[{"data":1,"prerenderedAt":932},["ShallowReactive",2],{"layer:sound:investigate":3},{"layer":4,"contentHash":907,"dependencyHashes":908,"approval":925,"releaseId":931},{"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":902,"reviewStatus":903,"authoring":904},1,"sound","en","investigate","Predict it, try it: resonance, echoes and everyday sound technology","Test resonance with a swing and a singing glass, then use echoes the way sonar, ultrasound, bats and dolphins do","Push a swing at the wrong rhythm, make a wine glass sing, and find the sympathetic strings that ring inside a sitar untouched. Time an echo the way sonar and a hospital scanner do, compare a bat's call with a dolphin's, and see why India's noise rules are stricter near a hospital than in a market.",[13,14,15,16,17],"Predict and test what resonance does, and identify natural frequency in swings, glasses and Indian instruments.","Explain reverberation as overlapping reflections, and use T = 0.161 × V ÷ A to find a room's reverberation time.","Use the echo formula in water and in soft tissue to find the depth of the sea floor or an organ from a timed echo.","Compare bat and dolphin echolocation, and explain why a dolphin's higher-frequency call still has a longer wavelength.","Describe how a microphone and a loudspeaker mirror each other, and combine two decibel levels correctly.",35,{"title":20,"rows":21},"Lesson plate",[22,25,28,31,34,37],{"label":23,"value":24},"Depth","Investigate",{"label":26,"value":27},"Reading time","≈ 35 minutes",{"label":29,"value":30},"Prior knowledge","Discover and Understand",{"label":32,"value":33},"Chapters","9",{"label":35,"value":36},"Labs","Sort games, match-pairs, two echo\u002Fsonar labs",{"label":38,"value":39},"Units used","Hz, dB, m, m\u002Fs, s, cubic metres",[41,45,51,57,60,65,70,74,92,107,112,137,186,191,194,207,228,255,259,264,269,272,276,289,301,315,327,332,337,340,353,357,386,390,403,419,424,427,439,459,463,467,472,475,503,516,521,526,529,568,574,578,582,587,590,618,631,644,649,685,690,735,869,873,887],{"id":42,"type":43,"markdown":44},"i-intro","prose","Discover found the vibration. Understand found the equations. Now it is time to become a sound scientist yourself: predict what will happen, try it, and see whether the prediction survives contact with reality.\n\nThis layer asks *what happens if...?* again and again. What happens if a room is empty instead of full? What happens if you push a swing out of rhythm? What happens if a bat's call bounced off water instead of air? Some of your predictions will be dead right. A few will surprise you, and those are the most useful lessons of all.\n\nYou will also meet three real technologies built entirely on echoes: sonar, medical ultrasound, and the sympathetic strings inside a sitar, plus the everyday physics of a microphone, a noisy festival, and a lawful decibel limit.",{"id":46,"type":47,"variant":48,"title":49,"markdown":50},"i-how-to-read","callout","observation","How to use this lesson","Every prediction box asks you to choose an answer *before* reading the explanation. Resist the urge to peek. Being wrong and finding out why is how this kind of understanding sticks.",{"id":52,"type":53,"title":54,"eyebrow":55,"navLabel":56},"i-ch01","chapter","Resonance: pushing at just the right moment","Chapter 01","1 Resonance",{"id":58,"type":43,"markdown":59},"i-ch1-p1","Every object that can vibrate has one or more speeds it vibrates at most easily, called its **natural frequency**. Push it gently at exactly that rate, again and again, and each little push adds to the last: the swing goes higher, the glass rings louder, the bridge deck sways further. Push it at any other rate and the pushes fight each other instead of adding up.\n\nThat building-up effect is **resonance**: a small, repeated push at the right frequency producing a big result. You have used it your whole life without the word. A child's swing is the clearest example there is.",{"id":61,"type":47,"variant":62,"title":63,"markdown":64},"i-def-resonance","definition","Resonance and natural frequency","The **natural frequency** of an object is the rate it vibrates at when disturbed and left alone, decided by its size, shape, stiffness and mass.\n\n**Resonance** happens when something is pushed, again and again, at (or very near) its own natural frequency, so that each push arrives in step with the last and the vibration grows large.",{"id":66,"type":47,"variant":67,"title":68,"markdown":69},"i-tryit-swing","try_it","The swing that will not go higher","Sit on a swing, or watch someone pushed on one.\n\n1. Ask the pusher to push at the *natural* rhythm: exactly when the swing returns to them each time. It climbs higher and higher with almost no effort.\n2. Now ask them to push at a random, uneven rhythm, sometimes helping, sometimes against the motion. The swing barely rises, however hard they push.\n\nA swing has one natural frequency, decided by the length of its chains (a longer swing is slower). No amount of force at the wrong moment beats a small amount of force at the right moment.",{"id":71,"type":47,"variant":67,"title":72,"markdown":73},"i-tryit-glass","Making a glass sing","You need a wine glass or a thin-walled drinking glass, and a clean, damp finger.\n\n1. Wet your index finger and wipe the rim of the glass clean.\n2. Rub your finger around the rim at a slow, steady speed, pressing lightly.\n3. Keep going. After a few seconds a clear, ringing note appears.\n\nYour finger is not squeaking; it is sticking and slipping on the glass thousands of times a second, and each slip is a tiny push. The glass rings at its own natural frequency, which depends on its size, shape and how much liquid is inside it. Add water and try again: the note drops, the same way a jal tarang bowl drops in pitch when you add water.",{"id":75,"type":76,"prompt":77,"options":78,"explanation":91},"i-predict-glass-water","prediction","You make a glass sing by rubbing its rim, and note down the pitch. You then pour in some water and rub again with the same technique. What happens to the pitch?",[79,82,85,88],{"id":80,"label":81},"a","It rises",{"id":83,"label":84},"b","It falls",{"id":86,"label":87},"c","It stays exactly the same",{"id":89,"label":90},"d","The glass stops ringing altogether","**It falls.** The glass wall plus the water inside it is now the vibrating system, and the water adds mass without adding much stiffness. More mass vibrating means a slower natural frequency, means a lower note, exactly as it did for the jal tarang bowls in the previous layer. Empty the glass and the note climbs back up.",{"id":93,"type":94,"title":95,"problem":96,"steps":97,"help":104},"i-we-resonance-box","worked_example","Sizing a resonance box for a tuning fork","A tuning fork stamped 256 Hz (middle C) is held over the open end of a tube that is closed at the bottom, so it behaves like a closed pipe. What length of air column will resonate loudest with the fork?",[98,99,100,101,102,103],"A closed pipe's fundamental frequency is f = v ÷ (4L), so rearrange for L: L = v ÷ (4f).","Put in the numbers: L = 343 ÷ (4 × 256).","4 × 256 = 1,024.","L = 343 ÷ 1,024 = **0.335 m**, about 33.5 centimetres.","Check: v = f × 4L = 256 × 4 × 0.335 = 343.04 m\u002Fs. ✓ (Rounding accounts for the tiny difference.)","This is exactly how a resonance-tube experiment works in a school lab: slide a water level up and down inside a tube until the ringing tuning fork sounds loudest, then measure the air column.",{"simplerExplanation":105,"anotherExample":106},"The air column has its own natural frequency, just like the swing. Set the tube length so that frequency matches the fork's 256 Hz, and the two reinforce each other into a loud note.","A concert-pitch fork at 440 Hz needs a shorter column: L = 343 ÷ (4 × 440) = 0.195 m, about 19.5 cm.",{"id":108,"type":47,"variant":109,"title":110,"markdown":111},"i-misconception-only-special","misconception","“Only special objects resonate”","It can look like resonance is a rare party trick reserved for wine glasses and tuning forks. It is not. **Every** object that can vibrate has a natural frequency: a ruler on a desk, a cupboard door, a bridge deck, the air inside a bottle, even a building.