Welcome dear students! Today we are going to learn about Sound from Class 8 Science.
How do you come to know that a period is over in your school? You come to know easily that someone is at your door when he knocks or you hear the sound of the doorbell. Most of the time you can make out that someone is approaching you by just hearing the foot steps. You might have played a game called hide and seek. In this game a person is blind-folded and has to catch the remaining players. How is the blind-folded person able to guess which player is closest to her? Sound plays an important role in our lives. It helps us to communicate with one another. We hear a variety of sounds in our surroundings. Make a list of sounds you hear in your surroundings. In the music room of your school you hear the sounds produced by musical instruments like flute, tabla, harmonium, and sitar. In Figure ten point one, we see illustrations of a harmonium, a sitar, a tabla, and a flute. 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? We shall discuss such questions in this chapter.
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Section ten point one: Sound is Produced by a Vibrating Body. Touch the school bell when not in use. What do you feel? Again touch it when producing sound. Can you feel it vibrating? Let us perform Activity ten point one. Take a metal plate or a pan. Hang it at a convenient place in such a way that it does not touch any wall. Now strike it with a stick. In Figure ten point two, we see a metal plate hanging from a support, and a stick is shown striking it. Do you hear a sound? Touch the plate or pan gently with your finger. Do you feel the vibrations? Again strike the plate with the stick and hold it tightly with your hands immediately after striking. Do you still hear the sound? Touch the plate after it stops producing sound. Can you feel the vibrations now? You will observe that the sound stops when you hold the plate tightly, and the vibrations also stop.
Next is Activity ten point two. Take a rubber band. Put it around the longer side of a pencil box. In Figure ten point three, we see a pencil box with a rubber band stretched around it, and two pencils are inserted between the box and the rubber band to keep it raised. Insert two pencils between the box and the stretched rubber. Now, pluck the rubber band somewhere in the middle. Do you hear any sound? Does the band vibrate? As you learnt in Class seven, the to and fro or back and forth motion of an object is called vibration. When a tightly stretched band is plucked, it vibrates and produces sound. When it stops vibrating, it does not produce any sound.
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Now let us do Activity ten point three. Take a metal dish. Pour water in it. Strike it at its edge with a spoon. In Figure thirteen point four, we see a metal dish filled with water, and a spoon is striking its edge. Do you hear a sound? Again strike the dish and then touch it. Can you feel the dish vibrating? Strike the dish again. Look at the surface of water. Do you see any waves there? Now hold the dish. What change do you observe on the surface of water? Can you explain the change? Is there a hint to connect sound with the vibrations of a body? In Figure ten point four, we see a vibrating dish producing waves in the water. We see that a vibrating object produces sound. In some cases, the vibrations are easily visible to us. But in most cases, their amplitude is so small that we cannot see them. However, we can feel them.
Activity ten point four: Take a hollow coconut shell and make a musical instrument ektara. You can also make it with the help of an earthen pot. In Figure ten point five, we see an ektara made from a coconut shell with a single stretched string and a wooden handle. Play this instrument and identify its vibrating part. Make a list of familiar musical instruments and identify their vibrating parts. A few examples are given in Table ten point one. The table lists Veena with a stretched string, and Tabla with a stretched membrane. The remaining rows are left blank for you to complete with your own observations.
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Many of you might have seen the manjira or cymbals, the ghatam, and the noot or mudpots, and the kartal. These musical instruments are commonly used in many parts of our country. These instruments are simply beaten or struck. In Figure ten point six, we see a ghatam which is a clay pot, and a pair of manjira cymbals. Can you name a few other musical instruments of this type? You too can make a musical instrument. Activity ten point five: Take six to eight 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. You will hear pleasant sounds. This is your jaltrang. In Figure ten point seven, we see several bowls arranged in a row with varying water levels, being struck with a pencil. When we pluck the string of an instrument, like the sitar, the sound that we hear is not only that of the string. The whole instrument is forced to vibrate, and it is the sound of the vibration of the instrument that we hear. Similarly, when we strike the membrane of a mridangam, the sound that we hear is not only that of the membrane but of the whole body of the instrument. When we speak, does any part of our body vibrate?
