Class 8 Science

Chapter 13 – Sound

Chapter Notes

Sound is a form of energy. Sound is that form of energy which makes us hear.

Each sound is special to the object which produces it. E.g., sound produced by beating a drum, clapping, plucking a guitar string, etc.

Sound Produced by Objects

When an object moves 'backwards and forwards' (to-and-fro) rapidly, we say that the object 'vibrates' or that the object is 'vibrating'.

The energy required to make an object vibrate and produce sound is provided by some outside source (like our hand, wind, etc.).

The vibrating air molecules pass on their motion to the next layer of air molecules due to which they also start vibrating back and forth. This process goes on and ultimately all the air molecules around the sound producing object start vibrating back and forth.

When the air molecules attached to our ears start vibrating, the ears feel these vibrations as sound.

Examples to show that sound is produced by vibrating objects

1. When we touch a ringing bell with our fingers, we observe that the bell is shaking back and forth rapidly and continuously. This rapid shaking is known as vibrating. If we hold the ringing bell tightly with our hand, it stops vibrating, and the sound also stops coming. From this we conclude that sound is produced by a vibrating bicycle bell. When the bicycle bell vibrates, it produces sound; and when the bicycle bell stops vibrating, the sound also stops coming from it. In general, we can say that a body must vibrate to produce sound. The sound of a school bell is produced by the vibrations of iron or brass plate when it is hit by a hammer.

2. Pluck a stretched rubber band in the middle, with finger. The rubber band starts vibrating and produces sound.

Sound is produced when a stretched rubber band vibrates. Now, if we hold the vibrating rubber band tightly with our hand, the rubber band stops vibrating and the sound being produced by it also stops.

3. If we pluck the string of a sitar in the middle, the sitar makes a sound. If we now put our finger gently on the sitar string, we can feel the string vibrating. Thus, sound is produced when a sitar string vibrates.

4. When we talk, we make sound. This sound is made by the vibrations of two vocal cords present in our voice box fixed in the throat. This can be shown as follows: Let us hold the fingers of our right hand gently on our throat and talk to one of our friends (or sing a song). When we are talking (making sound) our fingers feel that something is moving or vibrating inside the throat. Actually, when we talk, air from the lungs passes up the wind-pipe. This air makes the vocal cords in our voice box to vibrate rapidly. And vibrating vocal cords produce the sound (of our talk). Thus, sound is produced when our vocal cords vibrate. Please note that the vibrations of our vocal cords are small, so we can only feel them with our hands. We cannot see these vibrations with our eyes. Mosquitoes and bees make a buzzing sound by vibrating their wings very, very rapidly.

5. If we blow across the mouth of an empty test-tube, then a whistling sound is produced. This sound is produced by the vibrations of air present in the test-tube. Thus, sound is produced when the air column enclosed in a tube vibrates. The sound of a flute (bansuri) is produced by the vibrations of air column enclosed in the flute tube. The sound of a bursting balloon is produced by the vibrations of air enclosed in the balloon (when it escapes).

6. If we hit the stretched membrane (or skin) of a tabla, the membrane starts vibrating and produces a sound. Now, if we put a few small pebbles on the membrane of this sound producing tabla, the pebbles will start jumping up and down showing that the tabla membrane is vibrating while producing sound. Thus, sound is produced when the membrane (or skin) of a tabla vibrates. When we strike at the membrane (or skin) of a drum, it vibrates to produce sound. Thus, sound is produced when the stretched membrane (or skin) of a drum vibrates.

From the above discussion we conclude that: Sound can be produced by the following methods:

 (i) by vibrating strings (as in a sitar)

 (ii) by vibrating air columns (as in a flute)

 (iii) by vibrating membranes (as in tabla)

 (iv) by vibrating plates (as in a bicycle bell)

Sound Produced by Humans

The human beings produce sound by using the voice box which is called 'larynx'. Voice box (or larynx) is situated in our throat at the top of the wind-pipe (or trachea). The human voice box (or larynx) contains two ligaments known as vocal cords. The vocal cords are a kind of strings.

