[{"data":1,"prerenderedAt":1057},["ShallowReactive",2],{"layer:sound:discover":3},{"layer":4,"contentHash":1035,"dependencyHashes":1036,"approval":1051,"releaseId":1056},{"schemaVersion":5,"conceptId":6,"locale":7,"depth":8,"revision":5,"title":9,"subtitle":10,"summary":11,"objectives":12,"estimatedMinutes":18,"plate":19,"blocks":40,"sourceIds":1030,"reviewStatus":1031,"authoring":1032},1,"sound","en","discover","Everything that sounds is shaking","Find the vibration behind every sound, follow it to your ear, and learn why space is silent","Feel your own throat buzz, watch a tuning fork throw water, and follow the shaking from a tabla skin across the room to the hair cells in your ear. Meet pitch, loudness, echoes and the thunder rule, and find out why nothing at all can be heard in space.",[13,14,15,16,17],"Find the vibrating part of any sound-maker, and name the four families: skin, string, air column and solid body.","Explain why sound needs a medium, and why a bell in a vacuum jar goes silent while you can still see it ringing.","Say what changes pitch (frequency) and what changes loudness (amplitude), and change each one on purpose.","Use the flash-to-thunder count, and the round-trip idea for an echo, to work out a distance.","Trace the path of a sound through your ear and give one reason loud noise causes permanent damage.",30,{"title":20,"rows":21},"Lesson plate",[22,25,28,31,34,37],{"label":23,"value":24},"Depth","Discover",{"label":26,"value":27},"Reading time","≈ 30 minutes",{"label":29,"value":30},"Prior knowledge","None: start here",{"label":32,"value":33},"Chapters","10",{"label":35,"value":36},"Labs","Tone labs, speed race, storm lab, two games",{"label":38,"value":39},"Units used","Hz, dB, m\u002Fs, m, s",[41,45,51,57,60,65,70,132,196,201,206,209,238,243,275,280,298,301,315,319,323,357,362,367,370,374,408,413,442,446,459,475,480,483,487,510,523,558,562,567,570,574,589,597,613,617,622,625,628,644,649,654,657,700,706,711,715,719,743,747,752,755,782,786,789,793,798,803,808,866,998,1002,1018],{"id":42,"type":43,"markdown":44},"intro-throat","prose","Put two fingers lightly on the front of your throat and hum a long, low **mmmmmmm**.\n\nFeel that? Your throat is **buzzing**. Stop humming and the buzz stops at exactly the same instant as the sound. Hum higher and the buzz gets faster and finer. Hum louder and the buzz gets stronger.\n\nThat is the whole secret of this topic, and you just found it with your own fingers: **every sound in the world is made by something shaking.** A drum skin, a guitar string, a car engine, a mosquito's wings, a slamming door, your own voice box. Nothing shakes, nothing sounds.\n\nIn this lesson you will meet the shaking, follow it through the air to your ear, find out why space is completely silent, and learn why a thin string sounds high and a big drum sounds low.",{"id":46,"type":47,"variant":48,"title":49,"markdown":50},"how-to-read","callout","observation","How to use this lesson","Read the chapters in order the first time. Whenever you meet a **try it** box, actually try it: this is a topic you can test with your own body, a ruler, a rubber band and a bucket of water. When you meet a **prediction**, decide your answer before reading on.",{"id":52,"type":53,"title":54,"eyebrow":55,"navLabel":56},"ch01","chapter","Every sound starts with something shaking","Chapter 01","1 Something shakes",{"id":58,"type":43,"markdown":59},"ch1-p1","Walk around your home and hunt for a sound you can *see* being made.\n\nA **tabla** is easy: bang it and the tight skin dips and springs back, over and over, so fast it blurs. Sprinkle a few grains of rice on the skin first and they will jump.\n\nA **rubber band** stretched between your thumbs is easier still. Pluck it and you see a fuzzy band instead of a sharp line, because the band is in two places at once as far as your eyes can tell. Press it with a finger and both the blur and the sound stop together.\n\nA **steel tumbler** tapped with a spoon does not look like it is moving at all. But hold it gently against your lip after tapping and you feel the tingle. Dip the ringing rim into a bowl of water and the water will shiver and splash outwards. The tumbler is shaking too; just by a tiny amount, very fast.",{"id":61,"type":47,"variant":62,"title":63,"markdown":64},"tryit-fork-water","try_it","The tuning fork in the water","You need a tuning fork (many schools have one), or a metal fork from the kitchen, and a glass of water.\n\n1. Tap the fork firmly on a **rubber slipper or your knee**, never on stone or metal, which damages it and is far too loud.\n2. Listen to the steady note. The two prongs look perfectly still.\n3. Now lower the tip of one prong so it just touches the surface of the water.\n\nThe water leaps. Splashes fly sideways. The fork was never still: it was flicking in and out hundreds of times every second, too fast and too small for your eyes to catch, but not too small for the water.\n\n**Then** grip the prongs with your fingers. The note dies instantly. Stop the shake, stop the sound.",{"id":66,"type":47,"variant":67,"title":68,"markdown":69},"def-vibration","definition","Vibration","A **vibration** is a fast back-and-forth movement about a resting position. It goes out, comes back past the middle, goes out the other way, and returns, over and over.\n\nThe thing that vibrates is the **source** of the sound. Stop the vibration and the sound stops: that is why you damp a tabla with your palm, or press your hand on a ringing bell.",{"id":71,"type":72,"title":73,"prompt":74,"options":75},"explorer-what-vibrates","explorer","What is actually shaking?","Pick a sound-maker and find the part that vibrates.",[76,88,99,110,121],{"id":77,"label":78,"chain":79,"badge":84,"note":87},"voice","Your voice",[80,81,82,83],"Air pushed up from lungs","Two vocal folds flutter","Throat and mouth shape it","Sound leaves your lips",{"text":85,"tone":86},"Two flaps of tissue","yes","Across the top of your windpipe lie two small folds of tissue, the vocal folds. To make a voiced sound you close them and push air up from your lungs. The air forces them apart, they snap back, and they open and close hundreds of times a second. That flutter is your buzz. Your tongue, teeth, lips and the hollow spaces of your mouth and nose then shape the buzz into 'aa', 'ee' or 'ma'. Whisper and the folds stay open: you hear rushing air, which is why a whisper has no tune.",{"id":89,"label":90,"chain":91,"badge":96,"note":98},"tabla","Tabla and dholak",[92,93,94,95],"Palm or finger strikes","Stretched skin dips","Skin springs back","Air above it is pushed",{"text":97,"tone":86},"A stretched skin","On a tabla the tight skin, the pudi, is the vibrating part. Strike it and the skin bends, springs back past flat, bends the other way, and keeps going until friction stops it. Tighten the leather straps and the skin springs back faster, giving a higher note; that is what the tuning blocks and the small hammer are for. The black patch in the middle, the syahi, is not decoration: it makes the tabla's overtones fit together so the drum sounds like a musical note rather than a thud.",{"id":100,"label":101,"chain":102,"badge":107,"note":109},"sitar","Sitar and veena",[103,104,105,106],"Finger plucks a string","String swings side to side","Bridge passes the shake on","Gourd body pushes air",{"text":108,"tone":86},"A stretched string","A plucked string vibrates, but a bare string is very quiet: it is too thin to push much air. The shake travels down the string to the bridge, and the bridge shakes the large wooden face and the hollow gourd. Those big surfaces push a lot of air, which is why the instrument is loud. The sitar's wide, curved bridge, the jawari, lets the string graze it and gives the sitar its famous buzzing shimmer.",{"id":111,"label":112,"chain":113,"badge":118,"note":120},"bansuri","Bansuri and shehnai",[114,115,116,117],"Air blown across a hole","The air column wobbles","The whole pipe of air rings","Sound leaves the open end",{"text":119,"tone":86},"A column of air","Here nothing solid does the singing. The bamboo is just a container. Blowing across the mouth hole sets the column of air inside the tube wobbling back and forth along its length. Cover more holes and the air column is longer, so it wobbles more slowly and the note is lower. In a shehnai the wobble is started by a double reed, two thin slips of cane that flutter against