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Section ten point two: Sound Produced by Humans. Speak loudly for a while or sing a song, or buzz like a bee. Put your hand on your throat as shown in Figure ten point eight. Do you feel any vibrations? In humans, the sound is produced by the voice box or the larynx. Put your fingers on the throat and find a hard bump that seems to move when you swallow. This part of the body is known as the voice box. It is at the upper end of the windpipe. 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. In Figure ten point eight, we see a cross section of the human throat showing the larynx, the windpipe, and the vocal cords with a narrow slit between them. 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. When the vocal cords are tight and thin, the type or quality of voice is different from that when they are loose and thick. Let us see how the vocal cords function.
Activity ten point six: Take two rubber strips of the same size. Place these two pieces one above the other and stretch them tight. Now blow air through the gap between them. In Figure ten point nine part a, we see two rubber strips held taut with air blowing through the gap. As the air blows through the stretched rubber strips, a sound is produced. You can also take a piece of paper with a narrow slit and hold it between your fingers as shown in Figure ten point nine part b. Now blow through the slit and listen to the sound. Our vocal cords produce sound in a similar manner. The vocal cords in men are about 20 mm long. In women these are about 15 mm long. Children have very short vocal cords. This is the reason why the voices of men, women and children are different.
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Section ten point three: Sound Needs a Medium for Propagation. When you call up your friend who is standing at a distance, your friend is able to hear your voice. How does the sound propagate or travel to her? Activity ten point seven: Take a metal or glass tumbler. Make sure that it is dry. Place a cell phone in it. Remember that the cell phone must not be kept in water. Ask your friend to give a ring on this cell phone from another cell phone. Listen to the ring carefully. Now, surround the rim of the tumbler with your hands. In Figure ten point ten, we see a person holding a glass tumbler with a cell phone inside, placing their mouth over the opening to suck air out. Put your mouth on the opening between your hands. Indicate to your friend to give a ring again. Listen to the ring while sucking air from the tumbler. Does the sound become fainter as you suck air? Remove the tumbler from your mouth. Does the sound become loud again? Can you think of an explanation? Is it possible that the decreasing amount of air in the tumbler had something to do with decreasing loudness of the ring? Indeed, if you had been able to suck all the air in the tumbler, you will not listen any sound. Actually, sound needs a medium to travel. When air has been removed completely from a vessel, it is said that there is a vacuum in the vessel. The sound cannot travel through a vacuum.
Does sound travel in liquids? Let us find out. Activity ten point eight: 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. In Figure ten point eleven, we see a person holding a bell underwater in a bucket, with their ear gently placed on the water surface. Place your ear gently on the water surface. Be careful, the water should not enter in your ear. Can you hear the sound of the bell? Does it indicate that sound can travel through liquids? Oh, that is how whales and dolphins might be communicating under water.
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Let us find out if sound can travel through solids also. Activity ten point nine: Take a metre scale or a long metal rod and hold its one end to your ear. Ask your friend to gently scratch or tap at the other end of the scale. In Figure ten point twelve, we see one student holding a metre scale to their ear while another student scratches the opposite end. Can you hear the sound of the scratching? Ask your friends around you if they were able to hear the same sound? You can also perform the above activity by placing your ear at one end of a long wooden or metallic table and asking your friend to gently scratch the other end of the table. In Figure ten point thirteen, we see a student with their ear pressed against a wooden table while another scratches the far end. We find that sound can travel through wood or metal. In fact, sound can travel through any solid. You can perform interesting activities to show that sound can also travel through strings. Have you ever made a toy telephone? In Figure ten point fourteen, we see two cups connected by a taut string, with one person speaking into one cup and another listening at the other end. Can you say that sound can travel through strings? We have learnt so far that vibrating objects produce sound and it is carried in all directions in a medium. The medium could be a gas, a liquid or a solid. How do we hear it?