\n\nMost of the time nothing pushes these objects at exactly their natural rate, so we never notice. Engineers who design bridges, tall buildings and aircraft wings spend enormous effort making sure that wind, footsteps, engines and earthquakes never find that one frequency and build it up dangerously.",{"id":113,"type":114,"tone":115,"items":116},"i-spec-natural-freq","spec","blue",[117,121,125,129,133],{"label":118,"big":119,"value":120},"Playground swing","0.5–1 Hz","One full swing every one to two seconds; a longer swing is slower.",{"label":122,"big":123,"value":124},"Tall building, swaying in wind","well under 1 Hz","Skyscrapers are deliberately built with a slow natural sway, and dampers are added to calm it further.",{"label":126,"big":127,"value":128},"Sitar string","roughly 100–1,000 Hz","Depends on which note it is tuned to and where it is stopped on the neck.",{"label":130,"big":131,"value":132},"Wine glass (empty, dry)","a few hundred hertz","Bigger, thinner glasses tend to ring lower; smaller, thicker ones ring higher.",{"label":134,"big":135,"value":136},"Tabla (tuned)","roughly 100–500 Hz","Set by the skin's tension and the mass added by the syahi patch.",{"id":138,"type":139,"component":140,"componentVersion":5,"config":141,"objective":184,"textAlternative":185},"i-lab-sort-resonance","interactive","sort-game",{"prompt":142,"bins":143,"items":150,"seconds":183},"Sort each situation by whether resonance is being used on purpose, or is a problem to be avoided.",[144,147],{"id":145,"label":146},"useful","Resonance used on purpose",{"id":148,"label":149},"problem","Resonance is a danger to avoid",[151,155,159,163,167,171,175,179],{"id":152,"label":153,"bin":145,"why":154},"radio-tuning","Tuning a radio to a station","A radio circuit is set to resonate at exactly one station's frequency and ignore the rest.",{"id":156,"label":157,"bin":145,"why":158},"swing","Pushing a swing in rhythm","Small, well-timed pushes build up into a large, comfortable swing.",{"id":160,"label":161,"bin":145,"why":162},"sitar-string","Sitar sympathetic strings ringing with the played note","Extra strings are tuned to ring in resonance, adding the instrument's shimmer on purpose.",{"id":164,"label":165,"bin":145,"why":166},"mri","A hospital MRI scanner","MRI works by resonating the nuclei of hydrogen atoms in the body with radio waves, tuned very precisely.",{"id":168,"label":169,"bin":148,"why":170},"soldiers","Soldiers ordered to break step crossing a footbridge","Marching in step for a long time can, in rare cases, push a bridge near a troublesome natural frequency, so many armies break step by rule as a precaution.",{"id":172,"label":173,"bin":148,"why":174},"washing-machine","A washing machine shaking violently at one spin speed","At that particular drum speed the machine's frame is being pushed near its own natural frequency.",{"id":176,"label":177,"bin":148,"why":178},"glass-shatter","A very loud, sustained note shattering a glass","If the note matches the glass's natural frequency and is loud enough for long enough, the glass can flex further than it can bear.",{"id":180,"label":181,"bin":148,"why":182},"car-rattle","A loose car part that only rattles at one particular engine speed","The part's own natural frequency is being matched by the engine's vibration at that speed.",0,"Sort eight real situations into resonance used deliberately and resonance treated as a hazard.","A sorting game with eight cards and two bins: **resonance used on purpose** and **resonance is a danger to avoid**.\n\nUseful: tuning a radio, pushing a swing in rhythm, a sitar's sympathetic strings, and an MRI scanner, which resonates hydrogen nuclei with radio waves.\n\nA danger: soldiers breaking step on a footbridge as a precaution, a washing machine shaking hardest at one particular spin speed, a sustained loud note shattering a glass, and a loose car part that rattles only at one engine speed.\n\nThe common thread in every card, useful or dangerous: **something is being pushed repeatedly near its own natural frequency**, and the two possible outcomes are a useful build-up or a destructive one.",{"id":187,"type":53,"title":188,"eyebrow":189,"navLabel":190},"i-ch02","Sympathetic strings: resonance inside Indian instruments","Chapter 02","2 Sympathetic strings",{"id":192,"type":43,"markdown":193},"i-ch2-p1","A sitar looks like it has far more strings than a player's ten fingers could ever need, and it does: alongside the strings that are plucked sit a row of thin **sympathetic strings**, called *tarab* strings, running underneath. Nobody touches them with a finger or a pick.\n\nEach sympathetic string is tuned to a particular note of the raga being played. When the played string sounds that same note, the sympathetic string starts to vibrate on its own, purely from resonance, adding a soft, shimmering halo to the sound. Pluck a different note and a different sympathetic string answers.",{"id":195,"type":76,"prompt":196,"options":197,"explanation":206},"i-predict-sitar","A sitar's sympathetic string is tuned to the note D. The player plucks a main string and sounds the note D. What happens to the sympathetic D string, and what happens if the player instead plays the note F?",[198,200,202,204],{"id":80,"label":199},"Both strings ring equally whichever note is played",{"id":83,"label":201},"Only the D sympathetic string rings when D is played; little happens when F is played",{"id":86,"label":203},"Neither sympathetic string ever moves; it is decoration",{"id":89,"label":205},"The sympathetic string rings louder than the plucked string","**Only the D sympathetic string rings, and strongly, when D is played; playing F does very little to it.