each other when you blow, and the air column takes it from there.",{"id":122,"label":123,"chain":124,"badge":129,"note":131},"ghatam","Ghatam and manjira",[125,126,127,128],"Strike the solid object","The whole body rings","Its surface pushes air","The ring slowly fades",{"text":130,"tone":86},"The object itself","A ghatam is a clay pot with no skin and no strings: the baked clay itself rings when struck. Manjira (small brass cymbals), ghungroo bells, a temple bell and a steel tumbler tapped with a spoon all work this way. The whole solid object flexes by an amount far too small to see and springs back thousands of times a second. Because metal loses energy slowly, a bell rings for many seconds; clay loses it fast, so a ghatam gives a short, dry note.",{"id":133,"type":134,"component":135,"componentVersion":5,"config":136,"objective":194,"textAlternative":195},"lab-sort-vibrates","interactive","sort-game",{"prompt":137,"bins":138,"items":148,"seconds":193},"Which part of each instrument or object is doing the vibrating?",[139,141,143,145],{"id":140,"label":97},"skin",{"id":142,"label":108},"string",{"id":144,"label":119},"aircol",{"id":146,"label":147},"solid","The solid body itself",[149,152,156,160,163,167,171,174,178,182,185,189],{"id":89,"label":150,"bin":140,"why":151},"Tabla","The tight goatskin pudi dips and springs back when struck.",{"id":153,"label":154,"bin":140,"why":155},"mridangam","Mridangam","A barrel drum with a skin at each end; both heads vibrate.",{"id":157,"label":158,"bin":140,"why":159},"dholak","Dholak","Two stretched skins, tuned by tightening the ropes or bolts.",{"id":100,"label":161,"bin":142,"why":162},"Sitar","Plucked steel and bronze strings vibrate; the gourd body makes them loud.",{"id":164,"label":165,"bin":142,"why":166},"veena","Veena","Four playing strings and three drone strings, all plucked.",{"id":168,"label":169,"bin":142,"why":170},"sarangi","Sarangi","Bowed gut strings, plus dozens of sympathetic strings that ring on their own.",{"id":111,"label":172,"bin":144,"why":173},"Bansuri","The bamboo holds still; the column of air inside it does the vibrating.",{"id":175,"label":176,"bin":144,"why":177},"shehnai","Shehnai","A double reed starts it, but the sound comes from the air column in the cone.",{"id":179,"label":180,"bin":144,"why":181},"whistle","A referee's whistle","Air swirls inside the small chamber and sets the air column ringing.",{"id":122,"label":183,"bin":146,"why":184},"Ghatam","A clay pot with no skin: the baked clay itself rings.",{"id":186,"label":187,"bin":146,"why":188},"manjira","Manjira (small cymbals)","The brass discs flex and spring back after they clash.",{"id":190,"label":191,"bin":146,"why":192},"tumbler","Steel tumbler tapped with a spoon","The steel wall of the tumbler flexes thousands of times a second.",0,"Sort twelve sound-makers by what actually vibrates inside them: a skin, a string, a column of air, or the solid object itself.","A sorting game with twelve cards and four bins labelled **a stretched skin**, **a stretched string**, **a column of air** and **the solid body itself**.\n\nThe skins are tabla, mridangam and dholak. The strings are sitar, veena and sarangi. The air columns are bansuri, shehnai and a referee's whistle. The solid bodies are ghatam, manjira and a steel tumbler tapped with a spoon.\n\nThe useful test is: *what would I have to hold to make the sound stop?* For a drum, the skin. For a sitar, the string. For a bansuri, nothing solid at all, which is why you stop the note by stopping your breath. For a ghatam, the pot itself.\n\nThese four families are exactly how Indian musicians have classified instruments for two thousand years: **avanaddh** (covered with skin), **tat** (stringed), **sushir** (blown) and **ghan** (solid).",{"id":197,"type":47,"variant":198,"title":199,"markdown":200},"misconception-air-makes","misconception","“The air makes the sound”","The air does not *make* the sound. The air **carries** it.\n\nThe sound is made by a solid thing shaking: a skin, a string, a bell, your vocal folds. The air is only the messenger that brings the news to your ear. Take the air away and the skin still shakes, but nothing reaches you.\n\nThe one exception looks like a puzzle and is not: in a flute or a whistle, the vibrating thing *is* a column of air. But even there, that column is a definite object of a definite length, shaking back and forth like everything else.",{"id":202,"type":53,"title":203,"eyebrow":204,"navLabel":205},"ch02","How the shake reaches your ear","Chapter 02","2 Through the air",{"id":207,"type":43,"markdown":208},"ch2-p1","Your ear is nowhere near the drum. So how does the news get across the room?\n\nThink about what a drum skin does to the air touching it. When the skin bulges **outwards**, it shoves the air right next to it. That air gets squashed and crowded. When the skin snaps **back**, it leaves a little extra room, and the air there becomes thinner and stretched out.\n\nSquash, stretch, squash, stretch. Each squashed patch shoves the next patch of air, which shoves the next, and so on across the room. What travels is not the air itself, but the **pattern** of squashed and stretched air, racing outwards in every direction.\n\nWhen that pattern of pushes reaches your ear, it pushes on your eardrum, and the whole story starts again in reverse.",{"id":210,"type":211,"title":212,"items":213},"steps-journey","steps","From tabla to brain, in six steps",[214,218,222,226,230,234],{"title":215,"tag":216,"text":217},"The player strikes","source","A palm hits the tabla skin and the skin starts flicking in and out, a few hundred times a second.",{"title":219,"tag":220,"text":221},"The skin shoves air","squash","Each outward bulge squashes the air touching it; each inward dip leaves a thinner patch behind.",{"title":223,"tag":224,"text":225},"The pattern travels","343 m\u002Fs","Squashed and thinned patches move outwards through the room at about 343 metres every second. The air itself barely moves.",{"title":227,"tag":228,"text":229},"The eardrum is pushed","your ear","The arriving pattern pushes your eardrum in and lets it spring out, at the same rate as the tabla skin.",{"title":231,"tag":232,"text":233},"Tiny bones pass it on","amplify","Three of the smallest bones in your body carry the movement across the middle ear and press on a fluid-filled spiral.",{"title":235,"tag":236,"text":237},"The brain hears “tabla”","signal","Hair cells in the spiral turn the movement into nerve signals, and your brain recognises the pattern in a fraction of a second.",{"id":239,"type":47,"variant":240,"title":241,"markdown":242},"aha-not-carried","aha","The air is not delivered to your ear","This is the idea most people get wrong, so meet it early.\n\nWhen someone shouts to you across a field, **their breath does not reach you**. Each patch of air only jiggles a tiny distance back and forth and stays roughly where it was, like each person in a stadium wave standing up and sitting down without leaving their seat.\n\nWhat crosses the field is the *wave*: the travelling pattern of pushes. If air itself had to travel, sound would arrive as a smell arrives, minutes later, and it would be blown away by the slightest breeze.",{"id":244,"type":134,"component":245,"componentVersion":5,"config":246,"objective":273,"textAlternative":274},"lab-match-words","match-pairs",{"prompt":247,"mode":248,"pairs":249},"Match each sound word to what it means.","connect",[250,252,255,258,261,264,267,270],{"a":68,"b":251},"A fast back-and-forth movement",{"a":253,"b":254},"Source","The thing that is shaking",{"a":256,"b":257},"Medium","The stuff sound travels through",{"a":259,"b":260},"Pitch","How high or low a sound is",{"a":262,"b":263},"Loudness","How strong a sound seems",{"a":265,"b":266},"Echo","A sound that bounces back to you",{"a":268,"b":269},"Vacuum","Space with nothing in it, not even air",{"a":271,"b":272},"Eardrum","The skin that the arriving sound pushes","Connect the eight key words of this lesson to their plain-English meanings.","A matching game with eight pairs.\n\n- **Vibration** is a fast back-and-forth movement.\n- **Source** is the thing that is shaking.\n- **Medium** is the stuff sound travels through, such as air, water, wood or steel.\n- **Pitch** is how high or low a sound is.\n- **Loudness** is how strong a sound seems.\n- **Echo** is a sound that bounces off a surface and comes back to you.