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Section ten point four: We Hear Sound through Our Ears. The shape of the outer part of the ear is like a funnel. When sound enters it, it travels down a canal at the end of which there is a thin stretched membrane. It is called the eardrum. It performs an important function. To understand what the eardrum does, let us build a tin-can model of the eardrum. Activity ten point ten: Take a plastic or 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. Now ask your friend to speak Hurrey, Hurrey from the open end. In Figure ten point fifteen, we see a tin can with a rubber membrane stretched over one end, cereal grains on top, and a person speaking into the open end. Observe what happens to the grain. Why do the grains jump up and down? The eardrum is like a stretched rubber sheet. Sound vibrations make the eardrum vibrate. In Figure ten point sixteen, we see a cross section of the human ear showing the outer ear funnel, ear canal, eardrum, middle ear bones, inner ear, and auditory nerve connecting to the brain. The eardrum sends vibrations to the inner ear. From there, the signal goes to the brain. That is how we hear. You have already learnt in earlier classes about the oscillatory motion and its time period. The number of oscillations per second is called the frequency of oscillation. Frequency is expressed in hertz. Its symbol is Hz. A frequency of 1 Hz is one oscillation per second. If an object oscillates 20 times in one second, what would be its frequency? It would be 20 Hz. You can recognise many familiar sounds without seeing the objects producing them. How is it possible? These sounds must be different to enable you to recognise them. Have you ever thought what factors make them different? Amplitude and frequency are two important properties of any sound. Can we differentiate sounds on the basis of their amplitudes and frequencies?
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Section ten point five: Amplitude, Time Period and Frequency of a Vibration. We have learnt that the to and fro motion of an object is known as vibration. This motion is also called oscillatory motion. Activity ten point eleven: Take a metallic tumbler and a tablespoon. Strike the tablespoon gently at the brim of the tumbler. Hear the sound produced. Now bang the spoon on the tumbler and hear the sound produced again. Is the sound louder when the tumbler is struck hard? Now suspend a small thermocol ball touching the rim of the tumbler. In Figure ten point seventeen, we see a thermocol ball suspended by a thread, gently touching the rim of a glass tumbler. Vibrate the tumbler by striking it. See how far the ball is displaced. The displacement of the ball is a measure of the amplitude of vibration of the tumbler. Now, strike the tumbler gently and then a little harder. Compare the amplitudes of vibrations of the tumbler in the two cases. In which case is the amplitude larger? Loudness of sound is proportional to the square of the amplitude of the vibration producing the sound. For example, if the amplitude becomes twice, the loudness increases by a factor of 4. The loudness is expressed in a unit called decibel, abbreviated as dB. The following table gives some idea of the loudness of sound coming from various sources. Normal breathing is 10 dB. Soft whisper at 5 m is 30 dB. Normal conversation is 60 dB. Busy traffic is 70 dB. Average factory is 80 dB. Above 80 dB the noise becomes physically painful. 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. Compare the sound of a baby with that of an adult. Is there any difference? Even if two sounds are equally loud, they differ in some way. Let us see how. I wonder why my voice is different from that of my teacher.
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The frequency determines the shrillness or pitch of a sound. If the frequency of vibration is higher we say that the sound is shrill and has a higher pitch. If the frequency of vibration is lower, we say that the sound has a lower pitch. For example, a drum vibrates with a low frequency. Therefore, it produces a low-pitched sound. On the other hand, a whistle has a high frequency and therefore, produces a sound of higher pitch. In Figure ten point eighteen, we see a drum and a whistle side by side, illustrating low pitch and high pitch respectively. A bird makes a high-pitched sound whereas a lion makes a low-pitched roar. However, the roar of a lion is very loud while the sound of the bird is quite feeble. Every day you hear the voices of children and adults. Do you find any difference in their voices? Can you say that the frequency of the voice of a child is higher than that of an adult? Usually the voice of a woman has a higher frequency than that of a man.
Section ten point six: Audible and Inaudible Sounds. We know that we need a vibrating body for the production of sound. Can we hear the sound of all vibrating bodies? The fact is that sounds of frequencies less than about 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 Hz are also not audible to the human ear. Thus, for human ear, the range of audible frequencies is roughly from 20 to 20,000 Hz. Some animals can hear sounds of frequencies higher than 20,000 Hz. Dogs have this ability. The police use high frequency whistles which dogs can hear but humans cannot. The ultrasound equipment, familiar to us for investigating and tracking many medical problems, works at frequencies higher than 20,000 Hz.
