs and a stem. Keeping the stem steady, if the prongs are struck, they start vibrating.

Figure 15.2 (a) shows a stationary tuning fork. To show the state of air around the tuning fork, vertical lines are used. Here, the vertical lines are equally spaced. It indicates that the average distance between the air molecules is the same everywhere and the average air pressure in three regions 'A', 'B' and 'C' is also the same.

If the stem is kept steady and the prongs are struck, the prongs are set into vibrations. It means that they are set into periodic motion in forward and backward direction. We will see, step by step, the result of such a motion.

If during the vibration, the prongs of the tuning fork go away from each other, as shown in Figure 15.2 (b), the air outside the prongs is compressed and the pressure there increases. Such a state of high pressure is created in region A in the figure. The region in which air is at high pressure and high density is called compression. In the next step of vibration, the prongs of the fork come close to each other, as shown in Figure 15.2 (c). In this case, the air molecules near the prongs get more space to move away from each other. As a result, the air pressure in this region (Region A) decreases. Such region in which air is at low pressure and low density is called rarefaction.

However, the air molecules in this region, which were in compressed state earlier (Figure 15.2 (b), region A) transfer their energy to the air molecules in the next region (region B). So, the air in that region goes to compressed state (See Figure 15.2 (c), region B). Such a periodic motion of the prongs creates compression and rarefaction in the air and these are propagated away from the prongs. These are nothing but the sound waves. When these waves reach our ear, the ear-drum vibrates. Accordingly, specific signals reach the brain and we get a sense of hearing a sound.

Teacher's Note

When you hit a drum, the drum skin vibrates and makes sound. Just like that, a tuning fork makes sound by vibrating back and forth.

Exam Trick

Remember: Compression = air pushed together (like squeezing). Rarefaction = air spread out (like stretching). Sound needs both to travel.

Points to Remember

Sound is made when things vibrate back and forth.
Compression is when air is pushed together and gets denser.
Rarefaction is when air spreads out and becomes less dense.
Sound moves as waves made of compressions and rarefactions.
Our ear drum vibrates when sound waves reach it.

Propagation Of Sound and Medium

We have learnt in the sixth standard that sound travels through some material medium like solid, liquid or gas and reaches us. But what if such medium does not exist between the source of sound and our ear?

Sound generation and propagation needs medium like air. A simple experiment can prove this. The experimental arrangement is shown in Figure 15.3. In this experiment, a vacuum tight bell jar is placed on a smooth horizontal surface. The bell jar is connected to a vacuum pump via a tube. We can remove air inside the bell-jar, using the vacuum pump. As shown in the figure, the bell jar contains an electric bell, which is connected to the power supply through the lid of the bell jar.

At the start of experiment, the vacuum pump is off and the bell-jar contains air. If the key to electric bell is now switched on, the bell start ringing and the ringing can be heard from outside the bell jar. Now, if the vacuum pump is switched on, it starts removing the air from the bell jar. As the quantity of air inside the bell jar decreases, the level of ringing sound heard outside also decreases. If the pump is operated for sufficiently long period, the quantity of air like inside the bell jar will be low. At this point the level of ringing sound will also be very low. This experiment demonstrates that sound generation and propagation needs a medium.

If the air inside the bell jar is totally removed, will we be able to hear the sound of ringing bell? Why?

Two astronauts on the moon talking to each other directly, will be unable to listen to each other, even if they are very close to each other. The moon does not have atmosphere. Since there is no medium which is necessary for generation and propagation of sound, between the astronauts, direct sound propagation between them is not possible. Therefore, the astronauts use some technology like the one used in our cell-phones to communicate with each other. The waves used in cell-phone do not need any medium for propagation.

Teacher's Note

Sound needs air to travel, just like how we need air to breathe. In a vacuum with no air, sound cannot travel.

Exam Trick

Remember: No air = no sound. Sound needs a medium like air, water, or solid to travel. That is why astronauts use radios on the moon.