Sound is produced by the vibrations of vocal cords. The vocal cords are attached to muscles which change the tension (stretching) in the cords and the distance between the cords.

 (i) Normally, the muscles of vocal cords are completely relaxed due to which the vocal cords are separated and loose so that air from the lungs passes through them without producing any sound. Thus, when we are not talking (or singing), the two vocal cords are far apart with a lot of gap between them.

 (ii) When we want to speak, the muscles of vocal cords contract due to which the two vocal cords become stretched and close together leaving only a narrow slit between them. 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. Thus, when we talk or sing (or make any other sound), we actually make our vocal cords vibrate. And vibrations of vocal cords by expelled air produce vocal sounds.

Activity to Explain Working of Vocal Cord

Take two rubber strips of the same size.

Place these two rubber strips one above the other. Hold the two ends of the rubber strips in your hands and stretch them tight.

Keep the stretched rubber strips in front of your mouth and blow air through the thin gap between them. As the air blows through the stretched rubber strips, a sound is produced.

This sound is produced by the vibrations of stretched rubber strips when air rushes through the thin gap between them. Our vocal cords produce sound in a similar way.

 

When we talk or sing, then the frequency of sound produced by us changes continuously. The changes in frequency of sound while talking or singing are brought about by the action of muscles attached to the vocal cords in the voice box.

When the muscles attached to vocal cords contract and stretch, the vocal cords become tight and thin, and a sound of high frequency is produced. On the other hand, 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 woman 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 (or pitch) of their voice is very high. This is why their voice is shrill.

Sound Needs a Medium for Propagation

Transmission of sound is called propagation of sound.

The substance through which sound travels is called medium.

The medium can be a solid substance, a liquid or a gas.

Sound can travel through solids, liquids and gases because the molecules of solids, liquids and gases carry sound waves from one place to another (through their vibrations).

Sound cannot travel through vacuum because vacuum has no molecules which can vibrate and carry sound waves.

Activity to show that sound can travel through solids like wood, metal and thread

If we press our ear on to one side of a wooden bench or metallic scale, and ask a friend to tap or scratch the other end of the bench lightly, we will hear the sound of tapping or scratching through the wooden bench or metallic scale quite loudly.

This means that sound can travel through wood or metal, which are solids.

As another example, pass one end of a thread into the hole of a tin-can and hold it inside the can by tying it to a pin. Similarly, pass the other end of thread into the hole of second tin-can and tie it to another pin.

Now, if we speak into one tin-can, other person can hear our sound in his ear through the other tin-can.

Activity to show that sound can travel through water or liquid

Place a toy, which makes some sort of sound, in a polythene bag, tie the bag’s opening tightly and immerse it in a bucket full of water. Now, if we put our ear to the side of this bucket, we can hear the sound of the toy clearly.

This shows that sound can travel through water, which is a liquid.

Dolphins and whales which live in the sea can communicate with one another under water because sound travels through sea water which is a liquid.

Activity to show that sound can travel through gases or air

When the telephone bell rings in our home, we can hear its sound even from a distance. In this case, the sound of ringing telephone bell travels to us through the air in the room, which is a gas (or rather a mixture of gases).

When we talk to a person standing near us, then the sound of our talk travels to the other person through the air around us. The sounds of radio, television, motor cars, buses, trains, aeroplanes, and the chirping of birds, all travel through the air and reach our ears. In fact, most of the sounds which we hear in our everyday life, reach us through the air. All the above observations show that sound can travel through air, which is a gas. In general, we can say that sound can travel through gases.

Activity to Show that Sound Cannot Travel Through Vacuum

1. A ringing electric bell is placed inside an airtight glass jar containing air as shown in the figure. We can hear the sound of ringing bell clearly. Thus, when air is present in the glass jar, sound can travel through it and reach our ears.