** This is resonance being selective: a string only responds strongly to a push at its own natural frequency. Because each sympathetic string is tuned to a different note of the raga, plucking different notes calls up different sympathetic strings in turn, which is exactly why a sitar's sustain has that complex, singing quality a plain guitar does not.",{"id":208,"type":209,"title":210,"items":211},"i-steps-tuning-by-resonance","steps","How musicians use resonance to tune Indian drums, without any electronic tuner",[212,216,220,224],{"title":213,"tag":214,"text":215},"Tabla dough (syahi and satham)","mass loading","The black paste patch on a tabla, and wet flour dough sometimes pressed onto the smaller drum before a show, add mass to part of the skin, changing its natural frequencies so the drum sounds a real note, not a thud.",{"title":217,"tag":218,"text":219},"Tightening the straps","raise the pitch","Pulling the leather straps, or tapping the wooden tuning blocks around a tabla's rim, stretches the skin tighter. A tighter skin has a higher natural frequency, exactly like a tightened rubber band.",{"title":221,"tag":222,"text":223},"Mridangam paste on both heads","two different notes","A mridangam has a temporary rice-and-tamarind paste on one head and a permanent black patch on the other, tuned to two different, related notes so the drum can carry a bass and a treble sound at once.",{"title":225,"tag":226,"text":227},"Checking by ear against a drone","resonance as a test","A player taps a drum softly while a tanpura or harmonium drone plays the target note. If the drum is at the right natural frequency, you can hear it answer the drone faintly, the same sympathetic-resonance idea as the sitar's tarab strings.",{"id":229,"type":139,"component":230,"componentVersion":5,"config":231,"objective":253,"textAlternative":254},"i-lab-match-instruments","match-pairs",{"prompt":232,"mode":233,"pairs":234},"Match each Indian instrument to the resonance trick used to tune or voice it.","connect",[235,238,241,244,247,250],{"a":236,"b":237},"Sitar and sarangi","Unplayed sympathetic strings tuned to the raga's notes",{"a":239,"b":240},"Tabla","The syahi paste tunes the overtones to a real pitch",{"a":242,"b":243},"Mridangam","Different pastes on each head give two different notes",{"a":245,"b":246},"Jal tarang","Adding water lowers each bowl's natural frequency",{"a":248,"b":249},"Ghatam","The size and thickness of the clay set its one natural note",{"a":251,"b":252},"Bansuri","Covering finger holes changes the resonating length of the air column","Connect six Indian instruments to the specific way each one is tuned by resonance.","A matching game with six pairs.\n\n- **Sitar and sarangi**: sympathetic strings tuned to the raga's notes, ringing only when that note is played.\n- **Tabla**: the syahi paste tunes the overtones so a struck skin gives a real musical pitch.\n- **Mridangam**: two different pastes on the two heads give two different notes from one drum.\n- **Jal tarang**: adding water to a bowl adds mass, lowering its natural frequency.\n- **Ghatam**: the clay pot's own size and wall thickness fix its one natural note; nothing is tunable once fired.\n- **Bansuri**: covering finger holes lengthens the resonating column of air, lowering the note.\n\nEvery single one of these is the same idea from Chapter 1, applied by instrument-makers centuries before the word *resonance* existed.",{"id":256,"type":47,"variant":67,"title":257,"markdown":258},"i-tryit-two-glasses","Two glasses, one push","You need two identical drinking glasses (same shape, same size, same amount of liquid) and a quiet room.\n\n1. Stand them a short distance apart on the same hard table.\n2. Flick or rub the rim of the first glass to make it ring.\n3. Immediately touch the first glass to stop it, then bring your ear close to the second, untouched glass.\n\nIf the two glasses are close enough in tuning, the second one can be heard humming very faintly, entirely on its own. The first glass's ringing pushed the table and the air at its own natural frequency, and the second glass, sharing very nearly that same natural frequency, picked up the push by resonance, exactly as a sitar's sympathetic string answers a matching note.",{"id":260,"type":47,"variant":261,"title":262,"markdown":263},"i-nuance-ghatam-fixed","nuance","Why a ghatam cannot be tuned once made","A ghatam's natural frequency is fixed by its clay, its wall thickness and its exact size and shape, all set during firing. Unlike a drum's skin or a string's tension, there is no strap to tighten and no peg to turn afterwards.\n\nThis is why ghatam players often own several pots of different sizes, tuned roughly to different notes, and choose the one that suits a performance, and why a skilled potter making ghatam is valued: getting a pot close to a wanted pitch has to happen in the making, not after.",{"id":265,"type":53,"title":266,"eyebrow":267,"navLabel":268},"i-ch03","Reverberation: when echoes overlap","Chapter 03","3 Reverberation",{"id":270,"type":43,"markdown":271},"i-ch3-p1","A single, clean echo needs a lone hard surface at least about 17 metres away. But clap inside an ordinary room and you do not hear one echo; you hear a fast, blurred tail of sound that dies away over a second or so. Every wall, the ceiling, the floor and the furniture all reflect a little of the clap, at slightly different times, and they all arrive too close together for your brain to separate. That blurred, decaying tail is **reverberation**.",{"id":273,"type":47,"variant":62,"title":274,"markdown":275},"i-def-reverberation","Reverberation and reverberation time","**Reverberation** is the persistence of sound in a room after the source stops, caused by many overlapping reflections rather than one clean echo.\n\n**Reverberation time**, often written **RT60**, is how many seconds it takes the sound to fade by a large, standard amount (60 decibels) after the source stops. A short RT60 sounds dry and clear; a long RT60 sounds boomy and washed out.",{"id":277,"type":278,"items":279},"i-formulas-sabine","formulas",[280,283,286],{"expression":281,"caption":282},"T = 0.161 × V ÷ A","Sabine's formula. V is the room's volume in cubic metres, A is its total absorption (bigger for soft surfaces), T is in seconds.",{"expression":284,"caption":285},"more absorption → shorter T","Carpets, curtains, sofas and people all add absorption and shrink the reverberation time.",{"expression":287,"caption":288},"bigger room, same surfaces → longer T","A bigger room has more space for sound to keep bouncing before it is absorbed.",{"id":290,"type":94,"title":291,"problem":292,"steps":293,"help":299},"i-we-classroom-rt","Furnishing a bare classroom","An empty classroom has a volume of 200 cubic metres. With its bare hard surfaces, its total absorption is about 40 (in the units Sabine's formula uses). What is its reverberation time, and what happens if curtains and a carpet double the absorption to 80?",[294,295,296,297,298],"Bare room: T = 0.161 × V ÷ A = 0.161 × 200 ÷ 40.","0.161 × 200 = 32.2, then 32.2 ÷ 40 = **0.805 seconds**.","Furnished room: T = 0.161 × 200 ÷ 80 = 32.2 ÷ 80 = **0.4025 seconds**, almost exactly half.","Notice the pattern: doubling the absorption exactly halves the reverberation time, because A sits on the bottom of the fraction.","This is precisely why a bare, empty classroom sounds harsh and echoey during the holidays, and calmer once thirty students, their bags and their clothes are back in it: bodies and fabric are excellent absorbers.",{"simplerExplanation":300},"More soft stuff to absorb the sound means the room lets go of it faster. Twice the absorption gives half the lingering time.",{"id":302,"type":139,"component":303,"componentVersion":5,"config":304,"objective":313,"textAlternative":314},"i-lab-speed-echo-hall","sound-speed",{"media":305,"distanceM":309,"modes":310},[306,307,308],"air","water","steel",500,[311,312],"echo","race","Send a clap to a hall wall and time its echo, then compare with the same distance through water and steel.","An echo lab set at 500 metres. In **air**, the round trip takes about 2.9 seconds (500 × 2 ÷ 343). In **water**, the same round trip takes only about 0.68 seconds, and in **steel** about 0.17 seconds, because both carry the push far faster.