Page 1 :
Sound, , he sensation felt by our ears is called sound. Sound is a form of energy. Sound is that form of, , energy which makes us hear. We hear many sounds around us in everyday life. At home we hear, , the sounds of our Parents talking to us. We also hear the sounds of telephone bell, radio, television,, stereo-system, mixer-grinder and washing machine. At school we hear the sounds of our teachers,, classmates, and the school bell. We hear the sounds of, , scooters, motorcycles, cars, buses and trucks on the, roads. And the sound of a flying aeroplane is heard from the sky., , At a music concert, we hear the sounds produced by various musical instruments like sitar, vena,, violin, guitar, fanpura, piano, harmonium, flute, shehnai, tabla and cymbals, etc. Ina garden we hear the, sounds of chirping of birds and rustling of leaves of the trees in breeze, In rainy season, we hear the sound, , of rain drops falling on the roof and the sound of thundering of clouds. Ina factory we hear the different, types of sounds made by various big and small machines. And at night, when most of the sounds cease, , (stop), we can still hear the faint sounds of the ticking of a clock and the buzzing of, Each sound is special to the object which Produces it. For ex, , an electric switch and the whirr (sound of rotation) of a cei], by a clock, an electric switch and a ceiling fan, Tespectively. We rarely forget, and quickly Tecognise this sound when we hear it again. We can even recognise a Person from the sound, of his voice even without seeing his face, For example, we can usually tell from the voice which singer is, singing a particular Song on radio even though we do Not see the face of the singer. Thus, every sound hes, a unique ‘quality’ which distinguishes it from other sounds, Sound plays an important role in our daily, life. It helps us to communicate (or talk) with others, We will now discuss how sound is produced. This is, , a mosquito., ample, the ticking of a clock, the click of, ing fan are the characteristic sounds produced, , a sound having heard it once, , , , called production of sound., , SOUND IS PRODUCED By VIBRATI, ————reED BY V \, , When an object moves ‘backwards and forwards’ (to-and-fro), , or that the object is ‘vibrating’. Sound is produced when 4n object yj, , words, sound is produced by vibrating objects, So, whe, , vibrating to produce that sound. The, , NG OBJECTS, , rapidly, we say that the object ‘vibrates, brates (moves to-and-fro rapidly). In othet, , Whenever we hear a sound, then some object must me, energy required to make an object vibrate and produce sound, , 234
Page 2 :
235, , y some outside source (like our h;, , o ea spject are quite large which we cq, ve uci object are so small that we, ‘it b touching it gently with the finge, : nee roduced by vibrating objects, a, i 7 us ting 2 bicycle bell and touch it, 1 roducing sound) is shaking b, , ions of & sound, , and, wind, e, ind, etc.) In some cases, the vibrat, of the, , iN See wi,, , Cannot ith our eyes. But in other cases, the vibration”, , rs of see them easily, we have to feel the vibration® of such, , ° ~ th, ur hand. We will now give some examples ehh, , , , , , , gently with our fingers. We will find that a ringin, niet . Now, if we hold the ringing bic i and forth continuously. We say that the PIC, tine coming. From this we conclud ve bell tightly with our hand, it stops vibrating, an, “03 i vibrates, it produces sound; € that sound is produced by a vibrating bicycle bell. When, gee it. In general, we can say th ‘and when the bicycle bell stops vibrating, the sound al, oi from the vibrations of ir 'y that a body must vibrate to produce sound. The sound of a sch¢, ‘sod ace’ by bee on on brass plate when it is hit by a hammer, , f siretch a rubber band and tie it tightly between two nails fixed ona table [as shown '9 F, position the rubber band is not vibrating and hence riot producing any sou, , , , »0] bell 1, , , , pnts, Stretched —, rubber band Hela na, , tao, , , , , , Nail + Nail, , , , , , jE, , , , , , (a) Stretched rubber band not vibrating : (b) Stretched rubber band, No sound produced Sound is prod, , , , Figure 1., , Let us pluck the stretched rubber band in the middle with our finger. The rubber band starts vib, and produces sound. We can see the vibrations of the stretched rubber band. The rubber band, wards and forwards rapidly) between the positions A and B [see Figure (by). TI, hed rubber band vibrates. Now, if we hold the vibrating rubber band, nd stops vibrating and the sound being produced by it also stops, any stretched strings, a sound. If we now put our finger gently, tring