2. Now the glass jar containing ringing bell is placed over the plate of a vacuum pump. Air is gradually removed from the glass jar by the vacuum pump. When all the air is removed from the glass jar, no sound can be heard at all. This shows that sound cannot travel through vacuum (and reach our ears).

Sound in Outer Space or on the moon

Sound cannot be heard on the surface of moon because there is no air or atmosphere on the moon to carry the sound waves (or sound vibrations).

If an astronaut talks to another astronaut on the moon, he would see the lips moving but no sound will be heard. This is because sound cannot travel through the vacuum which exists on the surface of moon.

Similarly, there is no air (or any other gas) in outer space to carry sound waves. It is all vacuum in outer space due to which sound cannot be heard in outer space. The astronauts in outer space, talk to each other through wireless sets using radio waves. This is because radio waves can travel even through vacuum.

Speed of Sound

Sound takes some time to travel. The speed of sound depends on the nature of medium/material through which it travels.

In general, sound travels slowest in gases, faster in liquids and fastest in solids.

Medium

Speed of Sound (m/s)

Air

340

Water

1500

Iron metal

5000

If a train is very far away from us, we cannot hear the sound of approaching train through the air. But if we put our ear to the railway line made of steel, then we can hear the sound of the coming train easily even if it is quite far away. This is due to the fact that sound travels much faster through the railway line made of steel than through air. In fact, sound travels about 15 times faster in steel than in air.

Sound Travels Slower Than Light

The speed of sound in air is about 340 m/ s and the speed of light in air is 300,000,000 m/s. This means that sound travels at a slow speed but light travels much, much faster than sound.

In rainy season, the flash of lightning is seen first and the sound of thunder is heard a little later, though both are produced at the same time in clouds. It is due to the very high speed of light that we see the flash of lightning first and it is due to comparatively low speed of sound that the thunder is heard a little later.

Human Ear

The ears are the sense organs which help us in hearing sound.

The shape of the outer part of ear (which we see outside the head) is like a funnel. The outer part of ear is called 'pinna' and it is attached to about 2 to 3 centimetre long passage called 'ear canal'.

At the end of ear canal a thin, elastic and circular membrane called 'eardrum' is stretched tightly. 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. One end of hammer touches the eardrum and its other end is connected to second bone anvil. The other end of anvil is connected to third bone called stirrup. And the free end of stirrup touches the membrane over the oval window.

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. The three tiny bones in the middle part of ear act as a system of levers and amplify sound vibrations coming from the eardrum before passing them on to the inner part of the ear (cochlea).

The sound waves (coming from a sound producing body) are collected by the pinna of outer part of ear. These sound waves pass through the ear canal and fall on the eardrum.

When the sound waves fall on the eardrum, the eardrum starts vibrating back and forth rapidly. The vibrating eardrum causes a small bone hammer to vibrate. From hammer, vibrations are passed on to second bone 'anvil' and then to the third bone 'stirrup'.

The vibrating stirrup strikes on the membrane of oval window and passes the amplified vibrations to the liquid in cochlea. Due to this, liquid in cochlea begins to vibrate.

The vibrating liquid of cochlea sets up electrical impulses in the nerve cells present in it. These electrical impulses are carried by auditory nerve to the brain. The brain interprets these electrical impulses as sound and we get the sensation of hearing.

 

Activity to demonstrate the working of eardrum

Stretch a piece of thin rubber sheet (from a burst balloon) across one end of the plastic can and fasten it tightly with a rubber band. Hold the plastic can vertically in your hand with the 'rubber sheet covered end' at the top. Put four or five small grains of a cereal (like rice) on the stretched rubber sheet.