\n\nUnderwater listening devices and structural-testing engineers use exactly this: the same echo idea from Chapter 7 of Discover, just measured in a faster medium.",{"id":316,"type":76,"prompt":317,"options":318,"explanation":326},"i-predict-audience","A concert hall is measured empty, then measured again full of an audience, using the same loudspeaker and the same claps. What happens to the reverberation time with people in the seats?",[319,321,323,324],{"id":80,"label":320},"It gets longer, because people talk and rustle",{"id":83,"label":322},"It gets shorter, because bodies and clothes absorb sound",{"id":86,"label":87},{"id":89,"label":325},"It becomes impossible to measure","**It gets shorter.** People, and the clothes and seat cushions they sit against, are surprisingly good sound absorbers, similar to heavy curtains. A hall's total absorption rises sharply once it fills up, so by Sabine's formula the reverberation time falls.\n\nThis is a real headache for concert-hall designers and sound engineers: a hall tuned to sound right when full can sound too echoey during an empty rehearsal, and one tuned right when empty can sound too dry and dead on the night of the concert.",{"id":328,"type":47,"variant":329,"title":330,"markdown":331},"i-example-halls","example","Different rooms, built for different reverberation times","A recording studio's vocal booth is deliberately built almost dead, with a reverberation time under half a second, so a singer's recorded voice carries no room sound at all and effects can be added afterwards.\n\nA large place of worship or a stone-built hall, by contrast, can have a reverberation time of two seconds or more, which suits slow chanting or organ music beautifully but makes fast, clear speech hard to follow, because one word's tail overlaps the next word's start.\n\nThere is no single \"best\" reverberation time. A lecture hall wants speech to stay clear (short RT); a concert hall for orchestral music wants some richness (longer RT); the right answer depends on what the room is for.",{"id":333,"type":53,"title":334,"eyebrow":335,"navLabel":336},"i-ch04","Sonar: seeing the sea floor with echoes","Chapter 04","4 Sonar",{"id":338,"type":43,"markdown":339},"i-ch4-p1","Light barely travels through sea water; a torch beam fades out within tens of metres. Sound does far better, so ships and submarines use **sonar** (SOund Navigation And Ranging): send a sharp pulse of sound downward or outward, time how long the echo takes to return, and use the speed of sound in water to work out the distance.",{"id":341,"type":94,"title":342,"problem":343,"steps":344,"help":350},"i-we-sonar-fishing","How deep is the water under a fishing boat?","A depth sounder on a fishing boat sends a downward pulse and the echo returns after 0.06 seconds. Sound travels at about 1,500 m\u002Fs in sea water. How deep is the water?",[345,346,347,348,349],"The pulse makes a round trip: down to the sea floor and back up.","Total distance travelled = speed × time = 1,500 × 0.06 = **90 metres**.","That is there *and* back, so the depth is half of it.","Depth = 90 ÷ 2 = **45 metres**.","Check the other way: 45 m down takes 45 ÷ 1,500 = 0.03 s, and 45 m back takes another 0.03 s, totalling 0.06 s. ✓",{"simplerExplanation":351,"anotherExample":352},"Distance there-and-back is speed × time. Halve it for the one-way depth: 1,500 × 0.06 ÷ 2 = 45 m.","An echo of 1.6 seconds over the continental shelf gives a depth of 1,500 × 1.6 ÷ 2 = 1,200 m.",{"id":354,"type":47,"variant":329,"title":355,"markdown":356},"i-example-mariana","Sounding the deepest place on Earth","The Mariana Trench, in the western Pacific, plunges to about **10,935 metres**, deeper than Mount Everest is tall. A sonar pulse aimed straight down there takes about **14.6 seconds** to return: 1,500 × 14.6 ÷ 2 ≈ 10,950 m, close enough given how the real speed of sound changes slightly with pressure and temperature at such extreme depth.\n\nBefore sonar, sailors measured deep water by lowering a weighted rope, metre by metre, which could take hours for a single deep-sea reading. A sonar ping gives the same answer in about fifteen seconds.",{"id":358,"type":359,"caption":360,"columns":361,"rows":365},"i-table-sonar-depths","table","Sonar round-trip times for some real depths (sound in sea water at about 1,500 m\u002Fs)",[362,363,364],"Location","Approx. depth","Round-trip echo time",[366,370,374,378,382],[367,368,369],"A harbour or river mouth","10 m","0.013 s",[371,372,373],"A fishing boat over the shelf","45 m","0.06 s",[375,376,377],"A busy shipping lane","200 m","0.27 s",[379,380,381],"The average ocean depth","about 3,700 m","4.9 s",[383,384,385],"The Mariana Trench","about 10,935 m","14.6 s",{"id":387,"type":47,"variant":329,"title":388,"markdown":389},"i-example-old-sailors","Before sonar: the sounding line","For centuries, sailors measured depth with a **sounding line**: a rope with a heavy lead weight, marked at regular intervals and lowered by hand until it touched bottom. A single deep-ocean reading could take a strong sailor an hour of hauling rope, and only told them the depth directly below the ship at that moment.\n\nOn 23 March 1875, the survey ship HMS Challenger lowered a sounding line in the western Pacific and, after checking twice because the number seemed impossible, recorded a depth of about **8,200 metres**: the first sign of what is now known as the Mariana Trench. A modern sonar pulse finds the same kind of answer, more precisely, in well under a minute.",{"id":391,"type":76,"prompt":392,"options":393,"explanation":402},"i-predict-sonar-air","Sonar works superbly underwater. A ship's crew wonders whether the same equipment, sending an ordinary sound pulse through open air instead, could find an aircraft ten kilometres away just as well. Is this a good idea?",[394,396,398,400],{"id":80,"label":395},"Yes, air and water work equally well for sonar",{"id":83,"label":397},"No, sound loses far more energy over long distances in air, so a useful echo would not return",{"id":86,"label":399},"No, sound cannot travel through air at all",{"id":89,"label":401},"Yes, but only at night","**No.** Sound is absorbed and spread out far more over long distances in air than in the denser, more tightly coupled particles of water, and ten kilometres in air is an enormous distance for a sound pulse to survive there and back. This is exactly why long-range detection through air uses **radar**, which sends out radio waves (a form of light) instead of sound. Sonar's home advantage is underwater, where sound travels well and light barely travels at all; radar's home advantage is in air, for the opposite reason.",{"id":404,"type":405,"itemId":406,"prompt":407,"check":408,"hints":413,"feedback":416},"i-prac-sonar","practice","sound.investigate-sonar-depth","A ship's sonar sends a pulse and the echo returns after **2.0 seconds**. Using 1,500 m\u002Fs for sound in sea water, how deep is the water, in metres?",{"kind":409,"answer":410,"tolerance":411,"unit":412},"number",1500,10,"m",[414,415],"Find the total round-trip distance first: speed × time.","Then halve it for the one-way depth.",{"correct":417,"incorrect":418},"Right: 1,500 × 2.0 = 3,000 m round trip, so the depth is 3,000 ÷ 2 = **1,500 m**.","Total distance = 1,500 × 