\n- **Vacuum** is space with nothing in it, not even air.\n- **Eardrum** is the thin skin inside your ear that the arriving sound pushes.\n\nIf you can say all eight of these without looking, you have the vocabulary for the whole topic.",{"id":276,"type":53,"title":277,"eyebrow":278,"navLabel":279},"ch03","Sound needs something to travel through","Chapter 03","3 Sound needs stuff",{"id":281,"type":282,"prompt":283,"options":284,"explanation":297},"predict-vacuum","prediction","A small electric bell is hung on a thread inside a thick glass jar and switched on. You can hear it ringing. Now a pump slowly sucks all the air out of the jar. Through the glass you can still see the hammer hitting the bell. What do you hear?",[285,288,291,294],{"id":286,"label":287},"a","The same loudness: the glass carries the sound",{"id":289,"label":290},"b","Louder, because nothing is in the way",{"id":292,"label":293},"c","Fainter and fainter, until you hear nothing at all",{"id":295,"label":296},"d","The pitch drops but the loudness stays the same","**Fainter and fainter, until silence.** The hammer is still hitting the bell; you can see it. But with no air inside the jar, there is nothing to squash and stretch, so nothing carries the pattern to the glass and out to your ear.\n\nLet the air back in and the ringing swells up again. This experiment, first done with the new air pumps of the 1660s, is the clearest proof that sound is not a thing in its own right: it is something that happens *to* matter.",{"id":299,"type":47,"variant":67,"title":256,"markdown":300},"def-medium","A **medium** is the material that a wave travels through. For sound it can be a gas (air), a liquid (water) or a solid (wood, steel, the ground, your skull).\n\nSound has no way to exist without one. Light, radio and heat from the Sun cross empty space perfectly well; sound cannot cross even a few centimetres of it.",{"id":302,"type":134,"component":303,"componentVersion":5,"config":304,"objective":313,"textAlternative":314},"lab-speed-race-basic","sound-speed",{"media":305,"distanceM":310,"modes":311},[306,307,308,309],"vacuum","air","water","steel",343,[312],"race","Race the same sound across 343 metres through four materials, including one with nothing in it at all.","A race. The same clap is released at the same instant into four lanes, each 343 metres long: **vacuum**, **air**, **water** and **steel**. A timer runs.\n\n- **Steel** arrives first, after about 0.06 seconds: sound travels 5,960 metres every second in steel.\n- **Water** is next, at about 0.23 seconds: 1,480 metres per second.\n- **Air** takes exactly 1.00 second: 343 metres per second, the value we use all through this topic.\n- **Vacuum** never arrives. The timer runs on and on and the lane stays empty, because there is nothing there to pass the push along.\n\nThe pattern to notice: the more tightly the particles of a material are joined to each other, the faster the push gets handed on. Steel is about **17 times** faster than air, and water about **4 times** faster.",{"id":316,"type":47,"variant":198,"title":317,"markdown":318},"misconception-space-sounds","“Space battles sound like that”","In films, spaceships roar past and explosions boom. In real space, nothing of the kind happens.\n\nBetween the planets there is almost no matter at all: a few atoms in a space the size of a room. With no medium, there is no sound. A real rocket engine firing beside you in orbit would be **completely silent** to a floating listener.\n\nAstronauts on a spacewalk talk by radio, which is a kind of light and needs no medium. Touch two helmets together, though, and you can hear each other through the solid plastic.",{"id":320,"type":47,"variant":62,"title":321,"markdown":322},"tryit-table-scratch","Hear through a table","1. Ask a friend to scratch the far end of a wooden table or desk very gently with a fingernail, so gently that you can barely hear it across the table.\n2. Now press your ear flat on the table top and ask them to scratch exactly as before.\n\nIt is suddenly much clearer. The wood is a better path for the sound than the air is.\n\nTry it again on the metal rail of a bed or a bench, and with your ear against a filled steel bucket while someone taps the far side. Every time, the solid path wins. Dogs and many animals put their heads to the ground for exactly this reason.",{"id":324,"type":325,"caption":326,"columns":327,"rows":332},"table-media-basic","table","How fast sound travels in different materials, and how far it gets in one second (dry air at 20 degrees Celsius)",[328,329,330,331],"Material","Speed","Distance in 1 second","Compared with air",[333,338,342,347,352],[334,335,336,337],"Vacuum (empty space)","no sound at all","nothing arrives, ever","sound cannot start",[339,224,340,341],"Air","343 m, about three cricket pitches end to end","1 time",[343,344,345,346],"Fresh water","1,480 m\u002Fs","1.48 km","about 4 times faster",[348,349,350,351],"Wood (along the grain)","about 3,800 m\u002Fs","3.8 km","about 11 times faster",[353,354,355,356],"Steel","5,960 m\u002Fs","almost 6 km","about 17 times faster",{"id":358,"type":47,"variant":359,"title":360,"markdown":361},"nuance-why-solid-fast","nuance","Why solids are the fast lane","It seems backwards: steel is far heavier than air, so should it not be slower?\n\nWhat matters is not weight but **how firmly the particles are joined**. In steel, every atom is locked to its neighbours. Nudge one and its neighbour moves almost instantly, like a row of coins pressed tightly together in a line.\n\nIn air the particles are far apart and free. A pushed particle has to fly across a gap before it bumps into the next one, and all that flying takes time.\n\nStiff joints beat loose ones, and that is why the order goes: solids fastest, then liquids, then gases, and nothing at all in a vacuum.",{"id":363,"type":53,"title":364,"eyebrow":365,"navLabel":366},"ch04","High and low: pitch","Chapter 04","4 High and low",{"id":368,"type":43,"markdown":369},"ch4-p1","A mosquito whines. A lorry rumbles. Both are sounds, but they sit at opposite ends of something musicians call **pitch**.\n\nPitch is decided by one thing only: **how many times the source vibrates each second**. We count those in **hertz**, written Hz. One hertz means one complete back-and-forth every second.\n\n- A mosquito's wings beat roughly 600 times a second: about 600 Hz, a thin whine.\n- A big drum's skin might flick about 60 times a second: 60 Hz, a deep boom.\n\nMore shakes per second means a **higher** pitch. Fewer shakes means a **lower** pitch. Nothing else in the sound changes: the mosquito is not louder than the lorry, just higher.",{"id":371,"type":47,"variant":67,"title":372,"markdown":373},"def-frequency","Frequency and pitch","**Frequency** is the number of complete vibrations every second, measured in **hertz (Hz)**. 1,000 Hz is also written 1 kHz (one kilohertz).\n\n**Pitch** is what you *hear* when the frequency changes: high or low.\n\nFrequency is the measurement; pitch is the experience. A tuning fork stamped 440 Hz vibrates 440 times a second, and musicians hear that as the note they tune to.",{"id":375,"type":134,"component":376,"componentVersion":5,"config":377,"objective":406,"textAlternative":407},"lab-wave-pitch","sound-wave",{"presets":378,"hertz":402,"allowAudio":405,"showWaveform":405,"quiz":405},[379,383,387,390,394,398],{"label":380,"hertz":381,"amplitude":382},"Low rumble (60 Hz)",60,0.6,{"label":384,"hertz":385,"amplitude":386},"A man's speaking voice (120 Hz)",120,0.5,{"label":388,"hertz":389,"amplitude":386},"The tuning note (440 Hz)",440,{"label":391,"hertz":392,"amplitude":393},"A mosquito (600 Hz)",600,0.35,{"label":395,"hertz":396,"amplitude":397},"A whistle (2,000 Hz)",2000,0.4,{"label":399,"hertz":400,"amplitude":401},"Almost too high (15,000 Hz)",15000,0.3,{"min":403,"max":404,"initial":389},20,16000,true,"Slide the frequency up and down and hear, and see, what changes when a sound gets higher.","A tone lab. A slider sets the frequency from 20 Hz to 16,000 Hz, a wave picture shows the vibration, and a **Play** button sounds the tone. Nothing sounds until you tap Play.\n\nIt starts at **440 Hz**, the note orchestras tune to. The drawn wave shows a certain number of humps across the screen.\n\nDrag down to **60 Hz** and the humps become long and stretched out: few vibrations per second, a deep rumble you feel in your chest.