Keeping the plastic can vertical, ask your friend to shout ‘hurray’, ‘hurray’, upwards from the open end (lower end) of the plastic can by bringing his mouth below it. We will observe that when the sounds of hurray, hurray fall on the stretched rubber sheet from below, the rice grains placed over it start jumping up and down. The up and down movement of rice grains placed on the stretched rubber sheet tells us that when sound waves fall on it from below the stretched rubber sheet starts vibrating (up and down). This is how the eardrum in our ear works.

We should not put anything (like pin, pencil or pen, etc.) inside our ears. This is because they can tear the eardrum. The tearing of eardrum can make a person deaf. Our ears are very delicate organs. We should take proper care of our ears and protect them from damage.

Amplitude, Time-Period and Frequency of a Vibration

When an object moves back and forth continuously, we say that it is making vibrations (or oscillations). For example, when a swing moves back and forth repeatedly, we say that the swing is making vibrations (or oscillations).

A simple pendulum is like a swing and can be made by tying about one metre long thread from a small metal ball and suspending it from a height as shown in the figure. The small metal ball of pendulum is called bob. When the pendulum is at rest (not vibrating), then its bob is in the normal position or central position A.

When the pendulum bob goes from one extreme position B to the other extreme position C, and then comes back to B, we say that it completes one vibration (or one oscillation). Every vibration (or oscillation) has three characteristics: amplitude, time-period, and frequency.

1. Amplitude of Vibrations

The maximum distance to which the bob of a vibrating pendulum goes from its central position is called amplitude of vibrations (or amplitude of oscillations). In the figure, the distance AB or AC is the amplitude of vibration of this simple pendulum.

We can increase the amplitude of vibrations of a simple pendulum by raising the height from which the pendulum bob is initially released. Similarly, we can decrease the amplitude of vibrations by releasing the pendulum bob from a smaller height.

2. Time-Period of Vibrations

One complete to-and-fro movement of the pendulum bob is called one vibration (or one oscillation).

The time taken by pendulum bob to complete one vibration (or one oscillation) is called the time-period of pendulum.

For example, in the figure, the time taken by the pendulum bob to travel from position B to position C, and back to B, will be the time-period of this pendulum.

3. Frequency of Vibrations

The number of vibrations made per second by a vibrating body is called the frequency of vibration.

The unit of frequency of vibrations (or oscillations) of a vibrating object is hertz.

When an object makes 1 vibration per second (or 1 oscillation per second), its frequency is said to be 1 hertz. And if an object makes 10 vibrations per second, then its frequency will be 10 hertz. The frequency actually tells us how fast the vibrating object repeats its motion.

To find the frequency of a simple pendulum, we measure the time taken by the pendulum to make a large number of vibrations. Dividing the 'number of vibrations' by the 'time taken' we get the number of vibrations made in one second. This will give us the frequency of the pendulum.

The frequency of vibrations (or oscillations) of a simple pendulum is very low. So, a vibrating simple pendulum produces a sound having very low frequency. The very low frequency sound produced by a vibrating simple pendulum cannot be heard by our ears. An object must vibrate at a frequency of at least 20 hertz to be able to produce audible sound which can be heard by our ears.

Relation between Time-Period and Frequency

Time-period is equal to the reciprocal (or inverse) of frequency. That is:

Characteristics of Sound: Loudness, Pitch and Quality

1. Loudness

If more energy is supplied to an object by plucking it or hitting it more strongly, then the object will vibrate with a greater amplitude and produce a louder sound. Thus, the loudness of sound depends on the amplitude of vibrations of the vibrating object.

Greater the amplitude of vibrations, louder the sound will be. For example, when a sitar string is plucked lightly, then it vibrates with a small amplitude and produces a faint/feeble sound. But, when a sitar string is plucked hard, then it vibrates with a large amplitude and produces a very loud sound.

Activity to demonstrate the dependence of loudness of sound on the amplitude

Take a stainless-steel tumbler and a stainless-steel spoon.

Tie a small thermocol ball to a thread and suspend this thermocol ball touching the rim of steel tumbler as shown in the figure.