vibrates. The sound ¢, , , , sound is, , , , (moves bac!, produced when a stretc, our hand, the rubber ba, , 3, Sitar is a musical instrument. It has m., (wire) of a sitar in the middle, the sitar makes, wecan feel the string vibrating. Thus, sound is produced when a sitar s|, vena is also produced by the vibrations of stretched strings., , 4, When we talk, we make sound. This sound is made by the vibrations of two vocal cords present 17, cut voice box fixed in the throat. This can be shown as fol, gently on our throat and talk to one of our friends (oT sing, our fingers feel that something, ig moving or vibrating, inside the, ‘helungs passes up the wind-pipe. This air makes the vocal cords in, vibrating vocal cords produce the sound (of our talk). Thus, sown, vibrate. Please note that the vibrations of our yocal cords are small, so we %, hands. We cannot see these vibrations with our eyes. Mosquitoes and bees make a buzzing sound by, ‘rating their wings very, Very rapidly., wn blow across the mouth of an : ney, When Is sound is produced by the vibrations, shai air column enclosed in @ tube vibra “, va ONS Of air column enclosed in the flute tube, ‘rat scapes), , "ations of air enclosed in the balloon on it escape, , the string, , (stretched wires). If we pluck, on the sitar, , , , dota, , , , lows: Let us hold the fingers of out right hand, a song). When we are talking (making sound), throat. Actually, when we talk: air trom, our voice box to vibrate rapidly. And, dis produced when our vocal cords, can only teel them witl, , , , then a whistling sound is produced [see Figure, , ube, sst-tube, Thus. sound is produced, , mpty test-t, air prese, The sound of @ thate, a bursting balloon is produced bv the, , nt in the te, hansurt) is produced by the, , , , The sound of, , (whe
Page 3 :
236, , , , is sed when the stretched, (a) Sound is produced when the air (b) Sound is prod ofthe cmt., column inside the test-tube vibrates. membrane for sin), Figure 2. Sound is produced by vibrations of objects. ;, the membrane starts vibrating and pr,, , 6. If we hit the stretch kin) of a tabla, i €, sound. Now, if we puta esa: _ membrane of this sound aircon eae the Pebbles ,, start jumping up and down showing that the tabla membrane is vibrating, while a Uclng sound, 7},, sound is produced when the membrane (or skin) of a tabla vibrates. When we strike at the Membran,, skin) of a drum, it vibrates to produce sound [see Figure 2(b)]- Thus, sound is produced when the Stretches, , the thin cone of its Speak..., , membrane (or skin) of a drum vibrates. If we switch on a transistor radio,, e our fingers gently on the cone g¢ ., wards and forwards’ rap; ;, , vibrates and produces sound. We can feel these vibrations if we plac, , speaker. So, in a radio or television, the thin cone of the speaker vibrates ‘back’ ind fo: i, and produces sound. Thus, the sound of a radio (or television) is produced by the vibrations of the cone, , , , of speaker., 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), and, , , , (iv) by vibrating plates (as in a bicycle bell)., We will now discuss how sound reaches from a sound producing object to our ears. This is, , transmission of sound or propagation of sound., , , , , Propagation of Sou, Sound is produced by the vibrations of an object. When an object vibrates back and forth in air,, the molecules of air close to this object also start vibrating back and forth with the same frequency. The, , vibrating air molecules pass on their motion to the next layer of air molecules due to which they also, , vibrating back and forth. This process goes on and on. And ultimately, all the air molecules aroun, , sound producing object start vibrating back and forth (just like the vibrating object). When the vibratin, molecules fall on our ears, the ears feel these vibrations as sound. Thus, when an object vibrates (and, , makes sound), then the air around it also starts vibrating in exactly the same way and carries sound *, our ears through the vibrations of its molecules. And we say that a sound wave travels from the sou, , , , producing object to our ears, through the air., , SOUND PRODUCED BY HUMANS, , The human beings produce sound by using the voice box which is called ‘larynx’. Voice bo \", larynx) is situated in our throat at the top of the wind-pipe (or trachea). The human voice box (or lan", contains two ligaments known as vocal cords. The vocal cords are a kind of strings. Sound is produced, , = th, , by the vibrations of vocal cords (see Figure 3). The vocal cords are attached to muscles which change, , tension (stretching) in the cords and the distance between the cords.