\n\nDrag up to **2,000 Hz** and the humps crowd together, packed more than thirty times as tightly as at 60 Hz. The sound is a thin, piercing whistle.\n\nAt **15,000 Hz** the humps are too close to count and many adults hear nothing while most children hear a faint, painful hiss.\n\nThrough all of this the height of the wave never changes, and the loudness never changes. Only how *often* it shakes.",{"id":409,"type":47,"variant":410,"title":411,"markdown":412},"careful-volume","careful","Before you press play","Turn the volume **down** before starting any tone lab, then raise it slowly. Never use headphones at a high setting for these, and never hold a phone speaker against your ear while a tone plays.\n\nHigh-pitched tones can seem quiet and still be strong enough to tire your ears. If a sound is uncomfortable, it is too loud. Stop.",{"id":414,"type":415,"tone":416,"items":417},"spec-pitch-examples","spec","blue",[418,422,426,430,434,438],{"label":419,"big":420,"value":421},"Deepest thunder","20 Hz","About as low as human ears go. Below this you feel it in your body more than you hear it.",{"label":423,"big":424,"value":425},"A big dhol","about 80 Hz","A deep boom you can feel in the ground at a procession.",{"label":427,"big":428,"value":429},"Speaking voice","85–255 Hz","Adult voices sit here; children's voices are higher because their vocal folds are shorter.",{"label":431,"big":432,"value":433},"Tuning note","440 Hz","The frequency instruments are tuned to all over the world.",{"label":435,"big":436,"value":437},"Mosquito whine","about 600 Hz","The wingbeat itself; the sound is exactly the wing rate.",{"label":439,"big":440,"value":441},"Highest we hear","20,000 Hz","A young ear's upper limit. Most adults have already lost the top of this.",{"id":443,"type":47,"variant":62,"title":444,"markdown":445},"tryit-ruler","The ruler on the table edge","1. Hold a plastic ruler firmly on a table so that most of it sticks out over the edge.\n2. Twang the free end. Watch it flap slowly and listen to the low buzz.\n3. Now slide the ruler in so that only a short piece sticks out and twang again.\n\nShort piece: it flaps **much faster** and the note is **much higher**. Slide it in and out while twanging and you can play a rough slide whistle.\n\nYou have just found the rule that runs every stringed instrument in the world: **shorter means faster means higher.** It is why a sitar player's fingers move up the neck for higher notes, and why the short strings of a veena sing higher than the long ones.",{"id":447,"type":282,"prompt":448,"options":449,"explanation":458},"predict-rubber-band","You stretch a rubber band gently between two fingers and pluck it: you hear a low note. Now you pull your fingers further apart so the band is much tighter, and pluck it again with the same strength. What happens to the note?",[450,452,454,456],{"id":286,"label":451},"It gets higher",{"id":289,"label":453},"It gets lower",{"id":292,"label":455},"It stays the same but gets louder",{"id":295,"label":457},"It stays exactly the same","**It gets higher.** Tightening the band makes it snap back to the middle faster after each pluck, so it completes more vibrations every second, so the frequency rises and the pitch rises with it.\n\nThis is the third of the three pitch controls, and every stringed instrument uses all three:\n\n- **Tighter** string means higher (that is what tuning pegs do).\n- **Shorter** string means higher (that is what your fingers do on the frets).\n- **Thinner** string means higher (that is why the strings of a sitar or guitar have different thicknesses).\n\nDrums use the same first rule: tighten a dholak's ropes and its pitch rises.",{"id":460,"type":211,"title":461,"items":462},"steps-three-pitch-rules","Three ways to change the pitch of a string",[463,467,471],{"title":464,"tag":465,"text":466},"Change the length","shorter = higher","Press a string against a fret and only the short part is free to vibrate, so it shakes faster. Halving the length doubles the frequency: that jump is called an octave.",{"title":468,"tag":469,"text":470},"Change the tightness","tighter = higher","Turning a tuning peg stretches the string, so it springs back more sharply. This is how every instrument is tuned before a concert.",{"title":472,"tag":473,"text":474},"Change the thickness","thinner = higher","A heavy string is sluggish and vibrates slowly. That is why the low strings of a veena or guitar are visibly fatter than the high ones.",{"id":476,"type":53,"title":477,"eyebrow":478,"navLabel":479},"ch05","Loud and soft: loudness","Chapter 05","5 Loud and soft",{"id":481,"type":43,"markdown":482},"ch5-p1","Now hit the same tabla twice: once gently, once hard. The **pitch** does not change; it is the same drum, the same tension, the same skin. What changes is **loudness**.\n\nHit it harder and the skin swings further from its resting place on each flick. A bigger swing shoves more air, squashes it more strongly, and delivers a stronger push to your eardrum.\n\nThe size of that swing has a name: **amplitude**. Big amplitude means loud. Small amplitude means soft. It is completely separate from how *often* the skin shakes.\n\nSo every sound carries two independent pieces of news: how fast it shakes (pitch) and how far it shakes (loudness).",{"id":484,"type":47,"variant":67,"title":485,"markdown":486},"def-amplitude","Amplitude","**Amplitude** is how far the vibrating thing swings from its resting position on each vibration. On a drawn wave, it is the height of the humps.\n\nBigger amplitude means more energy carried, means louder. A quiet whisper and a shout can be the same pitch; the shout simply moves the air further.",{"id":488,"type":134,"component":376,"componentVersion":5,"config":489,"objective":508,"textAlternative":509},"lab-wave-loud",{"presets":490,"hertz":505,"allowAudio":405,"showWaveform":405,"quiz":405},[491,494,496,499,502],{"label":492,"hertz":389,"amplitude":493},"Whisper-soft (440 Hz)",0.1,{"label":495,"hertz":389,"amplitude":386},"Normal (440 Hz)",{"label":497,"hertz":389,"amplitude":498},"Loud (440 Hz)",0.95,{"label":500,"hertz":501,"amplitude":386},"Same loudness, low note",110,{"label":503,"hertz":504,"amplitude":386},"Same loudness, high note",1760,{"min":506,"max":507,"initial":389},50,4000,"Change the height of the wave without changing how often it shakes, and hear loudness change while pitch stays put.","The same tone lab, now with a loudness control as well as a frequency control.\n\nStart with the three **440 Hz** presets. All three draw waves with exactly the same number of humps across the screen, because the frequency has not changed. What changes is their **height**: the whisper-soft wave is a low ripple, the normal wave is about five times taller, and the loud wave nearly fills the screen. Your ear hears the same note three times, quietly then normally then loudly.\n\nNow compare the last two presets. Both have medium height, so both sound about equally loud, but one draws four long humps and the other draws sixty-four short ones in the same space. Same loudness, very different pitch.\n\nThe conclusion: **height of the wave means loudness, closeness of the humps means pitch.** They can be changed one at a time.",{"id":511,"type":282,"prompt":512,"options":513,"explanation":522},"predict-loud-vs-pitch","Two identical steel tumblers are on a table. You tap the first one gently and the second one hard, with the same spoon. Which statement is true?",[514,516,518,520],{"id":286,"label":515},"The second sounds louder and higher",{"id":289,"label":517},"The second sounds louder, at the same pitch",{"id":292,"label":519},"The second sounds higher, at the same loudness",{"id":295,"label":521},"They sound identical","**Louder, at the same pitch.** The pitch of a tumbler is set by its size, shape and the metal it is made of, and none of those changed when you hit it harder. What changed is the amplitude: the tumbler wall now flexes further on each vibration, pushing the air harder.\n\nThis is a useful test for any sound-maker. Ask: *did I change what is vibrating, or only how hard I started it?