2.0 = 3,000 m. That is there and back, so the depth is half: 1,500 m.",{"id":420,"type":53,"title":421,"eyebrow":422,"navLabel":423},"i-ch05","Ultrasound: echoes small enough to see inside a body","Chapter 05","5 Ultrasound",{"id":425,"type":43,"markdown":426},"i-ch5-p1","A medical ultrasound scanner uses the very same echo idea as sonar, shrunk enormously in both distance and time. A small probe presses against the skin, sends ultrasound pulses (millions of vibrations a second, far above hearing) into the body, and times the echoes bouncing off the boundaries between different tissues, such as skin, muscle, fluid and bone. A computer turns thousands of these tiny timed echoes into a moving picture.",{"id":428,"type":94,"title":429,"problem":430,"steps":431,"help":437},"i-we-ultrasound-timing","Timing an echo inside the body","An ultrasound pulse in soft tissue, where sound travels at about 1,540 m\u002Fs, reflects off an organ 12 centimetres (0.12 m) below the skin. How long does the echo take to return, in microseconds (millionths of a second)?",[432,433,434,435,436],"Convert to the round-trip idea: time = 2 × distance ÷ speed.","time = 2 × 0.12 ÷ 1,540 = 0.24 ÷ 1,540 seconds.","= 0.0001558 seconds.","Multiply by 1,000,000 to convert to microseconds: **about 155.8 microseconds**.","That is roughly a six-thousandth of the time it takes you to blink. A scanner performs thousands of these timed pulses every second to build one image.",{"simplerExplanation":438},"The pulse goes 12 cm down and 12 cm back, 24 cm total, at 1,540 m\u002Fs. That trip takes under two ten-thousandths of a second.",{"id":440,"type":359,"caption":441,"columns":442,"rows":446},"i-table-ultrasound-freq","Choosing an ultrasound frequency: detail against depth",[443,444,445],"Frequency","Wavelength in tissue","Typical use",[447,451,455],[448,449,450],"2 MHz","0.77 mm","Deep abdominal scans, where reach matters more than fine detail",[452,453,454],"5 MHz","0.31 mm","General-purpose scanning, including checking a baby before birth",[456,457,458],"15 MHz","0.10 mm","Very shallow, fine-detail scans of skin, eyes or small blood vessels",{"id":460,"type":47,"variant":261,"title":461,"markdown":462},"i-nuance-tradeoff","Why scanners do not simply use the highest frequency","A shorter wavelength reveals finer detail, the same rule that explains a bat's high-pitched call from earlier in this topic. So why not always use 15 MHz?\n\nHigh-frequency ultrasound loses energy to the body's tissues faster than low-frequency ultrasound does, so it cannot reach deep structures with a usable echo. A doctor scanning a deep organ chooses a **lower** frequency to reach it, accepting a coarser picture; a doctor scanning something shallow, such as an eye, chooses a **higher** frequency for sharp detail, because reach is not the problem there.\n\nThe same trade-off, detail against range, appears again with bats and dolphins in the next chapter, and with sonar frequencies used to find fish shoals versus map the whole sea floor.",{"id":464,"type":47,"variant":109,"title":465,"markdown":466},"i-misconception-ultrasound-radiation","“Ultrasound is a kind of radiation, like an X-ray”","It is not. An X-ray is a very high-energy form of light, which can damage living cells and is used carefully and sparingly, especially in pregnancy.\n\n**Ultrasound is entirely ordinary sound**, simply pitched above human hearing. It carries no ionising radiation at all, which is exactly why it is the preferred way to look at a baby before birth: as far as decades of use have shown, it carries none of an X-ray's risks.",{"id":468,"type":53,"title":469,"eyebrow":470,"navLabel":471},"i-ch06","Bats and dolphins: echolocation compared","Chapter 06","6 Bats and dolphins",{"id":473,"type":43,"markdown":474},"i-ch6-p1","A bat flying in pitch darkness and a dolphin swimming in murky water solve the same problem with the same trick: shout, and listen for the echo. But they shout into completely different media, at different speeds, and that changes the numbers.",{"id":476,"type":359,"caption":477,"columns":478,"rows":482},"i-table-bat-dolphin","Bat and dolphin echolocation compared",[479,480,481],"Feature","Bat (in air)","Dolphin (in water)",[483,487,491,495,499],[484,485,486],"Typical call frequency","up to about 120,000 Hz","up to about 150,000 Hz",[488,489,490],"Speed of sound in its medium","343 m\u002Fs","about 1,500 m\u002Fs",[492,493,494],"Wavelength at a high call frequency","about 3.4 mm at 100,000 Hz","about 1 cm at 150,000 Hz",[496,497,498],"What the short wavelength is for","Detecting a flying insect only millimetres across","Detecting a fish, or telling one object's texture from another",[500,501,502],"Extra trick","Some bats shift their call frequency as they close in on prey","Dolphins can focus a beam of clicks using a fatty organ in the forehead, the melon",{"id":504,"type":76,"prompt":505,"options":506,"explanation":515},"i-predict-dolphin-mud","A dolphin swims into water so muddy it cannot see its own fin. It still finds and catches a fish nearby. How?",[507,509,511,513],{"id":80,"label":508},"It smells the fish through the water",{"id":83,"label":510},"It sends out clicks and listens for the echo bouncing off the fish",{"id":86,"label":512},"It waits until the water clears",{"id":89,"label":514},"It uses the same light-based eyesight, just more slowly","**It sends out clicks and listens for the returning echo**, exactly like a bat in darkness. Muddy water blocks light just as effectively as darkness does, but it barely affects sound. This is why dolphins, and toothed whales generally, rely on echolocation as their main sense for hunting, far more than their eyes, even in perfectly clear water.",{"id":517,"type":47,"variant":518,"title":519,"markdown":520},"i-aha-different-speeds","aha","Same trick, different speeds, same physics","It looks odd at first that a dolphin's call has a *longer* wavelength (about 1 cm) than a bat's call (about 3.4 mm) even though the dolphin's frequency is *higher* (150,000 Hz against 120,000 Hz). The reason is the medium: sound in water travels more than four times faster than in air, so at any given frequency the wavelength is more than four times longer in water.\n\nA dolphin's targets, mostly fish, are typically bigger than a bat's targets, mostly insects, so a slightly longer wavelength still does the job. Nature has tuned each animal's call to what it actually needs to detect, in the medium it actually swims or flies through.",{"id":522,"type":53,"title":523,"eyebrow":524,"navLabel":525},"i-ch07","Microphones and speakers: sound becomes current, and back again","Chapter 07","7 Mics and speakers",{"id":527,"type":43,"markdown":528},"i-ch7-p1","A phone call, a loudspeaker, a recording: all of them cross from air to electricity and back at least once. The two machines that do the crossing are close cousins of each other, built almost the same way but running in opposite directions.",{"id":530,"type":531,"component":532,"componentVersion":5,"config":533,"textAlternative":567},"i-anim-mic-speaker","animation","process-steps",{"title":534,"diagram":535,"steps":536},"How a dynamic microphone and a loudspeaker mirror each other","none",[537,542,547,552,557,562],{"id":538,"label":539,"description":540,"highlight":541},"mic-diaphragm","Microphone: sound arrives","Arriving compressions and rarefactions push