Sounds and Vibrations- Class 8 Science Guide

Sounds and Vibrations- Physics Guide for Class 8

Information about Sounds and Vibrations

Title	Sounds and Vibrations
Class	Class 8
Subject	Class 8 Physics
Topics Covered	Sounds Vibrations Sounds produced by Humans Sounds produced by Animals

We live in a world full of sounds. Sound and music are a regular part of our everyday sensory experience. We hear sound from various sources — humans, birds, animals, vehicles, machines, television sets, music systems and so on. We communicate with one another by speaking, hearing and replying through speaking. 

Science recognises 'sound' as an integral and essential part of nature.

The basic concepts concerning 'sound' are the:

• causes of generation of sound
• reasons for differences in sounds from different sources
• mechanism of propagation of sound from one place to another
• mechanism of hearing of sound by the human ears. 

Sounds

We usually think of the word 'sound' whenever we have a sense of hearing in our ears. 

• Sound is a form of energy which produces the sensation of hearing in our ears. 

We all are familiar with different forms of energy like mechanical energy, heat energy and so on. We also know that one form of energy can be converted into another form. When we clap our hands, a sound is produced.

We now know that we can produce sound by striking, shaking, blowing, rubbing or plucking, different objects. A close look at all these situations shows that in all these cases, we actually set one, or another object(s), into vibrations. It is these vibrations that produce sound.

• Whenever a source produces a sound, there are always some vibrations linked with it. We can, therefore, think of vibrations as the basic cause of all sounds. 

Activity 1

Take a 'Dafli' and lightly strike its top layer, or 'membrane', with your palm. You can feel the skin vibrating as it produces some sound. 

• We can clearly see such vibrations in the case of a drum.
• Take a drum and strike it with its stick, or with hand. The drum produces sound.
• Now, put some coloured very tiny paper balls on the drum membrane.
• The paper balls are seen to 'dance up and down the membrane' when the drum is producing sound. 

Activity 2

• Take a tin can.
• Cut its ends.
• Now, cut the neck off a big balloon with scissors and stretch it over one end of the can. Make sure that balloon is stretched flat.
• Put a rubber band over the balloon to hold it in its place.
• Put some dry rice (grains) on the stretched balloon, while holding the can vertically.
• Ask your friend to shout from the open end of the can.

Observe what happens. Why do the rice grains jump up and down?

It is sound vibrations that make the rice grains jump 'up and down'. These simple activities help us to realise that the basic cause of all sounds is vibrations. 

Vibrations

• Vibrations can be thought of as the 'to and fro', or 'back and forth' motion of a given object.
• In some cases, the extent of vibrations of a source may be so very small that it may not be possible to detect them.
• Vibrations are easily seen to be produced by drums, bells, strings of guitar and so on. 

Sounds Produced by Humans

As human beings, we are all fond of hearing the sound of our family members, friends, neighbours and of course, great singers, both past and present.

How do we humans produce sound? What vibrates in our body system that helps everyone of us to produce our characteristic sound? 

• It is the voice box, or the larynx, of the human body system that helps us to produce sound.
• The voice box has two vocal chords stretched across it.
• They are stretched across the voice box in such a way that there is a narrow gap, or slit between them. 
• Air can pass through this slit.
• The lungs force air through the slit between the vocal chords and this make them vibrate and produce sound.
• The human body has muscles attached to the vocal chords.
• These muscles can pull the attached ligaments and make the vocal chords:
  (i) tighter or loosend up
  (ii) thinner or thicker.
• These changes, in the vocal chord, help to change the pitch of the sound produced and make it shriller or flatter.
• There are differences in the sizes, tautness and thickness of vocal chords in men, women and children. 
• This difference results in each group having a characteristic sound of its own. 

The vibrations, produced by our vocal cords, get converted into speech with the help of the tongue, the nose and the lips. Speech is the production of that sound (vibrations) that has a definite and clear meaning for us.

Sounds Produced by Animals

Our earth has an amazingly large variety of birds and animals living on it. There are endless varieties of sound produced by them. Some of them are very pleasant to our ears; others are not so pleasant while some of them are definitely unpleasant. Different types of birds and animals also have different mechanisms, and set-ups, in their bodies, for producing their characteristic sounds. 

We now look at some of these mechanisms:

1. Like human beings, there are quite a few animals that produce sound by means of vocal chords. When dogs bark, cats mew, horses neigh, lions roar or cows moo, they are all using their vocal chords.
2. Birds have a ring of cartilage, called syrinx, in their wind pipe. They use it (instead of the vocal chords) in their voice box to produce their characteristics sounds. This characteristic sound is a jarring, or unpleasant one, in case of birds like the crow, but it is a pleasant and soothing one in case of birds like the koel, parrot and mynah.
3. Insects, like mosquitoes and bees, make their characteristic sounds by vibrating their wings rapidly. The more rapid the vibration of their wings, the higher is the pitch of the sound produced by them.
4. Frogs, while croaking, are known to puff their 'vocal sacs, up their throat like a balloon. Additional inflation of their throat helps them to make their croaking louder.

We thus realise that our world of sound is a really diverse and amazing one. It is perhaps one of the reasons that makes our earth such a unique, lively and interesting planet. 

• Bees, when returning to their hive, are loaded with honey and, therefore, heavier. They then vibrate their wings less rapidly compared to the case when they are on their way for collecting honey. This difference, in the pitch of their buzzing sound, helps trained persons to decide whether the bees are returning back to their hive (after collecting honey) or are on their way to collect honey.