VIII-SCIENCE & TECHNOLOGY CHAPTER-13:SOUND

VIII-SCIENCE & TECHNOLOGY

CHAPTER-13:SOUND

FROM 10-11-2010 TO 17-11-2010


How is sound produced? How does it travel from one place to another? How do we
hear sound? Why are some sounds louder than others? Definitions of Amplitude,
frequency and Vibration-Audible and unaudible range-Effects of noise-Measures
to Limit Noise Pollution

Sound is Produced by a Vibrating Body-strike metal plate with a stick . Touch the
plate orpan gently with finger. strike the plate with the stick and hold it tightly with
hands immediately after striking

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.

Vibrating object produces sound. In some cases, the vibrations are easily visible to us.
But in most cases, their amplitude is so small that cannot seen,can be felt.
Take metal bowls or tumblers. Fill them with water up to different levels, increasing
gradually from one end to the other. Now take a pencil and strike the bowls gently.

Strike all of them in succession. Hear a pleasant sound- Jaltrang
In humans, the sound is produced by the voice box or the larynx
Two vocal cords, are stretched across the voice box or larynx in such a way that it
leaves a narrow slit between them for the passage of air
When the lungs force air through the slit, the vocal cords vibrate, producing sound.
Muscles attached to the vocal cords can make the cords tight or loose.

Vocal cords produce sound
The vocal cords in men are about 20mm long. In women these are about 5mm shorter.
Children have very short vocal cords.

Take a metal glass tumbler. Make sure that it is dry. Place a cell phone in it. Give a ring
on this cell phone from another cell phone. Listen to the ring carefully.

Surround the rim of the tumbler with hands Put mouth on the opening between hands.
Give a ring again. Listen to the ring while sucking air from the tumbler.

Sound become fainter as you suck-Remove the tumbler from mouth,Sound become loud
Decreasing amount of air in the tumbler had something to do with decreasing loudness
of the ring
Sound Needs a Medium for Propagation-Sound needs a medium to travel-Sound can
travel through wood or metal,solids and liquids.

When air has been removed completely from a vessel, it is said that there is vacuum in
the vessel.sound cannot travel through vacuum.

Take a bucket or a bathtub. Fill it with clean water. Take a small bell in one hand.
Shake this bell inside the water to produce sound. Make sure that the bell does not
touch the body of the bucket or the tub. Place ear gently on the water surface-hear
the sound of the bell
Take a metre scale or a long metal rod and hold its one end to ear. Gently scratch or
tap the other end of the scale- hear the sound of the scratch
Take a tin can. Cut its ends. Stretch a piece of rubber balloon across one end of the
can and fasten it with a rubber band. Put four or five grains of dry cereal on the
stretched rubber. Speak from the open end -Observe what happens to the grain.

Grains jump up and down.
The eardrum is like a stretched rubber sheet. Sound vibrations make the eardrum
vibrate. The eardrum sends vibrations to the inner ear. From there, the signal goes to
the brain.

To and fro motion of an object is known as vibration.
The number of oscillations per second is called the frequency of oscillation. Frequency
is expressed in hertz. Its symbol is Hz
Displacement of the object is a measure of the amplitude of vibration of the tumbler.
Loudness of sound is proportional to the square of the amplitude of the vibration
producing the sound.

The loudness is expressed in a unit called decibel (dB)
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.

The frequency determines the shrillness or pitch of a sound
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.The range of audible
frequencies is roughly from 20 to 20,000 Hz.

Unpleasant sounds are called noise.Musical sound is one which is pleasing to the ear.
Effects:Presence of excessive noise in the surroundings may cause many health
related problems. Lack of sleep, hypertension (high bloodpressure), anxiety and many
more health disorders may be caused by noise pollution. Person exposed to a loud
sound continuously may get temporary or even permanent impairment of hearing.

Measures to minimize:Noisy operations must be conducted away from any residential
area. Noise producing industries should be set up away from such areas. Use of
automobile horns should be minimised. TV and music systems should be run at low
volumes. Trees must be planted along the roads and around buildings to cut down on
the sounds reaching the
Residents.