--- title: "Sound Around Us" book: "Primary & Middle School Physics" subject: physics language: en chapter: 19 exercises: 11 source: https://one-course.com/books/physics/1/en/chapter/19-sound-around-us --- # Chapter 19 — Sound Around Us Hum your favorite song — and while you hum, lay two fingers gently on the front of your throat. Feel that trembling? You have just touched a [sound](#def-g3-sound-around-us-sound) at its birthplace. Every voice, every drum, every squeaky door makes its [sound](#def-g3-sound-around-us-sound) the very same way, and this chapter goes hunting for the trick. ## 19.1 Where sounds are born **Definition 19.1 (Vibration).** A *vibration* is a very fast trembling — a back-and-forth movement repeated many times each second, often too fast for the eye to follow. A plucked guitar string vibrates: you see only a blur, but your fingertip feels the buzzing. **Definition 19.2 (Sound).** A *sound* is what we hear when something vibrates and its trembling travels through the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) to our ears. The vibrating thing is the sound’s *source* — a string, a drum skin, a bell, your own throat. **Method 19.3 (Making rice dance).** To *see* a [sound](#def-g3-sound-around-us-sound) being born: 1. scatter a few grains of rice on the skin of a drum (or on a tin’s plastic lid, or a stretched balloon skin); 2. tap the skin near the grains — boom: the grains jump! 3. now only *shout* at the skin from very close: the grains tremble again, without being touched. The skin vibrates whenever it [sounds](#def-g3-sound-around-us-sound) — and even a [sound](#def-g3-sound-around-us-sound) arriving from your mouth sets it trembling. ![A real drum for the rice experiment: the white skin is the part that trembles. Scatter a few grains on it, tap near them — a sounding skin is a trembling skin, and the rice dances to prove it. Photo: Vladimir Morozov, CC BY 2.0.](https://one-course.com/images/onecourse/chapters/physics-1/g3-sound-around-us/img-ef3708c3dbca.jpg) *A real drum for the rice experiment: the white skin is the part that trembles. Scatter a few grains on it, tap near them — a sounding skin is a trembling skin, and the rice dances to prove it. *Photo: Vladimir Morozov, CC BY 2.0.** **Proposition 19.4 (No vibration, no sound).** Every [sound](#def-g3-sound-around-us-sound) comes from a [vibration](#def-g3-sound-around-us-vibration) — and when the [vibration](#def-g3-sound-around-us-vibration) stops, the [sound](#def-g3-sound-around-us-sound) stops with it, instantly. Strike a cymbal and let it ring; then grab it firmly with both hands: silence, the moment your grip stops the trembling. **Example 19.5 (Sound sources everywhere).** Hunt the [vibration](#def-g3-sound-around-us-vibration) behind each [sound](#def-g3-sound-around-us-sound): the guitar — plucked strings tremble; the drum — the struck skin trembles; your voice — two little folds in your throat tremble as [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) from your lungs rushes past (that is the buzz your fingers felt); the buzzing fly — its wings tremble hundreds of times a second; the slammed door — the whole door and its frame shudder for a moment. ## 19.2 Loud and quiet, high and low **Example 19.6 (Loud is a big trembling).** Pluck a stretched rubber band gently: a small blur, a quiet twang. Pluck it hard: a wide blur, a loud twang. The harder the source trembles — the wider its back-and-forth — the *louder* the [sound](#def-g3-sound-around-us-sound). Watch the rice grains too: a gentle tap makes them shiver, a mighty boom throws them in the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air). **Example 19.7 (High and low).** Stretch a rubber band over an open box and pluck; then pinch it in the middle and pluck the short half: the twang turns *higher*, like a bird’s chirp. Short, tight, thin things tremble faster and sing high; long, loose, thick things tremble slower and sing low. That is why the little flute chirps and the big tuba growls — and why the guitar’s thinnest string sings highest. ![The rubber-band guitar: the whole band swings widest at its middle, anchored at its two ends, and sings low; the pinched, shorter half trembles faster and sings higher.](https://one-course.com/images/onecourse/chapters/physics-1/g3-sound-around-us/fig-4ab7747169e6.svg) *The rubber-band guitar: the whole band swings widest at its middle, anchored at its two ends, and sings low; the pinched, shorter half trembles faster and sings higher.* **Remark 19.8 (Protect the little drum in your ear).** Deep in your ear, a tiny skin — thinner than paper — trembles with every arriving [sound](#def-g3-sound-around-us-sound), just like the rice-grain drum. [Sounds](#def-g3-sound-around-us-sound) that are too loud shake it too hard: very loud music, machines and fireworks can damage it for good, and it does not grow back. Cover your ears near loud noise, and never shout into anyone’s ear — ears have no eyelids. ## 19.3 From the source to the ear **Example 19.9 (The journey of a clap).