a thin diaphragm in and out, at exactly the rate the source is vibrating.",[],{"id":543,"label":544,"description":545,"highlight":546},"mic-coil","Microphone: a coil moves","The diaphragm is attached to a tiny coil of wire wrapped around a fixed magnet. As the diaphragm moves, the coil moves with it, through the magnet's field.",[],{"id":548,"label":549,"description":550,"highlight":551},"mic-current","Microphone: a current appears","A coil of wire moving through a magnetic field has a voltage induced in it, the same electromagnetic induction used inside a generator. The result is a tiny, rapidly changing electric current that mirrors the sound wave's shape.",[],{"id":553,"label":554,"description":555,"highlight":556},"speaker-current","Speaker: current arrives","An amplified version of that changing current, or a recorded and replayed one, is sent into a loudspeaker's own coil.",[],{"id":558,"label":559,"description":560,"highlight":561},"speaker-coil","Speaker: the coil is pushed","The speaker's coil sits inside a strong fixed magnet's field. A current through it now creates a force that pushes the coil, forwards and back, following the current exactly.",[],{"id":563,"label":564,"description":565,"highlight":566},"speaker-cone","Speaker: the cone moves the air","The coil is attached to a large paper or plastic cone. As the coil is pushed and pulled, the cone shoves and releases the air in front of it, recreating compressions and rarefactions: sound again.",[],"A six-step comparison, three steps each way.\n\n**Microphone (sound to current):** 1) Arriving sound pushes a diaphragm in and out. 2) A tiny coil attached to the diaphragm moves through a fixed magnet's field. 3) That movement induces a changing electric current shaped exactly like the sound wave.\n\n**Loudspeaker (current back to sound):** 4) An amplified current flows into the speaker's own coil. 5) The coil, sitting inside a magnet, is pushed forwards and back by that current. 6) A cone attached to the coil pushes the air, recreating the pattern of compressions and rarefactions.\n\nA microphone and a loudspeaker are built the same way, coil plus magnet plus diaphragm or cone, and run in exactly opposite directions. This is the same coil-in-a-magnetic-field idea used in the generators of the Electricity topic, just working at audio speed instead of 50 times a second.",{"id":569,"type":570,"conceptId":571,"relation":572,"explanation":573},"i-conn-electricity","connection","electricity","applied_in","A microphone induces current the same way a generator does: a coil moving through a magnetic field. A loudspeaker runs the idea backwards, using current to move a coil and push air.",{"id":575,"type":47,"variant":109,"title":576,"markdown":577},"i-misconception-mic-louder","“A microphone makes a sound louder”","A microphone does not make anything louder. It only **converts** a pattern of air pressure into a matching pattern of tiny electric current, typically a current far too weak to do anything on its own.\n\nMaking a sound loud again needs an **amplifier**, an electronic circuit that copies the microphone's pattern faithfully but with far more electrical power, feeding a loudspeaker strong enough to move a lot of air. A microphone without an amplifier and a speaker is like a messenger with a message but no voice.",{"id":579,"type":47,"variant":67,"title":580,"markdown":581},"i-tryit-string-phone","The string telephone: sound through a solid, without any electricity at all","You need two empty paper or plastic cups, a length of thin, taut string about 5–10 metres long, and two paperclips or matchsticks.\n\n1. Make a small hole in the bottom of each cup and thread the string through, tying it to a paperclip inside so it cannot pull out.\n2. Walk apart until the string is taut, not sagging.\n3. One person talks quietly into their cup while the other holds their cup to an ear, **without touching the string**.\n\nThe voice carries clearly, even as a whisper, over a distance where it would be inaudible through air alone. The talker's cup vibrates the taut string directly; because string is a solid, it carries the vibration far more efficiently than air does, exactly as the wooden table and the steel bucket did back in Discover. Pinch the string in the middle and the sound stops instantly, proving the string, not the air around it, is carrying the message.",{"id":583,"type":53,"title":584,"eyebrow":585,"navLabel":586},"i-ch08","Noise pollution and festival firecrackers","Chapter 08","8 Noise pollution",{"id":588,"type":43,"markdown":589},"i-ch8-p1","Loudness does not simply add up the way you might expect. Two identical machines running together are not twice as loud in decibels; they are only about 3 dB louder, because decibels are built from ratios, not plain counts, as you found in Understand. That single fact matters enormously for how India regulates noise.",{"id":591,"type":359,"caption":592,"columns":593,"rows":598},"i-table-cpcb-zones","India's ambient noise standards by zone (Noise Pollution Rules, 2000)",[594,595,596,597],"Zone","Day limit","Night limit","Typical area",[599,604,609,613],[600,601,602,603],"Silence zone","50 dB","40 dB","Within 100 m of hospitals, schools and courts",[605,606,607,608],"Residential","55 dB","45 dB","Housing colonies",[610,611,606,612],"Commercial","65 dB","Markets and shops",[614,615,616,617],"Industrial","75 dB","70 dB","Factory areas",{"id":619,"type":94,"title":620,"problem":621,"steps":622,"help":628},"i-we-combine-noise","Two machines together are not twice as loud","A generator alone measures 70 dB where you stand. A compressor alone, at the same spot, also measures 70 dB. What does a meter read with both running together?",[623,624,625,626,627],"Decibels are built from a ratio to a reference, so you cannot simply add 70 + 70 = 140.","Doubling the sound *energy* only adds about 3 dB to the level, as you found with two identical loudspeakers in Understand.","So two 70 dB machines together read about **73 dB**, not 140 dB.","Using the precise combination formula gives 73.0 dB, confirming the rule.","Now try two *different* levels: a 70 dB machine with a 75 dB one together give about **76.2 dB**, only a little above the louder one alone, because the quieter source adds comparatively little energy.",{"simplerExplanation":629,"anotherExample":630},"Adding a second, similar noise source barely changes the total decibel reading, because decibels compress huge changes in energy into small changes in level.","Three identical 90 dB machines together give about 94.8 dB, not 270 dB.",{"id":632,"type":76,"prompt":633,"options":634,"explanation":643},"i-predict-generator-compressor","A residential street's daytime noise limit under India's rules is 55 dB. A single motorbike passing measures 70 dB, well above the limit. If a second, identical motorbike passes at the very same moment, does the combined noise breach the limit by roughly twice as much?",[635,637,639,641],{"id":80,"label":636},"Yes, it becomes about 140 dB, twice the breach",{"id":83,"label":638},"No, it becomes about 73 dB, only a little louder than one bike alone",{"id":86,"label":640},"No, the two sounds cancel out",{"id":89,"label":642},"The limit no longer applies with two sources","**No: about 73 dB**, only roughly 3 dB above a single bike, because combining two similar sound sources adds about 3 dB, not double the number. It is still well over the 55 dB residential limit either way, which is the real point: even one badly silenced vehicle can be several times louder, in energy, than the legal limit allows.",{"id":645,"type":47,"variant":646,"title":647,"markdown":648},"i-careful-firecrackers","careful","The firecracker limit, and why distance is your best protection","India's rules cap firecrackers at **125 dB(AI)** measured 4 metres from where they are burst. That is already far above the 85 dB level where long exposure damages hearing, and a bystander standing much closer than 4 metres experiences a far higher level, because sound falls off quickly with distance: roughly 6 dB quieter every time the distance doubles.