The Fairy-Land of Science · Lecture VI, part 2 · Part 6: Lecture VI: The voices of nature and how we hear them
17. Sound waves, echoes, and the ear
Goal: Your child can describe a sound wave as a traveling push of crowded air, explain an echo, and name the main parts of the ear.
You'll need
- A Slinky or other spring toy (if you have one)
- A large wall outdoors, or a big empty room
- A hand mirror
- Science notebook and pencils
What's in the book
Pages 130 to 139. Air is springy, so a sound wave is a crowding and spreading of air passed along, like railway trucks bumping; sounds spread out on all sides and grow fainter; sound bounces back from walls, making echoes; a dog fooled by an echo; and a careful tour of the ear: the shell of the ear, the canal, the eardrum, the three tiny bones, and the coiled part inside.
The lesson, step by step
Hook question
SayWhen you shout in a big empty room or near a cliff, sometimes the sound comes back to you. Why?
Let your child guess.
Read together
Read page 130.
You may see an excellent example of this in a luggage train in a railway station, when the trucks are left to bump each other till they stop.
If you have a Slinky, stretch it along the floor between you. Push one end sharply forward once: watch a bunch of crowded coils travel to the other end. That is what a sound wave in the air is like.
Spreading and bouncing
Read page 132 and page 134. A clap sends waves out on all sides, like rings on a pond, so a sound gets fainter farther away. When the waves hit a wall far enough away, they bounce back:
and here you have an echo, "Ha, ha."
Tell Buckley's story of her dog barking at its own echo.
Try it: stand well back from a big wall (Buckley says at least 56 feet), clap once sharply and listen for the echo.
Your wonderful ear
Read page 136.
Put your finger round your ear and feel how the gristly part is curved towards the front of your head.
Look at your ears in the mirror. Look at the drawing of the ear (Fig. 33) together and trace the path: the shell of the ear catches the sound, it travels down the canal, shakes the thin eardrum, which moves three tiny bones (Buckley calls them the hammer, the anvil and the stirrup), which knock on a little window into a coiled tube full of liquid and fine hairs, and a nerve carries the message to the brain.
Try it: a bigger ear
Cup your hands behind your ears, like a big shell, and listen to something quiet. Now take them away. Buckley says this is what a person hard of hearing does to catch the sound.
Safety: never poke anything into your ear, and never hit anyone on the ear. Buckley warns that a blow can hurt the eardrum.
Tell it back and check
Your child traces a sound from a clap to the brain, in order. Then ask:
1. Why does a sound get quieter far away?
2. What is an echo?
3. Name the three tiny bones in the ear.
What we know now
Buckley says there are about three thousand fine threads in the coiled part of the ear. Today we know it holds thousands of tiny hair cells, each tuned to its own pitch, rather like the strings of a harp, just as she describes. Loud noise can harm them, so keep music and headphones at a gentle volume.
Afterwards
Notice it this week: listen for echoes in a tunnel, a stairwell, a gym or under a bridge. Swallow with your mouth shut and listen for the little click Buckley mentions.
Tip
Your child doesn't need to remember every part of the ear. The path (shell, canal, drum, three little bones, coiled tube, nerve, brain) is plenty.
Answers to the book's problems
1. The wave spreads out on all sides and has more and more air to move, so it gets weaker. 2. Sound that bounces back from a wall or cliff far enough away that you hear it a second time. 3. The hammer, the anvil and the stirrup.
Worked out for this site from the scan. If a number in your copy differs, trust the book.