** A friend claps across the playground. The clap sets the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) trembling; the trembling spreads out through the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) in every direction, like the rings around a pebble dropped in a pond; some of it reaches your ear and sets your ear’s little skin trembling — and you hear the clap. Source, journey, ear: every [sound](#def-g3-sound-around-us-sound) you have ever heard made this trip. How fast is the journey, and what can it travel through? Next year’s [sound](#def-g3-sound-around-us-sound) chapter takes exactly those questions. ## 19.4 Exercises **Exercise 19.1 ★.** What is a [vibration](#def-g3-sound-around-us-vibration)? Name the vibrating part in: a guitar; a drum; your voice; a buzzing fly. **Solution of Exercise 19.1.** A very fast back-and-forth trembling. Guitar: the plucked string. Drum: the struck skin. Voice: the two little folds in the throat. Fly: its beating wings. **Exercise 19.2 ★.** In the rice experiment, what do the jumping grains prove about a sounding drum skin? **Solution of Exercise 19.2.** That a sounding skin is a trembling skin: the grains jump because the skin under them moves rapidly up and down — even when only a shout, not a touch, sets it going. **Exercise 19.3 ★.** Why does grabbing a ringing cymbal silence it instantly? Which law of the chapter is at work? **Solution of Exercise 19.3.** The grip stops the cymbal’s trembling, and no [vibration](#def-g3-sound-around-us-vibration) means no [sound](#def-g3-sound-around-us-sound) — the law “no [vibration](#def-g3-sound-around-us-vibration), no [sound](#def-g3-sound-around-us-sound)”. **Exercise 19.4 ★.** To make the rubber band twang *louder*, what do you change about your pluck? What does the blur of the band look like then? **Solution of Exercise 19.4.** Pluck harder: the band swings wider — a wider blur — and the twang is louder. Bigger trembling, bigger [sound](#def-g3-sound-around-us-sound). **Exercise 19.5 ★.** Which sings higher: the short pinched band or the whole band? The little flute or the big tuba? The guitar’s thinnest or thickest string? **Solution of Exercise 19.5.** The short pinched band; the little flute; the thinnest string. Short, small, thin things tremble faster and sing higher. **Exercise 19.6 ★.** Hum with your fingers on your throat, then stop humming. What do your fingers feel, and exactly when does the feeling stop? **Solution of Exercise 19.6.** They feel a buzzing — the throat’s folds vibrating. The buzzing stops the instant the humming stops: no [vibration](#def-g3-sound-around-us-vibration), no [sound](#def-g3-sound-around-us-sound). **Exercise 19.7 ★.** Tell the journey of a clap from your friend’s hands to your ear, in three steps. **Solution of Exercise 19.7.** The clapping hands set the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) trembling; the trembling spreads through the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) like pond rings; it reaches the ear and sets the ear’s little skin trembling — and the clap is heard. **Exercise 19.8 ★.** Why must very loud [sounds](#def-g3-sound-around-us-sound) be kept away from ears? What tiny thing inside the ear is in danger, and what does it resemble from this chapter? **Solution of Exercise 19.8.** Very loud [sounds](#def-g3-sound-around-us-sound) shake the ear’s tiny skin — the ear’s own rice-grain drum, thinner than paper — hard enough to damage it for good; it never grows back. **Exercise 19.9 ★.** A music box’s comb has teeth of different lengths. Which teeth play the high notes and which the low ones? Answer with the rule of [Example 19.7](#ex-g3-sound-around-us-pitch). **Solution of Exercise 19.9.** The short teeth play the high notes, the long teeth the low ones: short things tremble faster and sing higher. **Exercise 19.10 ★★.** Pour water into a glass bottle and blow across its mouth: a hoot! Add more water and blow again: the hoot turns higher. What is trembling to make the hoot — the glass, the water, or the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) inside? Use the short-sings-high rule to defend your answer. **Solution of Exercise 19.10.** The [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) inside the bottle is the trembler — your breath sets it shivering. Adding water leaves a *shorter* column of [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) above the water, and short sings high: that is why the hoot rises. If the glass itself were the singer, adding water would hardly change the note. **Exercise 19.11 ★★.** Maya says: “When the television is on mute, the screen still moves — so movement alone must make [sound](#def-g3-sound-around-us-sound).” Use [Definition 19.2](#def-g3-sound-around-us-sound) and the rice experiment to explain what kind of movement makes [sound](#def-g3-sound-around-us-sound), and why the pictures on a screen make none. **Solution of Exercise 19.11.** [Sound](#def-g3-sound-around-us-sound) needs a *[vibration](#def-g3-sound-around-us-vibration)*: a to-and-fro trembling fast enough to shake the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air), like the drum skin that made rice dance. The pictures on a muted screen change, but nothing there trembles the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) — changing colors shake nothing, so there is nothing for an ear to catch.