Characteristics of sound - Class 8 PDF Download

Loudness

Sounds are produced by the vibrating object.If more energy is supplied to an object by plucking it or hitting it more strongly ,then the object will vibrate with the greater amplitude and produce a louder sound.

The loudness of sound depends on the amplitude of vibration of the vibrating object. Greater the amplitude of vibration, louder the sound will be.

When are Sitar string is plucked lightly ,then it vibrates with a small amplitude and produces a faint sound or feeble sound. When a sitar string is plucked hard, then it vibrates with a large amplitude and produces a very loud sound.

Activity

Take a stainless steel tumbler and a stainless steel spoon.Also tie a small thermocol ball to a thread and suspend thermocol ball touching the rim of Steel tumbler.

1)Strike the Steel tumbler gently at the rim with a spoon.We will observe that the tumbler produces of feeble sound and at the same time the suspended thermocol ball is pushed away to a small distance by the vibration of the tumbler.Since the thermocol ball is pushed away by a small distance, this means that the tumbler is vibrating with a small amplitude.When the amplitude of vibration of Steel tumbler is small the sound produced is feeble.

2)Now strike the Steel tumbler hard at the rim with the spoon. We will observe that the tumbler produces a very loud sound and at the same time the suspended thermocol balls is pushed away to a large distance by the vibration of the tumbler. Since the thermocol ball is pushed away by a large distance, this means that the tumbler is vibrating with large amplitude. When the amplitude of vibration of Steel tumbler is large, the sound produced is very loud.

The loudness of sound depends on the amplitude of vibration of sound producing objects.

a)When the amplitude of vibration is large ,the sound produced is loud.

b)If the amplitude of vibration is small ,the sound produced is feeble.

The loudness of sound is directly proportional to the square of amplitude of vibration.

If the amplitude of vibration is doubled ,then the loudness will become 4 times.

If the amplitude of vibration is halved ,then the loudness will become one fourth.

The loudness of sound is expressed in the units called decibel. The symbol of decibel is Db.

At a loudness of above 80 Db ,the sound become physically painful. And at about 140 Db level ,sound hurts too much.

Pitch

Pitch is that characteristic of sound by which we can distinguish between different sounds of the same loudness.

We can distinguish between a man’s voice and a woman voice even without seeing them. This is because man’s voice and woman’s voice differ in their pitch.A man’s voice is flat having low pitch whereas woman’s voice is shrill  having a high pitch.

The pitch of sound depends on the frequency of vibration.The pitch of sound is directly proportional to its frequency.

a) If the frequency of vibration is low, the sound produced has a low pitch.

b) If  the frequency of vibration is high is high, the sound produced has a high pitch.

The voice of a small baby has a higher frequency so the pitch of baby’s voice is higher than that of a woman. The sound of low frequency are said to have a low pitch while the sounds of higher frequency are said to have a high pitch.

A sound having high frequency  is said to be Shrill.

The voice of a woman is shriller than that of a man. The voice of man having low frequency or low pitch is said to be deep or flat.

As the frequency of vibration of an object increases, the pitch or shrillness of sound produced by it also increases.

1)When we go from a  man to a woman and then a small baby, the frequency of vibrations of their vocal cords increases due to which the pitch of shrillness of the voice increases.

2)When we switch on a table fan ,we can hear the increase in the pitch of sound produced as the speed of rotation of fan or frequency of rotation of fan increases.

3)The membrane of a drum vibrates with a low frequency ,therefore a drum produces a low pitched sound.

4)The air in a whistle vibrates with high frequency due to which the whistle produces a sound having a higher pitch.

5)A bird makes a high pitched sound whereas a lion makes a low pitched roar.The roar of a lion is very loud whereas the sound produced by a bird is comparatively quite feeble.

Activity

We take a metal ruler and place it near the edge of a table in such a way that the large part of the ruler projects out of the tablet top. The other end of the ruler is pressed firmly on the table with our hands.We now press the free end of ruler downwards with our other hand and then let it go. The free end of ruler starts vibrating and produces a low pitch sound.

Let us now decrease the vibrating length of the ruler which is projecting out of the table. We again press the free end of the ruler and then let it go. The ruler now starts vibrating faster and producers of high pitch sound. When we decrease the length of vibrating ruler, it vibrates faster and the pitch of sound produced by it becomes higher.

Shorter the projection of vibrating ruler, higher is the pitch of the sound produced by it. As we go on decreasing the length of the vibrating part of the ruler the frequency of its vibrations goes on increasing. And this increase in frequency of vibration leads to the production of higher pitched sound.

Quality

We can distinguish between the sounds or notes produced by a Sitar and a flute even without seeing these musical instrument. This is because the sound produced by a Sitar and flute differ in quality.

Quality is that characteristic of a sound which enables us to distinguish between the sound produced by different sound producing objects even if they are of same loudness and pitch.

Sound produced by different musical instruments like Sitar ,flute, piano, violin ,guitar , veena , shehnai,  harmonium, trumpet and tabla can be distinguished by their quality.

The quality of sound produced by different musical instruments or different singers is different because they produce sound waves of different shapes.