* Change what is vibrating and the pitch moves. Change only the push and only the loudness moves.",{"id":524,"type":525,"title":526,"note":527,"scale":528,"rungs":529},"ladder-loudness","ladder","How loud is that, in decibels?","Loudness is measured in decibels (dB). The scale is squashed on purpose: every extra 10 dB means about twice as loud to your ears, and ten times as much energy arriving.","log",[530,533,536,540,543,547,551,554],{"label":531,"value":403,"display":532},"Rustling leaves","20 dB",{"label":534,"value":18,"display":535},"A whisper, one metre away","30 dB",{"label":537,"value":538,"display":539},"A quiet library",40,"40 dB",{"label":541,"value":381,"display":542},"Normal conversation","60 dB",{"label":544,"value":545,"display":546},"A busy Indian road",80,"80 dB",{"label":548,"value":549,"display":550},"A motorbike without a silencer",95,"95 dB",{"label":552,"value":501,"display":553},"A loudspeaker at a wedding","110 dB",{"label":555,"value":556,"display":557},"A firecracker, close by",150,"150 dB or more",{"id":559,"type":47,"variant":410,"title":560,"markdown":561},"careful-hearing","Your ears do not grow back","Inside your ear are about sixteen thousand tiny hair cells that turn movement into nerve signals. Very loud sound bends and breaks them, and **broken hair cells never grow back**. A person with damaged hearing at twelve will still have it at eighty.\n\nThree rules that cover almost everything:\n\n- **Above about 85 dB, time matters.** An hour of very loud music does more harm than a minute.\n- **A single very loud bang can do permanent damage on its own.** Firecrackers set off close by are the commonest cause among children in India.\n- **Ringing in your ears after a loud event is a warning, not a joke.** It means cells were hurt.\n\nStand well back from crackers and speakers, keep headphones at about half volume, and use plugs or even fingers when you know a bang is coming.",{"id":563,"type":53,"title":564,"eyebrow":565,"navLabel":566},"ch06","Fast, but far slower than light","Chapter 06","6 Light wins the race",{"id":568,"type":43,"markdown":569},"ch6-p1","Sound in air travels **343 metres every second**. That is fast: about 1,235 kilometres per hour, faster than most aeroplanes.\n\nBut light travels **299,792,458 metres every second**, which is about **874,000 times faster**. For any distance you can see across, light arrives effectively instantly, while sound plods along behind.\n\nYou already know this without being told. At a cricket match far from the boundary, you see the bat hit the ball and the *thock* arrives a moment later. On Diwali, the flash of a cracker comes first and the bang follows. During a storm, lightning always beats thunder.\n\nAnd that gap is not a nuisance. It is a free measuring tape.",{"id":571,"type":47,"variant":240,"title":572,"markdown":573},"aha-thunder-rule","Count the seconds to the storm","The rule in one line: **count the seconds between the flash and the thunder, and divide by 3. That is how many kilometres away the lightning struck.**\n\nWhy 3? Sound covers 1,000 metres in 1,000 divided by 343, which is 2.92 seconds. Close enough to 3 for counting out loud.\n\n- Flash, then **3 seconds**, then thunder: about **1 km** away.\n- Flash, then **9 seconds**: about **3 km** away.\n- Flash and bang almost together: the storm is **overhead**. Go indoors immediately.\n\nThe light takes about three millionths of a second to cover that kilometre, far too little to matter, so the whole delay belongs to the sound.",{"id":575,"type":576,"title":577,"problem":578,"steps":579,"help":586},"we-thunder-discover","worked_example","How far away was that lightning?","You see a flash of lightning. You count steadily: one, two, three, four, five, six. Then the thunder rolls in. How far away did the lightning strike?",[580,581,582,583,584,585],"The flash arrives essentially instantly, so the whole 6 second delay belongs to the sound.","Sound travels 343 metres in each of those seconds.","Distance = 343 × 6 = **2,058 metres**.","That is 2,058 ÷ 1,000 = **about 2 kilometres**.","Check with the quick rule: 6 ÷ 3 = 2 km. ✓ The rule and the full calculation agree.","Worth knowing: if that count had been 3 seconds or less, the storm would be within a kilometre and you should already be inside a building.",{"simplerExplanation":587,"anotherExample":588},"Six seconds, and sound covers 343 m each second. 343 + 343 + 343 + 343 + 343 + 343 = 2,058 m, which is about 2 km.","Count 15 seconds and the strike was 343 × 15 = 5,145 m, about 5 km away: the far edge of the storm.",{"id":590,"type":134,"component":303,"componentVersion":5,"config":591,"objective":595,"textAlternative":596},"lab-speed-lightning",{"media":592,"distanceM":396,"modes":593},[307,308,309],[594,312],"lightning","Set a storm at a chosen distance, watch the flash arrive at once, and count the seconds until the thunder.","A storm-distance lab. You place a lightning bolt anywhere from 100 metres to 2,000 metres away and press **Strike**.\n\nThe flash fills the screen instantly. A counter starts, and a ring of sound spreads outwards across the map at 343 metres per second. When the ring reaches you, the thunder sounds and the counter stops.\n\n- Set it at **343 m** and the counter stops at **1.0 second**.\n- Set it at **1,000 m** and it stops at **2.9 seconds**, which is the 3-seconds-per-kilometre rule appearing on its own.\n- Set it at **2,000 m** and it stops at **5.8 seconds**.\n\nThere is also a **guess** mode: the lab strikes at a secret distance, you count the seconds, divide by 3, and type your estimate in kilometres. The lab then shows how close you were. With practice most people get within a few hundred metres, using nothing but counting.",{"id":598,"type":599,"itemId":600,"prompt":601,"check":602,"hints":607,"feedback":610},"prac-thunder","practice","sound.discover-thunder-9s","You see lightning and count **9 seconds** before the thunder arrives. Roughly how many **kilometres** away was the strike? (Use 3 seconds per kilometre.)",{"kind":603,"answer":604,"tolerance":605,"unit":606},"number",3,0.2,"km",[608,609],"Divide the seconds by 3.","9 ÷ 3 = ?",{"correct":611,"incorrect":612},"Right: 9 ÷ 3 = **3 km**. The full calculation agrees: 343 × 9 = 3,087 m, which is about 3.1 km.","Divide the count by 3: 9 seconds ÷ 3 = 3 kilometres. (Exactly, 343 × 9 = 3,087 m.)",{"id":614,"type":47,"variant":410,"title":615,"markdown":616},"careful-lightning-safety","The 30-30 rule for storms","Counting is fun, but it is also a safety tool, and India records many lightning deaths every year, mostly in open fields.\n\n- If the gap between flash and thunder is **30 seconds or less** (about 10 km), the storm is close enough to be dangerous. Go inside a proper building or a closed vehicle.\n- Wait **30 minutes** after the last thunder before going back out. Lightning can strike from a storm that seems to have passed.\n\nNever shelter under a lone tree, and stay off open ground, rooftops and water.",{"id":618,"type":53,"title":619,"eyebrow":620,"navLabel":621},"ch07","Echoes: sound that comes back","Chapter 07","7 Echoes",{"id":623,"type":43,"markdown":624},"ch7-p1","Shout into a well, or clap in an empty hall, or call out across a valley, and your own voice comes back to you a moment later. That returning sound is an **echo**.\n\nSound bounces. When the travelling pattern of pushes meets a hard, flat surface such as a wall, a cliff or the water at the bottom of a well, most of it turns around and comes back, exactly as a ball bounces off a wall.\n\nSoft things behave differently. A curtain, a mattress, a heap of clothes or a crowd of people soak the sound up instead of bouncing it. That is why a bare empty room echoes and the same room full of furniture and people does not.",{"id":626,"type":47,"variant":67,"title":265,"markdown":627},"def-echo","An **echo** is a sound you hear twice: once directly from the source, and again after it has bounced off a distant surface and travelled back to you.