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Section ten point seven: Noise and Music. We hear different types of sounds around us. Is the sound always pleasing? Does a sound sometimes cause discomfort to you? Some sounds are pleasant to the ear, whereas some are not. Suppose construction work is going on in your neighbourhood. Are the sounds coming from the construction site pleasing? Do you enjoy the sounds produced by horns of buses and trucks? Such unpleasant sounds are called noise. In a classroom, if all the students speak together, what would the sound produced be called? On the other hand you enjoy sounds from musical instruments. Musical sound is one which is pleasing to the ear. Sound produced by a harmonium is a musical sound. The string of a sitar also gives out a musical sound. But, if a musical sound becomes too loud, would it remain melodious?
Section ten point eight: Noise Pollution. You already know about air pollution. Presence of unwanted gases and particles in air is called air pollution. Similarly, presence of excessive or unwanted sounds in the environment is called noise pollution. Can you list some sources of noise pollution? Major causes of noise pollution are sounds of vehicles, explosions including bursting of crackers, machines, loudspeakers and so on. What sources in the home may lead to noise? Television and transistor radio at high volumes, some kitchen appliances, desert coolers, air conditioners, all contribute to noise pollution.
What are the Harms of Noise Pollution? Do you know that presence of excessive noise in the surroundings may cause many health related problems. Lack of sleep, hypertension which is high blood pressure, anxiety and many more health disorders may be caused by noise pollution. A person who is exposed to a loud sound continuously may get temporary or even permanent impairment of hearing.
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Measures to Limit Noise Pollution. To control noise, we must control the sources of noise. How can this be achieved? For this, silencing devices must be installed in air craft engines, transport vehicles, industrial machines and home appliances. How can the noise pollution be controlled in a residential area? All 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, thus reducing the harmful effects of noise pollution.
Hearing Impairment. Total hearing impairment, which is rare, is usually from birth itself. Partial disability is generally the result of a disease, injury or age. Children with impaired hearing need special care. By learning sign language, such children can communicate effectively. Because speech develops as the direct result of hearing, a child with a hearing loss may have defective speech also. Technological devices for the hearing-impaired have made it possible for such persons to improve their quality of life. Society can do much to improve the living environment for the hearing-impaired and help them live normal lives.
Let us review the keywords for this chapter. They are amplitude, eardrum, frequency, hertz, larynx, loudness, noise, oscillation, pitch, time period, vibration, voice box, and wind pipe.
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What You Have Learnt. Sound is produced by vibrating objects. In human beings, the vibration of the vocal cords produces sound. Sound travels through a medium which can be gas, liquid or solid. It cannot travel in vacuum. The eardrum senses the vibrations of sound. It sends the signals to the brain. This process is called hearing. The number of oscillations or vibrations per second is called the frequency of oscillation. The frequency is expressed in hertz. Larger the amplitude of vibration, the louder is the sound. Higher the frequency of vibration, the higher is the pitch, and shriller is the sound. Unpleasant sounds are called noise. Excessive or unwanted sounds lead to noise pollution. Noise pollution may pose health problems for human beings. Attempts should be made to minimise noise pollution. Plantation on the roadside and elsewhere can reduce noise pollution.
Now let us solve the exercises. Exercise one: Choose the correct answer. Sound can travel through gases only, solids only, liquids only, or solids, liquids and gases. The correct answer is solids, liquids and gases. Exercise two: Voice of which of the following is likely to have minimum frequency? Baby girl, baby boy, a man, or a woman. The correct answer is a man. Exercise three: In the following statements, tick T against those which are true, and F against those which are false. Part a: Sound cannot travel in vacuum. True. Part b: The number of oscillations per second of a vibrating object is called its time period. False. It is called frequency. Part c: If the amplitude of vibration is large, sound is feeble. False. It is loud. Part d: For human ears, the audible range is 20 Hz to 20,000 Hz. True. Part e: The lower the frequency of vibration, the higher is the pitch. False. Lower frequency means lower pitch. Part f: Unwanted or unpleasant sound is termed as music. False. It is termed as noise. Part g: Noise pollution may cause partial hearing impairment. True.
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Exercise four: Fill in the blanks with suitable words. Part a: Time taken by an object to complete one oscillation is called time period. Part b: Loudness is determined by the amplitude of vibration. Part c: The unit of frequency is hertz. Part d: Unwanted sound is called noise. Part e: Shrillness of a sound is determined by the frequency of vibration. Exercise five: A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency. Time period is the time taken for one oscillation. Total time is 4 seconds for 40 oscillations. So, time period equals 4 divided by 40, which is 0.1 second. Frequency is the number of oscillations per second. So, frequency equals 40 divided by 4, which is 10 Hz. Exercise six: The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration? Time period equals one divided by frequency. Frequency is 500 Hz. So, time period equals one divided by 500, which is 0.002 seconds.
