Sound is Produced by a Vibrating Body. Try to touch the school bell when not in use. What do you feel? Again touch it when it is producing sound. Can you feel its vibration? Yes, you feel the effect of vibration! The to and fro or back and forth motion of an object is termed as vibration. When a tightly stretched band is plucked, it vibrates and produces sound. When it stops vibrating, it does not produce any sound

 

Let us try to observe this with an activity!

 

Take a metal dish. Pour water into it. Strike it at its edge by a spoon. Do you hear a sound? Yes, you will hear a sound! Again strike the dish and then touch it. You will feel the dish vibrating! Now strike the dish again. Look at the surface of the water.

 

You will find small waves, which are due to the vibration, produced the striking of the spoon with the dish. From this you find that the vibrating body produces sound In some cases, the vibrations are easily visible to us. But in most cases, their amplitude is so small that we cannot see them. However, we can feel them.

 

There are many musical instruments that are played by vibrating their parts, the manjira (cymbals), the ghatam, and the not (mud pots), and the Kartal are some examples. These instruments are commonly used in many parts of our country.

 

Sound needs a medium for propagation: Air

 

When you call your friend from a distance, your friend is able to hear your voice but we know that we can�t hear a clock ticking in a bell jar placed in a vacuum.

 

To understand the propagation of sound, we will perform some activities. Remember that sound is produced through vibrations.

 

- Take a metal glass tumbler
- Make sure that it is dry
- Place a cell phone in it

 

• Ask your friend to give a ring on this cell phone from some other phone

 

You can hear the ring. Now put your mouth on the opening of the tumbler. Ask your friend to give a ring again, listen to the ring while sucking the air from the tumbler. As you suck the air, the volume of the heard sound keeps reducing and if you are able to suck all the air from the tumbler i.e. you are able to create a vacuum in the tumbler, you will not be able to hear any ringing sound whatsoever.

 

And when you remove the tumbler from your mouth the volume of the heard sound increases again. Why does this happen? This is because sound needs a medium to travel, so when the air in the tumbler is reduced, the loudness of the sound also reduced. When the air is completely removed from the tumbler i.e. there is a vacuum, no sound is heard because the sound does not travel through the vacuum.

 

Sound needs a medium for propagation: Liquid

 

Does sound travel through liquids? Let us perform an activity:

 

1. Take a small bell in one hand.
2. Shake this bell inside the water.
3. You will be able to listen to the sound.
4. If you place your ear gently on the surface of the water you will still be able to listen to the sound of the bell. But there was no air near the bell and still, you could hear the ring of the bell.

 

This indicates that sound can travel through liquids.

 

Sound needs a medium for propagation: Solid

 

Does sound travel through solids?

 

Take a meter scale or a long metal rod. Hold one of its ends near to your ear. Ask your friend to gently scratch or tap at the other end of the scale. Can the people standing nearby hear the sound clearly? No, but you can hear this sound. You can hear the sound because you have placed the scale near your ear. Thus we can say that sound travels through the scale and reaches you.

 

So, we can conclude that sound can travel through solids.

 

Frequency, Loudness, and Pitch of a Sound

 

We have learned that the to and fro motion of an object is known as vibration. This motion is also called oscillatory motion.

 

Frequency: The number of oscillations per second is called the frequency of oscillation. Frequency is expressed in hertz. Its symbol is Hz. A frequency of 1 Hz is one oscillation per second. If an object oscillates 20 times in one second, what would be its frequency? You can recognize many familiar sounds without seeing the objects producing them. How is it possible?

 

These sounds must be different to enable you to recognize them. Have you ever thought about what factors make them different? Amplitude and frequency are two important properties of any sound. Can we differentiate sounds on the basis of their amplitudes and frequencies?

 

Loudness and Pitch

 

The loudness of sound is proportional to the square of the amplitude of the vibration producing the sound. For example, if the amplitude becomes twice, the loudness increases by a factor of 4. The loudness is expressed in a unit called decibel (dB).

 

The following table gives some idea of the loudness of sound coming from various sources.

 

Normal breathing	10 dB
Soft whisper (at 5m)	30 dB
Normal conversation	60 dB
Busy traffic	70 dB
Average factory work	80 dB

Loudness of sound

 

Above 80 dB the noise becomes physically painful. The loudness of sound depends on its amplitude. When the amplitude of vibration is large, the sound produced is loud. When the amplitude is small, the sound produced is feeble.

 

Pitch

 

Compare the sound of a baby with that of an adult. Is there any difference? Even if two sounds are equally loud, they differ in some way. Let us see how. The frequency determines the shrillness or pitch of a sound.

 

If the frequency of vibration is higher we say that the sound is shrill and has a higher pitch. If the frequency of vibration is lower, we say that the sound has a lower pitch. For example, a drum vibrates with a low frequency.

 

Therefore, it produces a low-pitched sound. On the other hand, a whistle has a high frequency and therefore, produces a sound of higher pitch. A bird makes a high-pitched sound whereas a lion makes a low-pitched roar. However, the roar of a lion is very loud while the sound of the bird is quite feeble.

 

Every day you hear the voices of children and adults. Do you find any difference in their voices? Can you say that the frequency of the voice of a child is higher than that of an adult? Usually, the voice of a woman has a higher frequency and is shriller than that of a man.

 

Audible and Inaudible Sounds

 

We know that we need a vibrating body for the production of sound. Can we hear the sound of all vibrating bodies? The fact is that sounds of frequencies less than about 20 vibrations per second
(20 Hz) cannot be detected by the human ear. Such sounds are called inaudible.

 

On the higher side, sounds of frequencies higher than about 20,000 vibrations per second (20 kHz) are also not audible to the human ear. Thus, for the human ear, the range of audible frequencies is roughly from 20 to 20,000 Hz.

 

Noise and Music

 

We hear different types of sounds around us. Is the sound always pleasing? Does a sound sometimes cause discomfort to you? Some sounds are pleasant to the ear, whereas some are not. Suppose construction work is going on in your neighborhood. Are the sounds coming from the construction site pleasing? Do you enjoy the sounds produced by the horns of buses and trucks? Such unpleasant sounds are called noise.

 

In a classroom, if all the students speak together, what would the sound produced be called? On the other hand, you enjoy sounds from musical instruments. Musical sound is one which is pleasing to the ear. The sound produced by a harmonium is a musical sound. The string of a sitar also gives out a musical sound.

 

Read More: Sound: Production, Propagation, Properties, Echoes and Reverberation