(a) Strike the steel tumbler gently at the rim with a spoon to make it vibrate. We will observe that the tumbler produces a feeble sound and, at the same time, the suspended thermocol ball is pushed away to a small distance by the vibrations of the tumbler. This means that the tumbler is vibrating with a small amplitude.

(b) 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 ball is pushed away to a large distance by the vibrations of the tumbler. This means that the tumbler is vibrating with a large amplitude.

From the above discussion we conclude that the loudness of sound depends on the amplitude of vibrations of sound producing objects.

The loudness of sound is directly proportional to the square of amplitude of vibrations of sound producing object. This means that:

 (i) If the amplitude of vibrations is doubled (made 2 times), then the loudness will become four times [ because (2)2 = 4 ].

 (ii) And if the amplitude of vibrations is halved (made ), then the loudness will become one-fourth [ because  ] .

The loudness of sound is expressed in the unit called decibel (dB). The softest sound which humans can hear is said to have a loudness of 0 dB (zero decibel). The loudness of sounds coming from some of the common sources of sound around us are given below:

Sound

Loudness

1. Normal breathing

10 dB

2. Whispering

30 dB

3. Normal conversation

60 dB

4. Busy street

70 dB

5. Average factory

80 dB

6. Very noisy factory

100 dB

7. Loud music in disco

110 dB

8. A jet aeroplane taking off

130 dB

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

2. Pitch

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

The pitch of a sound depends on the frequency of vibration (of the sound producing object). In fact, the pitch of a sound is directly proportional to its frequency. If the frequency of vibration is low, the sound produced has a low pitch. On the other hand, if the frequency of vibration is high, the sound produced has a high pitch.

A sound having high frequency (or high pitch) is said to be shrill.

It is the frequency of vibration of the sound producing object which determines the pitch (or shrillness) of a sound. As the frequency of vibration of an object increases, the pitch (or shrillness) of sound produced by it also increases.

The membrane of a drum vibrates with a low frequency; therefore, a drum produces a low-pitched sound. On the other hand, the air in a whistle vibrates with high frequency due to which the whistle produces a sound having a higher pitch (than that of a drum).

Activity to show the relation between the frequency and pitch of a sound

Take a metal ruler and place it near the edge of a table in such a way that a large part of the ruler projects out of the table top. The other end of ruler is pressed firmly on the table with our hand. 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 produces a high pitch sound. Thus, when we decrease the length of the 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 sound produced by it.

3. Quality

We can distinguish between the sounds (or notes) produced by singers like Arijeet Singh and Honey Singh by listening to their songs on radio, even without seeing them. This is because the sounds produced by Arijeet Singh and Honey Singh differ in quality.

We can say that, Quality is that characteristic of sound which enables us to distinguish between the sounds produced by different sound producing objects even if they are of same loudness and pitch. 'Quality' of sound is also known as 'timbre'.

Audible and Inaudible Sounds

An object must vibrate at the rate of at least 20 times per second (20 Hz) to be able to produce audible sound.

The sounds having too low frequencies which cannot be heard by human ear are called infrasonic sounds (or just infrasonics). Rhinoceros can produce infrasonic sounds having frequencies less than 20 Hz. They can also hear infrasonic sounds.

Also, if the frequency of a sound is more than 20,000 hertz, even then it cannot be heard by us. The human ear can hear sounds which have frequencies between 20 hertz and 20,000 hertz. Thus, the range of audible frequencies of sound for human hearing is from 20 hertz to 20,000 hertz.

The sounds having too high frequencies which cannot be heard by human ear are called ultrasonic sounds (or just ultrasonics). Thus, the sounds of frequencies greater than 20,000 hertz are called ultrasonics.

The human beings can neither produce ultrasonic sound nor can they hear ultrasonic sound.

Some animals can produce ultrasonic sounds as well as hear ultrasonic sounds. For example, bat is an animal which screams at a very high frequency, much beyond the limit of our hearing. In other words, bat produces ultrasonic sound during screaming.