Points to Remember

Sound needs a medium to travel through.
Medium can be air, water, or any solid material.
Without a medium, sound cannot be heard.
In space or on the moon, there is no air so no sound travels.
Astronauts use radios because sound waves cannot travel in empty space.

Frequency Of Sound Waves

Using figure 15.2, we learnt about how the vibrations of tuning fork result in generation of compressions and rarefactions in air. More detail observation shows that, the actual variation in the air density and pressure are as shown in Figure 15.4. If any object vibrates in the air, such sound waves are produced in the air.

As shown in the Figure 15.4, one compression and one rarefaction together forms one cycle of the wave. The number of cycles formed in the air in one second will be decided by the number of times the prongs of the tuning fork (or any other vibrating body) vibrates back and forth i.e. the number of vibrations of the tuning fork in one second.

Number of such cycles that are produced in the air (or other medium) per second is called as the frequency of the sound wave. The frequency is measured in Hertz. If one cycle is completed in one second, the frequency is said to be 1 Hz. For example, the tuning fork in Figure 15.1 shows its frequency to be 512 Hz. It means that the fork vibrates 512 times per second. These vibrations will set 512 cycles of compression and rarefaction in the air, per second. Thus, the sound generated by the fork will have a frequency of 512 Hz. The frequency of a tuning fork is decided by the dimensions of the prongs (length, thickness) and the material used for making the fork.

Take 6-7 glass cups. Arrange them in a line and fill them with water with gradually increasing water level from one end to other. Take a pencil and strike the cups sequentially. The sound generated by each cup will be different. Why it is so? When a cup is struck, waves are set up in the air column above the water level in the cup. The frequency of the generated wave depends on the height of the air column inside the glass cup. Since the water level in each glass is different, the height of the air column in each glass is also different. Therefore, the frequency of sound generated by each glass cup will also be different. So, the sound generated is different.

An 'app' for measurement of sound frequency may be available on cell-phones. With the help of your teacher, use the app to measure the frequency of the sound generated from each glass cup. Do you observe any relation between the frequency of generated sound and the height of the air column in the glass-cup? This is your simple 'Jaltarang'! Can this experiment be performed with stainless steel pots of different sizes?

Teacher's Note

Jaltarang is an Indian musical instrument made with water-filled cups. Higher water level gives lower frequency, and lower water level gives higher frequency.

Exam Trick

Remember: Frequency = number of cycles per second. Higher frequency = higher sound. Unit is Hertz (Hz). One vibration per second = 1 Hz.

Points to Remember

One compression and one rarefaction make one complete cycle.
Frequency is the number of cycles made in one second.
Frequency is measured in Hertz (Hz).
Higher frequency means more vibrations per second.
The tuning fork dimensions decide its frequency.

Sound and Music

From the above activity, it is clear that if the frequency of sound wave is changed, different sound is produced. Sound waves of different frequencies produce different sound notes. In the field of music, various musical instruments are used for creation of sound notes. This includes instruments like sitar, violin, guitar which use strings for production of sound and instruments like flute, shehnai which use air blown into pipes for the production of sound.

In string based instruments, the frequency of vibration of the string is changed by changing the tension on the string and/or by changing the vibrating length of the string using fingers. This results in generation of different notes. In musical instruments like flute, the holes on the flute are opened or closed to change the length of vibrating air column in the flute. The frequency of waves, therefore, changes and it results in the production of different notes. In flute, different notes can be generated by changing the way of air-blowing also.

Teacher's Note

In Indian music, Sa Re Ga Ma Pa Dha Ni are seven notes. Each note has a different frequency. Higher frequency notes are higher pitch.

Exam Trick

Remember: Change tension on string = change frequency = change note. Tighter string = higher note. Looser string = lower note.

Points to Remember

Different frequencies make different sound notes.
String instruments change frequency by changing string tension.
Wind instruments change frequency by changing air column length.
Higher frequency = higher pitched note.
Lower frequency = lower pitched note.

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