\n\nFor your brain to hear it as a separate sound rather than as a smear, the return trip must take at least about **0.1 second**. Since sound covers 343 metres in a second, that means the reflecting wall has to be at least about **17 metres** away. Nearer than that and the echo merges with the original.",{"id":629,"type":576,"title":630,"problem":631,"steps":632,"help":639},"we-echo-discover","How far is the cliff?","You stand in a valley, clap once, and hear the echo come back exactly **2 seconds** later. How far away is the cliff that bounced it?",[633,634,635,636,637,638],"In those 2 seconds the sound made a **round trip**: out to the cliff and back again.","Total distance travelled = 343 × 2 = **686 metres**.","But that is there *and* back, so the one-way distance is half of it.","Distance to the cliff = 686 ÷ 2 = **343 metres**.","Check: out 343 m takes 1 second, back 343 m takes another second, total 2 seconds. ✓","The trap to avoid: never forget to halve. The commonest mistake in echo questions is answering 686 m.",{"simplerExplanation":640,"hints":641},"Sound goes 343 m every second, so in 2 seconds it goes 686 m. Half of that trip was the journey out: 343 m.",[642,643],"The sound goes there and comes back, so it covers the distance twice.","Find the total distance first, then halve it.",{"id":645,"type":47,"variant":646,"title":647,"markdown":648},"example-gol-gumbaz","example","The whispering dome at Gol Gumbaz","At Gol Gumbaz in Vijayapura (Bijapur), Karnataka, stands one of the largest domes in the world, about 44 metres across. Inside runs a circular gallery, and the hard curved stone bounces sound superbly.\n\nA whisper against the wall on one side can be heard clearly on the opposite side, dozens of metres away, because the sound skims around the curved wall instead of spreading out and fading. A single clap is famously said to come back several times over as it bounces around the dome.\n\nThe same trick, deliberately used, is why an open-air amphitheatre carries an actor's voice to the back row, and why a bathroom makes everyone sound like a singer: hard tiles bounce, they do not absorb.",{"id":650,"type":53,"title":651,"eyebrow":652,"navLabel":653},"ch08","Your ears","Chapter 08","8 Your ears",{"id":655,"type":43,"markdown":656},"ch8-p1","The visible part of your ear is only the funnel. The real machinery is inside your skull, and it is astonishing: it can detect a movement of your eardrum smaller than the width of an atom, and it does it in both ears at once so that you know which direction the sound came from.\n\nFollow the path in. Sound is collected by the outer flap, travels down a short tube, and pushes on a tight little skin. Three tiny bones carry that push across an air-filled gap. The last bone presses on a coiled tube full of liquid, and inside that coil, thousands of microscopic hairs sway. Each sway becomes a nerve signal, and the nerve carries it to your brain.\n\nThe whole journey takes a few thousandths of a second.",{"id":658,"type":659,"component":660,"componentVersion":5,"config":661,"textAlternative":699},"anim-ear-path","animation","process-steps",{"title":662,"diagram":663,"steps":664},"How your ear turns shaking air into a thought","none",[665,670,675,679,684,689,694],{"id":666,"label":667,"description":668,"highlight":669},"pinna","Outer ear (pinna)","The flap you can see and touch. Its folds and curves gather sound and funnel it inwards, and they also change the sound slightly depending on whether it came from in front, behind or above. That is part of how you know where a sound is coming from.",[],{"id":671,"label":672,"description":673,"highlight":674},"canal","Ear canal","A short tube about two and a half centimetres long that carries sound to the eardrum. It is also a trap: the wax and fine hairs in it keep dust and insects away from the delicate parts. Never push anything into it, not even a cotton bud.",[],{"id":676,"label":271,"description":677,"highlight":678},"eardrum","A thin, tight skin stretched right across the end of the canal, about the size of a small fingernail. The arriving pattern of squashed and thinned air pushes it in and lets it spring out, at exactly the same rate as the original source was vibrating.",[],{"id":680,"label":681,"description":682,"highlight":683},"ossicles","Three tiny bones","The hammer, the anvil and the stirrup: the three smallest bones in your whole body, the stirrup no bigger than a grain of rice. They form a lever chain that carries the eardrum's movement across the middle ear and concentrates it onto a much smaller spot, making the push about twenty times stronger.",[],{"id":685,"label":686,"description":687,"highlight":688},"cochlea","Cochlea","A tube about the size of a pea, coiled like a snail shell and filled with liquid. The last bone presses on a small window into it, and ripples run along the liquid inside. Different places along the coil respond to different pitches: high notes near the entrance, low notes deep inside.",[],{"id":690,"label":691,"description":692,"highlight":693},"hair","Hair cells","About sixteen thousand microscopic cells stand along the cochlea with fine bristles on top. When the ripples sway their bristles, each cell fires an electrical signal. These are the cells that very loud sound destroys, and they do not grow back.",[],{"id":695,"label":696,"description":697,"highlight":698},"nerve","Auditory nerve and brain","The nerve carries a stream of electrical signals to the hearing part of the brain, which sorts them into words, music, a dog barking or your name. Comparing what the left and right ears heard, and when, tells your brain which direction the sound came from.",[],"A seven-step walk-through of hearing, from the outside in.\n\n1. **Outer ear (pinna).** The flap you can see gathers sound and funnels it into the canal. Its folds also colour the sound slightly depending on direction, which helps you tell front from behind.\n2. **Ear canal.** A tube about 2.5 cm long. Wax and fine hairs guard it. Never put anything into it.\n3. **Eardrum.** A tight skin about the size of a small fingernail. The arriving pushes move it in and out at the same rate as the source shook.\n4. **Three tiny bones.** Hammer, anvil and stirrup, the smallest bones in your body, form a lever that carries the movement across the middle ear and makes the push roughly twenty times stronger.\n5. **Cochlea.** A pea-sized coil of liquid. Ripples run along it, and different places along the coil answer to different pitches: high near the entrance, low deep inside.\n6. **Hair cells.** About sixteen thousand cells with fine bristles. A sway becomes an electrical signal. Loud noise breaks them permanently.\n7. **Auditory nerve and brain.** Signals reach the brain, which recognises the sound and, by comparing the two ears, works out where it came from.",{"id":701,"type":702,"conceptId":703,"relation":704,"explanation":705},"conn-anatomy","connection","human-body-anatomy","part_of","The ear is one of the sense organs in the body's anatomy: see where it sits in the skull and how its parts compare in size with the rest of you.",{"id":707,"type":702,"conceptId":708,"relation":709,"explanation":710},"conn-body-systems","body-systems","related_to","Hearing is the nervous system at work: hair cells make signals, the auditory nerve carries them, and the brain interprets them.",{"id":712,"type":47,"variant":198,"title":713,"markdown":714},"misconception-eardrum-hears","“The eardrum hears the sound”","The eardrum does not hear anything. It is a thin skin that simply moves when it is pushed, like a tiny trampoline.\n\nAll it does is pass the movement along to the three bones, which pass it to the liquid in the cochlea, where hair cells turn the movement into nerve signals. **Hearing happens in the brain**, using those signals.\n\nThis is why a person whose eardrum is damaged can sometimes still hear through bone conduction, and why hearing aids and cochlear implants can help: the job is to get a signal to the nerve, and the eardrum is only one possible route.",{"id":716,"type":47,"variant":359,"title":717,"markdown":718},"nuance-two-ears","Why you have two ears","Close your eyes and ask a friend to clap somewhere around you. You can point at them surprisingly well. How?\n\nA clap from your right reaches your right ear a fraction of a millisecond before your left, and slightly louder, because your head is in the way of the far ear. Your brain reads those two tiny differences and turns them into a direction.