Some other animals like dogs, monkeys, deer and leopards can also hear ultrasonic sounds.

Since dogs can hear ultrasonic sounds, therefore, some dog owners use special high frequency whistles which only dogs can hear. The crime-branch police use special high frequency whistles which produce ultrasonic sounds, to give commands to their dogs.

The sounds having too high frequency (greater than 20,000 Hz) which cannot be heard by human beings is also called just 'ultrasound'. For example, a sound of frequency 100,000 hertz is an ultrasound.

Due to its very high frequency, ultrasound has a greater penetrating power than ordinary sound. The ultrasound is reflected just like ordinary sound waves and produces echoes. But the echoes produced by ultrasound cannot be heard by our ears, they can only be detected by special equipment. These days, ultrasound is used for a large number of purposes. Some of the important uses of ultrasound are given below:

 (i) Ultrasound is used as a diagnostic tool in medical science to investigate inside of the human body.

 (ii) Ultrasound is used to study the growth of foetus (developing baby) inside the mother's womb.

 (iii) Ultrasound is used in the treatment of muscular pain and a disease called arthritis (which is inflammation of joints).

 (iv) Ultrasound is used to measure the depth of sea (or ocean). It is also used to locate under-water objects like shipwrecks, submarines and shoals of fish, etc.

Speed of infrasonic sound waves as well as ultrasonic sound waves is the same as that of audible sound waves.

Noise and Music

The unpleasant sounds around us are called noise. Noise is produced by the irregular vibrations of the sound producing source.

Some of the examples of noise are: Running of mixer and grinder in the kitchen; Blowing of horns of motor vehicles; Bursting of crackers; Barking of dogs; Shattering of glass.

The sounds which are pleasant to hear are called musical sounds (or music). Musical sounds (or music) are produced by the regular vibrations of the sound producing source.

Some of the examples of musical sounds are: The sound produced by strings of a sitar; the sound produced by a harmonium and other musical instruments.

The speakers of radio, stereo-systems and television also produce musical sounds (or music). When a person sings a melodious song, he (or she) also produces musical sounds. We enjoy the musical sounds produced by the musical instruments or singers because they give pleasant sensation in our ears. If, however, a musical sound becomes too loud, it would become noise.

Musical Instruments

The arrangement of sounds of different frequencies called 'notes' (or swara) in a way that is pleasant to hear, is called music. The instruments which make musical sounds are called musical instruments.

Types of musical instruments

 (i) Stringed musical instruments

 (ii) Wind musical instruments

 (iii) Membrane musical instruments

 (iv) Plate type musical instruments

1. Stringed musical instruments produce musical sounds by the vibrations of stretched strings (or stretched wires). When the stretched string of a musical instrument is plucked or bowed with the fingers of our hand, the string starts vibrating and produces sound. The strings are fixed tightly on a large sounding box (which is usually made of wood). The air present in the sounding box increases the loudness of sound produced by vibrating strings.

The examples of stringed musical instruments are: Sitar, Veena, Violin, Tanpura, Santoor, Guitar, Piano and Ektara.

2. Wind musical instruments produce musical sounds by the vibrations of air columns inside them. In a wind musical instrument, a column of air enclosed in a wooden tube (or metal tube) vibrates and produces musical sound. We have to pump air (usually from our mouth) into the wind instrument to make it work and produce sounds. Some of the wind musical instruments are:

Shehnai, Flute (Bansuri), Nadaswaram and Trumpet. Harmonium is a keyboard wind musical instrument.

3. Membrane is a thin sheet of skin. In a membrane type musical instrument, a thin membrane fixed tightly over a hollow wooden drum vibrates and produces sound. We have to strike the stretched membrane of the instrument with our hands or with sticks to make it vibrate and produce sound. The examples of membrane type musical instruments are: Mridangam, Tabla, Dholak, Drum and Dhapli.

