notes prepared by subhankar Karmakar
SOUND
Sound is a form of energy. Sound is that form of energy which makes us hear. A vibrating object always produces sound. Each sound is special to the object which produces it.
In some cases, the vibrations of a sound producing object are quite large which we can see with our eyes. But in other cases, the vibrations of the sound producing object are so small that we cannot see them easily, we have to feel the vibrations of such an object by touching it gently with the fingers of our hand.
Like, sound is produced by a vibrating bicycle Bell. Sound is produced when a stretched rubber band vibrates. Sound is produced when a sitar string vibrates. Sound is produced when our vocal cords vibrate. Sound is produced when the air column enclosed in a tube vibrates. Sound is produced when the membrane of a tabla vibrates. Sound is produced when the stretched membrane of a drum vibrates. Sound of a radio or television or speakers is produced by the vibrations of the the cone of the speakers.
Sound can be produced by the following methods:
1. By vibrating strings (in guitar, sitar etc)
2. By vibrating air columns (in flutes)
3. By vibrating membranes (in tabla, drums)
4. By vibrating plates (in bicycle bells)
Propagation of sound:
When an object vibrates then the air around it also starts vibrating in exactly the same way and carries sound to our ears through the vibrations of its molecules.
Sound produced by humans:
The human beings produce sound by using the voice box which is called larynx. It is also known as the voice box. The human voice box or larynx contains two ligaments known as vocal cords. The vocal cords act like a kind of strings. Sound is produced by the vibrations of vocal cords.
The lungs pass a current of air between the two vocal cords. This air makes the vocal cords vibrate and the vibrating vocal cords produce sound.
When the muscles attached to the vocal cords contract and stretch, the vocal cords become tight and thin, and a sound of high frequency is produced. When the muscles relax, the vocal cords become loose and thick and a sound of low frequency is produced. The vocal cords of a man are about 20 mm long. The vocal cords of a omen are about 5 mm shorter than man. Due to the shorter vocal cords, the frequency or pitch of a woman's voice is higher than that of a man. Small children have very short vocal cords due to which the frequency of each of their voice is also very high.
SOUND NEEDS A MEDIUM FOR PROPAGATION:
Sound needs a solid, liquid or gas for transmission. Therefore, we can say, sound needs a material medium like solid liquid or gas to travel and be heard. Sound cannot travel through vacuum or empty space, as there is no molecules which can vibrate and carry sound waves in empty space.
Sound cannot be heard on the surface of moon because there is no Air on the moon to carry the sound waves. The astronauts who land on moon talk to each other through wireless sets using radio waves.
SPEED OF SOUND
We know sound can travel through solids, liquids and gases. The speed of sound is different in different materials. Sound travels slowest in gases, faster in liquids and fastest in solids. Sound travels at a speed of 340 m/s, in water at 1500 m/s and in Iron 5000 m/s.
The speed of sound in air is 340 m/s whereas light travels at 300000 m/s. It is due to the very high speed of light that we see the flash of lightning first and it is due to comparative low speed of sound that the thunder is heard a little later.
STRUCTURE OF HUMAN EAR
• Pinna: The funnel shaped outer part of the ear is called pinna. Sound waves enter our ear through pinna.
• Ear canal: Pinna is attached to about 2 to 3 centimetre long passage called ear canal.
• Ear drum: Ear canal is attached with a thin elastic and circular membrane called ear drum.
• Three tiny ear bones: There are three small and delicate bones called Hammer, anvil and stirrup in the middle part of the ear which are linked to one another. The three tiny bones in the middle ear act as a system of levers and amplify sound vibrations coming from the ear drum before passing them on to the inner part called cochlea.
Cochlea: The inner part of ear has a coiled tube called "cochlea". One end of cochlea is connected to middle part of ear through the elastic membrane over the oval window. Cochlea is filled with a liquid. The liquid present in cochlea contains nerve cells which are sensitive to sound. The other end of cochlea is connected to auditory nerve which goes into the brain.
WORKING OF EAR:
The sound waves are collected by the pinna. Then it passes through ear canal and fall on ear drum. It makes ear drum vibrating back and forth, which makes hammer to vibrate. Vibrations then amplified and passes through anvil and stirrup. This amplified vibrations passes through oval window to cochlea. This makes liquids in cochlea to vibrate and generates electrical impulses in the nerve cells. These electrical impulses are carried by auditory nerve to brain and the brain interprets these impulses as sound and thus we get the sensation of hearing.
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