Page 4 :
237, , Larynx, , Vocal, cords, , , , “i a vibrating (b), 5 = 3+ Sound is produced when our vocal cords vibrate, Normally, the muscles of vocal cords are completely relaxed due hich the vocal cords are, nd loose so that air from the lungs passes Hague thon ae producing any sound [see, or singing), the two vocal cords are far apart with a lot of gaP, , ed a, xa)| Thus, when we are not talking (, , Been tems, . (i) When > won pees. be muscles of vocal cords contract due to which the two vocal cords, pecome stretchee a i . ogether leaving only a narrow slit between them [see Figure 3(0)]- The lungs, ' ga current 0 me e! auce. the two vocal cords. This air makes the vocal cords vibrate. And the, vibrating vocal cor is he MICE sound. Thus, when we talk or sing (or make any other sound), we actually, ake oUF vocal Soe prate. And vibrations of vocal cords by expelled air produce vocal sounds., , ACTIVITY, , | We can demonstrate the working of vocal cords to produce sound as, , follows: Take two rubber strips of the same size. Place these two rubber, | grips one above the other. Hold the two ends of the rubber strips in your, [hands and stretch them (ight. Keep the stretched rubber strips in front of, | your mouth and blow air through the thin gap between them (see Figure, , 4), As the air blows through the stretched rubber strips, a sound is, , , , | |, | produced. This sound is produced by the vibrations of stretched rubber, |srips when air rushes thee ugh the thin gap between them. Our voc a) Figure 4. Activity to demonstrate the, | ; working of vocal cords to produce, cords produce sound in similar way. :, , sound., , Nera ———————, , 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, ‘han man. Due to the shorter vocal cords, the frequency (or pitch) of a woman’s voice is higher than that, faman. Small children have very short vocal cords due to which the frequency (or pitch) of their voice is, ay high. This is why their voice is shrill. So, it is due to the different frequencies (or different pitch), , als ' 2 » voice: on, wome sila, lies by the different lengths of their vocal cords that the voices of men, women and children are, “erent,, , SOUND NEEDS A MEDIUM FOR PROPAGATION, , . The substance through which sound travels is called medium. The medium can be a solid substance, , legs or a gas. And transmission of sound is called propagation of sound. So, by saying that sound, , he * Medium for pro ae ation, we mean that sound needs a solid, liquid or gas for transmission. In, , “words, sound needs er atavial medium like solid, liquid or gas to tray el and be heard. Sound can, rs seds a ria, , Pa
Page 5 :
238, , vacuum (or empty space),, travel through solids, liquids and gases but it cannot _ ia els liquids and gases cnt 7, travel through solids, liquids and gases because the molecules of s ‘, , Y 8, , Sh eau nnot travel through vacy ty, , waves from one place to another (through their vibrations). Sound ca valent 4 bec, a ‘0 another (throug) y sound waves. So, a materia Jum like ai, : Ms,, , vacuum has no molecules which can vibrate and carry 80 the ‘source of sound’ to our ‘earg: “Ay, Wood, ete., is necessary for the transmission of aoune from - oe some activities which wil é oh,, , i : ‘ » will now desc! s Show ,, words, sound needs a medium for propagation. We will through vacuum W thay, sound can travel through solids, liquids and gases, but not 8, , Sound Can Travel Through Solids, Liquids and Gases, , ees, ACTIVITIES, , Let us press our ear on to one side of a wooden bench and ask a fia e sil ea Other eng, of the bench lightly. We will hear the sound of tapping or scratching through the “ na Auite loud),, This means that sound can travel through wood, which is a solid. Now take a = re - a le of metal and, hold its one end close to your ear very carefully. Ask your friend to scratch the other end of metre Scale, lightly. You will be able to hear the sound of scratching through the metallic metre scale quite loudly (thoug,, other persons around you cannot hear the same sound of scratching). This means that, sound can traye!, through a metal, which is a solid. in general, we can say that sound travels through solid substances, We, can also make a toy telephone as follows to show that sound travels through solids., , Take two Open tin-cans, each having a small hole at the centre of its bottom. Also take about 20 Metres, long thick thread. Pass one end of the thread into the hole of one tin-can and hold it inside the can by tying, , it toa pin. Similarly, pass the other end of thread into the hole of second tin-can and tie it to another bin, Tin-can, , 20 metres long, thick thread, , , , , , , , Tin-can, , , , Figure S, A toy telephone (Here sound travels through the thread, which is a solid substance)., The two tin-cans connected by the thread are now held by two children and taken as far as possible so the, the thread gets stretched tightly (as shown in Figure 5)., , Now, if one child speaks into one tin-can, he can be heard by the child at the other end who pul!, ear to the other tin-can. For example, in Figure 5, the child at the left end is speaking into the toy telephow, and the child at the right end (who is 20 metres away) can hear his sound clearly. In this case of, telephone, sound made by the child while speaking, travels through the thread, which is a solid substan, , , , |, , , , The above activities show that sound can travel through solids like wood, metal and thread. In!, sound can travel through all the solid substances like metals (iron, steel, etc.), wood, bricks, stone *", glass, etc. We will now describe an activity to show that sound can also travel through liquids. We I, water as liquid in the following activity.