\n\nThis is why a sound directly in front of you and directly behind you are the hardest to tell apart: both reach the two ears at the same moment. You tilt your head automatically to break the tie, and the shape of your outer ear helps too.",{"id":720,"type":415,"tone":721,"items":722},"spec-hearing-range","copper",[723,727,731,735,739],{"label":724,"big":725,"value":726},"Human, young ears","20 Hz – 20 kHz","The full range at its best. It shrinks with age from the top down.",{"label":728,"big":729,"value":730},"Human, most adults","up to 15–17 kHz","By the twenties the very top is usually gone, which is completely normal.",{"label":732,"big":733,"value":734},"Dog","up to 45 kHz","Far beyond us, which is why a dog whistle sounds silent to you and loud to a dog.",{"label":736,"big":737,"value":738},"Bat","up to 120 kHz","Bats shout in this range and listen for the echoes to fly and hunt in the dark.",{"label":740,"big":741,"value":742},"Elephant","down to 14 Hz","Elephants call below our range and can hear each other kilometres away.",{"id":744,"type":47,"variant":67,"title":745,"markdown":746},"def-ultra-infra","Ultrasound and infrasound","**Ultrasound** is sound too *high* for human ears: above 20,000 Hz. Bats, dolphins and dog whistles use it, and hospitals use it to look at babies before they are born.\n\n**Infrasound** is sound too *low* for human ears: below 20 Hz. Elephants and whales use it for long-distance calls, and earthquakes and volcanoes make it.\n\nNeither one is mysterious or magic. They are ordinary sound, just outside the window that human ears happen to open on.",{"id":748,"type":53,"title":749,"eyebrow":750,"navLabel":751},"ch09","Music, noise and a quieter world","Chapter 09","9 Music and noise",{"id":753,"type":43,"markdown":754},"ch9-p1","What makes the sound of a bansuri pleasant and the sound of a scooter horn unpleasant? Both are vibrations in air, both reach the ear the same way.\n\nThe difference is **pattern**. A musical sound repeats: the same shape of vibration, over and over, at a steady rate. Your ear finds a clear pitch in it, and your brain enjoys the order.\n\nA noise does not repeat. It is a jumble of many frequencies arriving in no particular arrangement, so there is no pitch to latch on to. A door slamming, gravel being poured, a horn blaring in traffic.\n\nThat is not the whole story, because a drum is musical and a tabla stroke is a wonderful sound with a definite pitch. But it is the right place to start: **music is ordered vibration, noise is disordered vibration.**",{"id":756,"type":325,"caption":757,"columns":758,"rows":762},"table-music-noise","Musical sound and noise compared",[759,760,761],"Feature","Musical sound","Noise",[763,767,770,774,778],[764,765,766],"Pattern of the vibration","Repeats regularly, the same shape each time","Irregular, no repeating shape",[259,768,769],"You can hum it: there is a definite note","No definite note to hum",[771,772,773],"Where it comes from","Instruments and voices built to vibrate in one clean way","Collisions, scrapes, engines, crowds, many frequencies at once",[775,776,777],"Effect on a listener","Usually pleasant; can be relaxing or exciting","Usually unpleasant; tiring and stressful over time",[779,780,781],"Examples","Bansuri, sitar, tabla, singing, a temple bell","Traffic horns, a generator, a slamming gate, a firecracker",{"id":783,"type":47,"variant":359,"title":784,"markdown":785},"nuance-one-persons-noise","The line is not perfectly sharp","A tabla is a drum, and a struck drum ought to be noisy, yet a well-made tabla gives a clear note. The black syahi patch is the reason: it tunes the skin so that its vibrations fit together neatly instead of clashing.\n\nAnd some of it is human, not physical. A neighbour's music at midnight is music to them and noise to you. A film's sound designer uses crashes and rumbles on purpose. Modern musicians use noise deliberately.\n\nSo use the definition as a tool, not a wall: ordered vibration is *usually* heard as music, disordered vibration is *usually* heard as noise, and your ear is the final judge.",{"id":787,"type":43,"markdown":788},"ch9-p2","India is a loud country, and the noise is not harmless. Constant traffic, horns, construction, loudspeakers and generators raise blood pressure, break sleep, make it harder for children to learn, and, at the top end, damage hearing permanently.\n\nThe law recognises this. India's noise rules set limits for different zones: quietest near hospitals and schools, stricter at night than in the day, with firecrackers limited to 125 decibels measured 4 metres away.\n\nYou cannot fix a city on your own. But you can do four things today: **do not use a horn unless it prevents an accident, stand well back from crackers and speakers, keep your headphones at about half volume, and give your ears quiet time after a loud event.**",{"id":790,"type":47,"variant":62,"title":791,"markdown":792},"tryit-noise-survey","A one-page noise survey","Spend twenty minutes being a sound scientist.\n\n1. Sit somewhere for **two minutes** with your eyes closed and list every sound you can hear. Most people find eight or more.\n2. Mark each one: **M** if it is musical, **N** if it is noise.\n3. Mark each one again: could you make it stop? Who is making it?\n4. Repeat in three places: your room, outside your gate, and a main road.\n\nThe road list will be much longer and almost all N. Now ask the useful question: **which of these sounds would nobody miss if it stopped?** That short list is exactly what noise rules try to deal with.",{"id":794,"type":702,"conceptId":795,"relation":796,"explanation":797},"conn-electricity","electricity","applied_in","A microphone turns sound into a changing electric current and a loudspeaker turns current back into sound, which is how phones, recordings and public address systems work.",{"id":799,"type":702,"conceptId":800,"relation":801,"explanation":802},"conn-light","light","contrasts_with","Light and sound are both waves that carry energy, but light needs no material, travels about 874,000 times faster, and is a very different kind of wave.",{"id":804,"type":53,"title":805,"eyebrow":806,"navLabel":807},"ch10","Check what you know","Chapter 10","10 Check yourself",{"id":809,"type":810,"title":811,"terms":812},"glossary-discover","glossary","Sound words to keep",[813,816,819,822,825,829,833,836,839,842,846,849,852,855,859,863],{"term":68,"meaning":814,"example":815},"A fast back-and-forth movement about a resting position. Every sound begins with one.","Fingers on your throat while you hum.",{"term":253,"meaning":817,"example":818},"The thing that is vibrating and making the sound.","The skin of a tabla.",{"term":256,"meaning":820,"example":821},"The material a sound travels through: a gas, a liquid or a solid.","Air, water, wood, steel.",{"term":268,"meaning":823,"example":824},"A space with no matter in it at all. Sound cannot travel through one.","Between the planets.",{"term":826,"meaning":827,"example":828},"Frequency","How many complete vibrations happen each second, measured in hertz (Hz).","A tuning fork stamped 440 Hz.",{"term":830,"meaning":831,"example":832},"Hertz (Hz)","The unit of frequency. 1 Hz is one vibration per second; 1,000 Hz is 1 kilohertz (kHz).","Human hearing runs from 20 Hz to 20,000 Hz.",{"term":259,"meaning":834,"example":835},"How high or low a sound seems. Higher frequency means higher pitch.","A mosquito is high, a dhol is low.",{"term":485,"meaning":837,"example":838},"How far the source swings on each vibration. Bigger amplitude means louder.","Hitting a drum harder.",{"term":262,"meaning":840,"example":841},"How strong a sound seems to a listener, measured in decibels (dB).","A whisper is 30 dB, traffic is 80 dB.",{"term":843,"meaning":844,"example":845},"Decibel (dB)","The unit of loudness. Every extra 10 dB means about twice as loud to your ears.","85 dB for a long time damages hearing.",{"term":265,"meaning":847,"example":848},"A sound heard again after it has bounced off a surface and returned.","A clap in an empty hall.",{"term":271,"meaning":850,"example":851},"The thin, tight skin at the end of the ear canal that the arriving sound pushes.","About the size of a small fingernail.",{"term":686,"meaning":853,"example":854},"The pea-sized liquid-filled coil in your inner ear where movement becomes nerve signals.","Contains about 16,000 hair cells.",{"term":856,"meaning":857,"example":858},"Ultrasound","Sound above 20,000 Hz: too high for humans to hear.","Bats, dolphins, hospital scans.",{"term":860,"meaning":861,"example":862},"Infrasound","Sound below 20 Hz: too low for humans to hear.","Elephant calls, earthquakes.",{"term":761,"meaning":864,"example":865},"Sound with no repeating pattern and no definite pitch; usually unwanted.","A horn in traffic.",{"id":867,"type":868,"title":869,"questions":870},"quiz-discover","quiz","Ten questions on the basics of sound",[871,884,897,908,920,933,946,959,972,985],{"itemId":872,"prompt":873,"options":874,"correct":289,"why":883},"sound.discover-q-start","What does **every** sound in the world begin with?",[875,877,879,881],{"id":286,"label":876},"Moving air",{"id":289,"label":878},"Something vibrating",{"id":292,"label":880},"An echo",{"id":295,"label":882},"A loudspeaker","Every sound starts with something shaking: a skin, a string, a column of air, a solid object or your vocal folds. Air only carries it.",{"itemId":885,"prompt":886,"options":887,"correct":292,"why":896},"sound.discover-q-vacuum","An alarm clock rings inside a jar. All the air is pumped out. What happens?",[888,890,892,894],{"id":286,"label":889},"It sounds the same",{"id":289,"label":891},"It sounds louder",{"id":292,"label":893},"It fades to silence",{"id":295,"label":895},"It sounds lower","With no air there is nothing to squash and stretch, so nothing carries the sound out of the jar. You can still see the hammer moving.",{"itemId":898,"prompt":899,"options":900,"correct":292,"why":907},"sound.discover-q-fastest","In which of these does sound travel fastest?",[901,902,904,905],{"id":286,"label":339},{"id":289,"label":903},"Water",{"id":292,"label":353},{"id":295,"label":906},"Empty space","Steel, at about 5,960 m\u002Fs, roughly 17 times faster than air. The particles in a solid are locked together, so the push is passed on almost instantly. In empty space, sound does not travel at all.",{"itemId":909,"prompt":910,"options":911,"correct":286,"why":919},"sound.discover-q-pitch","You tighten a rubber band and pluck it again. What changes?",[912,914,915,917],{"id":286,"label":913},"It sounds higher",{"id":289,"label":895},{"id":292,"label":916},"Only louder",{"id":295,"label":918},"Nothing","A tighter band snaps back faster, so it vibrates more times per second. Higher frequency means higher pitch.",{"itemId":921,"prompt":922,"options":923,"correct":292,"why":932},"sound.discover-q-loud","You hit the same drum harder. What changes?",[924,926,928,930],{"id":286,"label":925},"The pitch rises",{"id":289,"label":927},"The pitch falls",{"id":292,"label":929},"The loudness rises, the pitch stays",{"id":295,"label":931},"Both rise","Hitting harder makes the skin swing further on each vibration (bigger amplitude), which is louder. How *often* it vibrates has not changed, so the pitch is the same.",{"itemId":934,"prompt":935,"options":936,"correct":289,"why":945},"sound.discover-q-thunder","You count 6 seconds between lightning and thunder. How far away was the strike?",[937,939,941,943],{"id":286,"label":938},"About 0.5 km",{"id":289,"label":940},"About 2 km",{"id":292,"label":942},"About 6 km",{"id":295,"label":944},"About 18 km","Divide the seconds by 3: 6 ÷ 3 = 2 km. In full: 343 × 6 = 2,058 m.",{"itemId":947,"prompt":948,"options":949,"correct":289,"why":958},"sound.discover-q-echo","An echo returns 2 seconds after your clap. How far away is the wall?",[950,952,954,956],{"id":286,"label":951},"171.5 m",{"id":289,"label":953},"343 m",{"id":292,"label":955},"686 m",{"id":295,"label":957},"1,029 m","In 2 seconds the sound covers 343 × 2 = 686 m, but that is *there and back*. The wall is half that: 343 m.",{"itemId":960,"prompt":961,"options":962,"correct":289,"why":971},"sound.discover-q-hz","A sound of 500 Hz means the source is vibrating",[963,965,967,969],{"id":286,"label":964},"500 times a minute",{"id":289,"label":966},"500 times a second",{"id":292,"label":968},"500 metres per second",{"id":295,"label":970},"500 decibels loud","Hertz counts complete vibrations per second. 500 Hz is 500 back-and-forth movements every second.",{"itemId":973,"prompt":974,"options":975,"correct":295,"why":984},"sound.discover-q-ear","Which part of the ear turns movement into nerve signals?",[976,978,980,982],{"id":286,"label":977},"The pinna",{"id":289,"label":979},"The eardrum",{"id":292,"label":981},"The three tiny bones",{"id":295,"label":983},"The hair cells in the cochlea","The pinna funnels, the eardrum moves, the bones pass the movement on, and the hair cells in the cochlea are where movement finally becomes an electrical nerve signal.",{"itemId":986,"prompt":987,"options":988,"correct":289,"why":997},"sound.discover-q-ultra","A dog whistle sounds silent to you but loud to a dog because",[989,991,993,995],{"id":286,"label":990},"It is too quiet for people",{"id":289,"label":992},"Its frequency is above 20,000 Hz",{"id":292,"label":994},"It makes no vibration",{"id":295,"label":996},"Dogs have bigger ears","It is ultrasound: real sound, but higher than the top of the human range. Dogs hear up to about 45,000 Hz, so to them it is loud and clear.",{"id":999,"type":1000,"prompt":1001},"reflect-discover","reflection","Sit somewhere quiet for two minutes and list every sound you can hear. For three of them, write down what you think is actually vibrating, and how you could test your guess without any equipment.",{"id":1003,"type":1004,"title":1005,"points":1006},"cheat-discover","summary","Cheat sheet",[1007,1008,1009,1010,1011,1012,1013,1014,1015,1016,1017],"**Every sound starts with a vibration.** Stop the shaking and the sound stops instantly.","**Four families of vibrator:** a stretched skin (tabla), a stretched string (sitar), a column of air (bansuri) and a solid body (ghatam).","**Sound needs a medium.** It travels through gases, liquids and solids, and not at all through a vacuum, which is why space is silent.","**Solids are fastest.** Air 343 m\u002Fs, water 1,480 m\u002Fs, wood about 3,800 m\u002Fs, steel 5,960 m\u002Fs. Tightly joined particles hand the push on quickly.","**Pitch comes from frequency**, counted in hertz (Hz). Shorter, tighter or thinner means faster means higher.","**Loudness comes from amplitude**, how far the source swings, measured in decibels (dB).","**Light beats sound by about 874,000 times.** Count the seconds from flash to thunder and divide by 3 to get kilometres.","**An echo is sound bounced back.** It needs about 0.1 s, so a reflecting wall at least about 17 m away.","**Your ear:** pinna, canal, eardrum, three tiny bones, cochlea, hair cells, nerve, brain. Hearing happens in the brain.","**We hear 20 Hz to 20,000 Hz.** Above is ultrasound, below is infrasound.","**Loud noise permanently kills hair cells.** They never grow back, so stand back from crackers and keep headphones low.",{"id":1019,"type":1020,"sourceIds":1021},"sources-discover","sources",[1022,1023,1024,1025,1026,1027,1028,1029],"sound-ncert-class9-sound","sound-britannica-sound","sound-hyperphysics-speed","sound-hyperphysics-ear","sound-nidcd-hearing","sound-nidcd-noise","sound-cpcb-noise","sound-wikipedia-instruments",[1022,1023,1024,1025,1026,1027,1028,1029],"needs_review",{"generatedBy":1033,"notes":1034},"claude-code","Draft generated locally; pending owner review. Every speed, delay and distance computed and asserted in scratchpad\u002Fsound\u002Fnumbers.py.","5fcac1e738d805e6b4456f368898059cb69dd829901ada1010cce792c1f56673",{"component:sort-game@1":1037,"component:match-pairs@1":1038,"component:sound-speed@1":1039,"component:sound-wave@1":1040,"logic:practice":1041,"component:process-steps@1":1042,"source:sound-britannica-sound":1043,"source:sound-cpcb-noise":1044,"source:sound-hyperphysics-ear":1045,"source:sound-hyperphysics-speed":1046,"source:sound-ncert-class9-sound":1047,"source:sound-nidcd-hearing":1048,"source:sound-nidcd-noise":1049,"source:sound-wikipedia-instruments":1050},"b164f45a2c8ca08f26c450768ff0231e113e9fe45381eddb34dc6d0548596c38","2a8ee4ac87460b4e1175a4bb13c96b03d577db06dde95670eb7fcfe4ad787899","22b6738152933cfccb190e600f1f387e7d7c3b1b36f37184363a3cd5f8dae8b6","c04a20203101dfc3dcfa0440d2d1ec936289920891a98545909218b09a36ac4b","3d6b0fe1b15255975a32b0fcd94e8019bc959ad45cbf12e136e86149549c6878","c2f918c426383c50d52939054780add1488f3f282c9d1a965c3a38345ddcb265","e63e8ac99c466c18f61ba209707bcf992538e2d87b16d8b6e7ceef147566ec7f","278297753fb9417bb45ce52bcbba4e42ccba48426815813568968b0ffaa42b34","0a3d2c27986cca40d06b5de949d3c7a13272672e9766ecf33e53ec27ee28c886","4cb248d8beea032b112212f11d2a3d10751a3a68da1b26d2ef93b11835467932","615e6ca7b7252ccb2523eea00746ef69215849397defe43c853a9fcb617799dd","562aa906d72517f7a88b5d3bae2937ed3678a02844460b91738fc93e493f3490","6c2548bc22d1e37e553f1f05a7b1c919c097e49ad319523b186e766828b576cd","0b24a438b7c5bb6e98dad1043334a28d32ba3aa4ef1eca60660933b537c66d0a",{"state":1052,"reviewer":1053,"selfReview":405,"reviewedAt":1054,"method":1055},"approved","The library owner","2026-09-20T10:18:37.581Z","owner_bulk","preview-7e1cbbcc4f",1789899597496]