Exercise seven: Identify the part which vibrates to produce sound in the following instruments. Part a: Dholak. The vibrating part is the stretched membrane. Part b: Sitar. The vibrating part is the stretched string. Part c: Flute. The vibrating part is the air column. Exercise eight: What is the difference between noise and music? Can music become noise sometimes? Noise is unpleasant or unwanted sound, while music is pleasant sound produced by musical instruments. Yes, music can become noise if it is played at a very high volume or if it is unwanted in a particular situation. Exercise nine: List sources of noise pollution in your surroundings. Sources include sounds of vehicles, bursting of crackers, industrial machines, loudspeakers, television and radio at high volumes, kitchen appliances, desert coolers, and air conditioners.
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Exercise ten: Explain in what way noise pollution is harmful to human. Noise pollution can cause lack of sleep, hypertension or high blood pressure, anxiety, and other health disorders. Continuous exposure to loud sounds can cause temporary or permanent hearing impairment. Exercise eleven: Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer. I would suggest buying the house three lanes away from the roadside. The roadside house will be exposed to heavy traffic noise, which causes noise pollution and health problems. The house three lanes away will be quieter and healthier. Exercise twelve: Sketch larynx and explain its function in your own words. The larynx, or voice box, is located at the upper end of the windpipe. It contains two vocal cords stretched across it, leaving a narrow slit for air passage. When air from the lungs passes through this slit, the vocal cords vibrate and produce sound. Muscles attached to the cords can make them tight or loose, changing the voice quality. Exercise thirteen: Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why? Yes. Light travels much faster than sound. The speed of light is approximately 300,000 km/s, while the speed of sound in air is only about 340 m/s. Therefore, we see the lightning flash almost instantly, but it takes time for the sound of thunder to travel to our ears.
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Now let us look at the Extended Learning Activities and Projects. Activity one: Visit the music room of your school. You may also visit musicians in your locality. Make a list of musical instruments. Note down the parts of these instruments that vibrate to produce sound. Activity two: If you play a musical instrument, bring it to the class and demonstrate how you play it. Activity three: Prepare a list of famous Indian musicians and the instruments they play. Activity four: Take a long thread. Place your hands over your ears and get some one to place this thread round your head and hands. Ask her to make the thread taut and hold its ends in one hand. Now ask her to draw her finger and thumb tightly along the thread. In Figure ten point nineteen, we see a person with hands over ears, a thread looped around their head and hands, being pulled taut. Can you hear a rolling sound like that of a thunder? Now repeat the activity while another friend stands near both of you. Can he hear any sound? You will hear the rolling sound through the thread and bone conduction, but the friend standing nearby will not hear it because the sound travels through the solid thread directly to your ears, not through the air. Activity five: Make two toy telephones. Use them as shown in Figure ten point twenty. Make sure that the two strings are taut and touch each other. Let one of you speak. Can the remaining three persons hear? See how many more friends you can engage in this way. Explain your observations. The sound travels through the taut string as vibrations. Only the person whose ear is connected to the other end of the string will hear it clearly. Others standing nearby will not hear it because the sound energy is confined to the string. Activity six: Identify the sources of noise pollution in your locality. Discuss with your parents, friends and neighbours. Suggest how to control noise pollution. Prepare a brief report and present it in the class.
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Did You Know? Golconda fort, near Hyderabad, is one of the most magnificent forts in India. It is famous for many engineering and architectural marvels. One of the marvels is the water supply system. But, perhaps, more astonishing is a dome near the entrance to the fort. A hand-clap at a particular point under the dome reverberates and can be heard at the highest point of the fort, about a kilometre away. This was devised as a warning system. If a guard saw a suspicious movement outside the fort, he clapped at the particular point under the dome, and the army inside the fort was alerted to the danger of the approaching enemy.
Thank you for listening! Keep revising and practicing. Goodbye! [CHAPTER_COMPLETE]