\n\nThe safest choices at a firecracker display: stand well back, cover your ears just before a loud burst, and never bend down close to light one yourself.",{"id":650,"type":139,"component":140,"componentVersion":5,"config":651,"objective":683,"textAlternative":684},"i-lab-sort-noise-zone",{"prompt":652,"bins":653,"items":666,"seconds":183},"Sort each real-world sound into the zone whose daytime limit it would most likely exceed.",[654,657,660,663],{"id":655,"label":656},"silence","Exceeds silence zone (50 dB)",{"id":658,"label":659},"residential","Exceeds residential (55 dB)",{"id":661,"label":662},"commercial","Exceeds commercial (65 dB)",{"id":664,"label":665},"within-industrial","Within industrial (75 dB)",[667,671,675,679],{"id":668,"label":669,"bin":655,"why":670},"hospital-chat","Quiet conversation outside a hospital gate (about 55 dB)","Even ordinary conversation can exceed the strict 50 dB silence-zone limit.",{"id":672,"label":673,"bin":658,"why":674},"loud-tv","A loud television heard through a house wall (about 60 dB)","Above the 55 dB residential limit but comfortably under 65 dB.",{"id":676,"label":677,"bin":661,"why":678},"market","A crowded, noisy market street (about 70 dB)","Above the 65 dB commercial limit but still under the 75 dB industrial one.",{"id":680,"label":681,"bin":664,"why":682},"factory-floor","Inside a working factory (about 72 dB)","Loud, but within the higher 75 dB limit set for industrial areas, which are expected to be noisier.","Sort four everyday sounds by which of India's four noise-zone daytime limits each one would exceed.","A sorting game with four cards and four bins, one for each CPCB zone's daytime limit: silence (50 dB), residential (55 dB), commercial (65 dB) and industrial (75 dB).\n\nQuiet conversation at 55 dB already exceeds a silence zone's 50 dB limit. A loud television at 60 dB exceeds the residential limit of 55 dB. A noisy market at 70 dB exceeds the commercial limit of 65 dB. A factory at 72 dB stays inside the industrial limit of 75 dB.\n\nThe lesson: the *same* sound level can be perfectly legal in one zone and a breach in another, because the rules recognise that hospitals and homes need more quiet than markets and factories do.",{"id":686,"type":53,"title":687,"eyebrow":688,"navLabel":689},"i-ch09","Check what you know","Chapter 09","9 Check yourself",{"id":691,"type":692,"title":693,"terms":694},"i-glossary","glossary","Investigate words to keep",[695,699,703,707,711,715,719,723,727,731],{"term":696,"meaning":697,"example":698},"Natural frequency","The rate an object vibrates at when disturbed and left alone.","A swing's natural frequency depends on the length of its chains.",{"term":700,"meaning":701,"example":702},"Resonance","A large vibration built up by repeated small pushes at (or near) an object's natural frequency.","A pushed swing climbing higher and higher.",{"term":704,"meaning":705,"example":706},"Sympathetic string","An unplayed string that vibrates by resonance when a nearby string or note matches its tuning.","The tarab strings under a sitar's neck.",{"term":708,"meaning":709,"example":710},"Reverberation","The lingering, blurred tail of sound in a room caused by many overlapping reflections.","A clap in an empty hall.",{"term":712,"meaning":713,"example":714},"Reverberation time (RT60)","How many seconds sound takes to fade by 60 decibels after the source stops.","A bare classroom: about 0.8 s.",{"term":716,"meaning":717,"example":718},"Sonar","SOund Navigation And Ranging: finding distance underwater by timing an echo.","A fishing boat's depth sounder.",{"term":720,"meaning":721,"example":722},"Ultrasound (medical)","Sound above hearing, used with timed echoes to build a picture inside the body.","A scan before a baby is born.",{"term":724,"meaning":725,"example":726},"Echolocation","Using self-made sound and its returning echo to sense surroundings.","Bats in the dark, dolphins in murky water.",{"term":728,"meaning":729,"example":730},"Diaphragm","A thin, flexible surface that moves with arriving sound, or that pushes air to make sound.","Inside a microphone and inside an eardrum.",{"term":732,"meaning":733,"example":734},"Decibel combination","Combining two sound levels by adding their energies, not their decibel numbers directly.","Two 70 dB sources together read about 73 dB, not 140 dB.",{"id":736,"type":737,"title":738,"questions":739},"i-quiz","quiz","Ten questions on resonance, echoes and everyday sound technology",[740,753,766,779,792,805,818,831,844,857],{"itemId":741,"prompt":742,"options":743,"correct":83,"why":752},"sound.investigate-q-resonance","Resonance happens when an object is pushed",[744,746,748,750],{"id":80,"label":745},"as hard as possible, regardless of timing",{"id":83,"label":747},"repeatedly at or near its own natural frequency",{"id":86,"label":749},"only once, very hard",{"id":89,"label":751},"at a frequency far from its own","Resonance needs repeated pushes in step with the object's own natural frequency, so each push adds to the last.",{"itemId":754,"prompt":755,"options":756,"correct":83,"why":765},"sound.investigate-q-glass-water","Adding water to a singing wine glass makes its rubbed note",[757,759,761,763],{"id":80,"label":758},"higher",{"id":83,"label":760},"lower",{"id":86,"label":762},"unchanged",{"id":89,"label":764},"silent","The added water adds mass to the vibrating system without adding much stiffness, lowering its natural frequency, exactly like a jal tarang bowl.",{"itemId":767,"prompt":768,"options":769,"correct":83,"why":778},"sound.investigate-q-sitar","A sitar's sympathetic strings vibrate mainly because of",[770,772,774,776],{"id":80,"label":771},"the player's fingers touching them",{"id":83,"label":773},"resonance with a matching note being played",{"id":86,"label":775},"the wind",{"id":89,"label":777},"electricity in the strings","Each sympathetic string is tuned to a particular note. It rings by resonance only when that note is sounded nearby.",{"itemId":780,"prompt":781,"options":782,"correct":83,"why":791},"sound.investigate-q-reverb","Reverberation is best described as",[783,785,787,789],{"id":80,"label":784},"one clean echo from a distant wall",{"id":83,"label":786},"many overlapping reflections blurring into a fading tail",{"id":86,"label":788},"sound getting higher in pitch",{"id":89,"label":790},"sound travelling through a vacuum","A single distinct echo needs one distant, isolated surface. Reverberation is many close reflections overlapping.",{"itemId":793,"prompt":794,"options":795,"correct":83,"why":804},"sound.investigate-q-rt-audience","A concert hall's reverberation time when full of an audience, compared