4. Plate type musical instruments produce musical sounds by the vibrations of thick plates or objects made of plate. The plate type musical instruments are simply beaten (or struck) to produce musical sounds.

The cymbals (manjira) is a plate type musical instrument. The cymbals consist of two concave brass plates. When the two metal plates of cymbals are struck together, they make a ringing musical sound.

The bell used in performing pooja or that in temples is also a plate type musical instrument. The bell is a hollow metal vessel which emits musical sound when struck with a hammer fixed inside it.

Noot (or matka) is also a plate type musical instrument which makes sound when struck. Ghatam and kartal are also plate type musical instruments.

               

Jal-tarang is also a kind of plate type musical instrument.

The 'Jal-tarang' instrument consists of a number of cups containing different amounts of water. When the cups containing water are struck with two sticks in a proper way, then musical sounds are produced. The frequency (or pitch) of the sound produced in jal-tarang is adjusted by putting the appropriate amount of water in each cup. The cup containing minimum water produces the sound of lowest frequency (or lowest pitch).

As the amount of water in the cups goes on increasing, the frequency (or pitch) of the sound produced, also goes on increasing. So, the cup having maximum amount of water will produce sound of highest frequency (or highest pitch).

Noise Pollution

The presence of loud, unwanted and disturbing sounds in our environment is called noise pollution. Some of the major sources of noise pollution in the environment around us are as follows:

 (i) The motor vehicles (like cars, buses and trucks, etc.) running on the road produce noise pollution by blowing horns and sounds of their engines.

 (ii) The bursting of crackers on various social and religious occasions produces noise pollution.

 (iii) The various machines in factories make loud sounds and cause noise pollution.

 (iv) The take-off, landings, and flying of aeroplanes produces noise pollution.

 (v) Loudspeakers and bands at marriages and other social functions cause noise pollution.

 (vi) The construction of buildings produces a lot of noise pollution in the surroundings.

The Major Sources of Noise Pollution in the Homes

 (i) The loud playing of radio, stereo-systems and televisions (at high volume) produces noise pollution.

 (ii) Some kitchen appliances (like mixer and grinder) cause noise pollution.

 (iii) The use of desert coolers and air conditioners produces noise pollution.

Harmful effects of Noise Pollution

 (i) Loud noise can cause great harm to our ears. Constant loud noise reduces the hearing power of our ears. Loud noise can even damage the ears permanently and cause deafness.

 (ii) Loud noise can cause a person to lose concentration in his work or studies.

 (iii) Loud noise can cause an ailment called hypertension (high blood pressure).

 (iv) Loud noise can cause irritation and headache.

 (v) Loud noise during night-time disturbs our sleep. Continued lack of sleep is bad for health.

Measures to Control Noise Pollution

 (i) We should not play radio, stereo-systems and television too loudly.

 (ii) The horns of motor vehicles should not be blown unnecessarily.

 (iii) The bursting of crackers should be avoided.

 (iv) The noise-making factories and airports should be shifted away from the residential areas of the city.

 (v) Loudspeakers should be played at low volume during marriages and other social functions. No loudspeakers should be allowed to be used late in the night.

 (vi) Trees should be planted along the roads and around buildings to reduce the noise pollution from the roads and other activities from reaching the residents of the area.

Hearing Impairment (Damaged Hearing)

Hearing impairment means that a person has damaged hearing ability and cannot hear properly.

Some people are born with poor hearing. The people with very poor hearing are said to be deaf Thus, deafness is the total hearing impairment. Most deaf people can still hear some sounds. Deaf people often use a sign language with their hands to communicate with others effectively.

Partial hearing loss is generally due to an illness, ear infection, injury or old age. Partial hearing loss (or partial hearing impairment) can also be caused by noise pollution. A person having partial hearing loss can hear sounds properly by using 'hearing aid'. Hearing aid is a small sound amplifying device worn on the ear by a partially deaf person (so as to hear properly).

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