with empty, is usually",[796,798,800,802],{"id":80,"label":797},"longer, because people talk",{"id":83,"label":799},"shorter, because people and clothing absorb sound",{"id":86,"label":801},"exactly the same",{"id":89,"label":803},"impossible to measure","Bodies and fabric absorb sound well, so a full hall has more absorption and a shorter reverberation time than an empty one.",{"itemId":806,"prompt":807,"options":808,"correct":83,"why":817},"sound.investigate-q-sonar-depth","A ship's sonar pulse returns after 4.0 seconds in sea water at 1,500 m\u002Fs. The depth is closest to",[809,811,813,815],{"id":80,"label":810},"1,500 m",{"id":83,"label":812},"3,000 m",{"id":86,"label":814},"6,000 m",{"id":89,"label":816},"750 m","Round-trip distance = 1,500 × 4.0 = 6,000 m. The depth is half of that: 3,000 m.",{"itemId":819,"prompt":820,"options":821,"correct":83,"why":830},"sound.investigate-q-ultrasound-freq","A doctor needs to scan a shallow structure, such as an eye, in fine detail. They should choose",[822,824,826,828],{"id":80,"label":823},"a lower ultrasound frequency, for reach",{"id":83,"label":825},"a higher ultrasound frequency, for a shorter wavelength",{"id":86,"label":827},"ordinary audible sound",{"id":89,"label":829},"an X-ray instead","A higher frequency gives a shorter wavelength and finer detail, and reach is not a problem for a shallow target.",{"itemId":832,"prompt":833,"options":834,"correct":83,"why":843},"sound.investigate-q-dolphin","A dolphin's echolocation call in water has a longer wavelength than a bat's call in air, even though the dolphin's frequency is higher, mainly because",[835,837,839,841],{"id":80,"label":836},"the dolphin is a bigger animal",{"id":83,"label":838},"sound travels much faster in water than in air",{"id":86,"label":840},"water absorbs high frequencies",{"id":89,"label":842},"dolphins do not really echolocate","Wavelength = speed ÷ frequency. Sound is over four times faster in water, so even a higher frequency there can give a longer wavelength.",{"itemId":845,"prompt":846,"options":847,"correct":83,"why":856},"sound.investigate-q-mic","A basic dynamic microphone turns sound into an electric current using",[848,850,852,854],{"id":80,"label":849},"a battery inside the microphone",{"id":83,"label":851},"a coil moving through a magnet's field, induced by the moving diaphragm",{"id":86,"label":853},"sunlight falling on the diaphragm",{"id":89,"label":855},"heat from the voice","Arriving sound moves a diaphragm, which moves an attached coil through a magnetic field, inducing a current, the same principle used in a generator.",{"itemId":858,"prompt":859,"options":860,"correct":80,"why":868},"sound.investigate-q-combine","Two identical machines, each 80 dB alone, run together. The combined level is closest to",[861,863,865,867],{"id":80,"label":862},"83 dB",{"id":83,"label":864},"160 dB",{"id":86,"label":866},"80 dB, no change",{"id":89,"label":602},"Two identical sources add about 3 dB, because decibels combine by energy, not by simple addition: 80 + 3 = 83 dB.",{"id":870,"type":871,"prompt":872},"i-reflect","reflection","Pick one everyday object (a cupboard door, a metal railing, a plastic ruler, a steel bucket) and predict what would happen if you tapped it repeatedly at different, steady rhythms. Try it if you can, and describe whether you found a rhythm where the object rang more, or vibrated more, than at other rhythms.",{"id":874,"type":875,"title":876,"points":877},"i-cheat","summary","Cheat sheet",[878,879,880,881,882,883,884,885,886],"**Resonance:** repeated pushes at (or near) an object's own natural frequency build a large vibration; pushes at other rates do not add up.","**Sympathetic strings** (sitar, sarangi) ring by resonance only when a nearby note matches their own tuning; nobody touches them.","**Reverberation** is many overlapping reflections, unlike a single clean echo; reverberation time (RT60) shortens as a room's absorption rises: T = 0.161 × V ÷ A.","**Sonar** times an echo in water (about 1,500 m\u002Fs) to find depth: depth = speed × time ÷ 2.","**Medical ultrasound** uses the same echo idea in soft tissue (about 1,540 m\u002Fs); higher frequency gives finer detail but less reach.","**Bats (air) and dolphins (water)** both echolocate with high-frequency calls; the medium's speed changes the wavelength for a given frequency.","**A microphone and a loudspeaker** are close cousins: a coil and a magnet, run in opposite directions, the same idea as a generator.","**Decibels combine by energy, not by simple addition.** Two identical sources add about 3 dB; a much quieter source barely changes a louder total.","**India's noise-zone limits** run from 50\u002F40 dB (silence, day\u002Fnight) up to 75\u002F70 dB (industrial); firecrackers are capped at 125 dB(AI) at 4 m.",{"id":888,"type":889,"sourceIds":890},"i-sources","sources",[891,892,893,894,895,896,897,898,899,900,901],"sound-ncert-class9-sound","sound-hyperphysics-standing-waves","sound-hyperphysics-speed","sound-noaa-sonar","sound-nidcd-noise","sound-cpcb-noise","sound-britannica-loudspeaker","sound-physicsclassroom-sound","sound-wikipedia-instruments","sound-wikipedia-challenger-deep","sound-who-listening-safe",[891,892,893,894,895,896,897,898,899,900,901],"needs_review",{"generatedBy":905,"notes":906},"claude-code","Draft generated locally; pending owner review. Every reverberation, sonar, ultrasound and decibel-combination number computed and asserted in scratchpad\u002Fsound\u002Fnumbers.py.","b7e635421960a17bc1e92ffa8989d936d9667ce445e57a3f7fe43e9b422bb7d3",{"component:sort-game@1":909,"component:match-pairs@1":910,"component:sound-speed@1":911,"logic:practice":912,"component:process-steps@1":913,"source:sound-britannica-loudspeaker":914,"source:sound-cpcb-noise":915,"source:sound-hyperphysics-speed":916,"source:sound-hyperphysics-standing-waves":917,"source:sound-ncert-class9-sound":918,"source:sound-nidcd-noise":919,"source:sound-noaa-sonar":920,"source:sound-physicsclassroom-sound":921,"source:sound-who-listening-safe":922,"source:sound-wikipedia-challenger-deep":923,"source:sound-wikipedia-instruments":924},"b164f45a2c8ca08f26c450768ff0231e113e9fe45381eddb34dc6d0548596c38","2a8ee4ac87460b4e1175a4bb13c96b03d577db06dde95670eb7fcfe4ad787899","22b6738152933cfccb190e600f1f387e7d7c3b1b36f37184363a3cd5f8dae8b6","3d6b0fe1b15255975a32b0fcd94e8019bc959ad45cbf12e136e86149549c6878","c2f918c426383c50d52939054780add1488f3f282c9d1a965c3a38345ddcb265","cafa69169bb48e537fa9ebdb5c9ff74885ea0add07f5bd93fb4ba60885f4f640","278297753fb9417bb45ce52bcbba4e42ccba48426815813568968b0ffaa42b34","4cb248d8beea032b112212f11d2a3d10751a3a68da1b26d2ef93b11835467932","aceb27d2545768b5ef61862c7c358c3c7c317288d7963dbd8712f8acc8af3529","615e6ca7b7252ccb2523eea00746ef69215849397defe43c853a9fcb617799dd","6c2548bc22d1e37e553f1f05a7b1c919c097e49ad319523b186e766828b576cd","89db468fdabca49aa3be9a157e9e0767aa5bb8e500bc451e3b1137895f11bb97","967196792bc122ee73ed66cefbf9d62ac069b2c688af723b4f4eb52fcdd8174a","66f6253ec4bd1db55054078b190fd82fc95ce974ec8c7588f5ab1227591d0cfc","806a62cb4a75c848e81a2531f00be631e3ef3b94d4be235d02b8eb34b8dc72a0","0b24a438b7c5bb6e98dad1043334a28d32ba3aa4ef1eca60660933b537c66d0a",{"state":926,"reviewer":927,"selfReview":928,"reviewedAt":929,"method":930},"approved","The library owner",true,"2026-09-20T10:18:37.581Z","owner_bulk","preview-7e1cbbcc4f",1789899598813]