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What's going on when I hear a sound?
You hear sounds when vibrations get inside your ears and stimulate your
nerves to send electrical signals to your brain.
Suppose, for instance, that you are pounding on a drum. The drumhead
starts vibrating. As the drumhead vibrates, it bumps into air molecules
and starts them bouncing to and fro. Those bouncing air molecules bump into
other air molecules and start them moving. This chain reaction of moving
air molecules carries sound through the air in a series of pulsating pressure
waves that we call sound.
Sound waves carry vibrations from the drum into your ears. Inside your
ear, moving air molecules push on your eardrum and start it vibrating. Your
eardrum, in turn, pushes on the bones of your middle ear, the tiniest bones
in your body. These bones act like a set of levers, pushing against the
thin membrane that covers the opening to your inner ear.
The movement of this membrane makes pressure waves in the fluid inside the
cochlea, where cells with tiny sensing hairs transform the waves into electrical
signals. These electrical signals travel along the auditory nerve to your
brain. When these electrical signals reach your brain, you hear a sound-the
beat of a drum.
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Why can you hear the music of the Head Harp only when the string is
around your head? How do the Secret Bells work?
When you pluck on the string that's wrapped around your friend's head,
the string starts vibrating. To reach your ears, the vibrations in the string
must push on the air molecules to make sound waves that travel through the
air. But the string isn't very large and it doesn't push on very many air
molecules. So sound vibrations don't travel easily from the string into
the air.
When the string is around your own head, the sound can take a more direct
route to your ears. Rather than traveling through the air, the vibrations
can travel through your hands and through the bone of your skull directly
to the fluid inside your cochlea in your inner ear. Instead of traveling
from solid to air and back to solid, the vibrations move from one solid
(the string) to another (your bones), and then into the fluid of your cochlea.
As a result, the sound you hear is much louder and richer.
The same thing happens with Secret Bells. When you put your hands over
your ears, you provide a path that lets more of the vibrations reach your
ears. When your hands aren't over your ears, you hear a faint, high-pitched,
tinny sound. When you put your hands over your ears, you hear deep, resonant,
bell-like tones. The hanger makes the same sound in both situations, but
in one you provide a path that lets more of the sound reach your ears.
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With your scissors, cut a piece of string about
3 feet long. (Grown-ups should cut a piece about 4 feet long.)
Hold the two ends of the string in one hand.
The rest of the string will make a loop.
Lay the loop over the hook part of the hanger.
Push the two ends through the loop, and pull them all the way through the
other side. (This is easier to undo than a knot.)
Wrap the loose ends of the string
two or three times around the first fingers on each hand.
Swing the hanger so it gently bumps against the
leg of a table, or against a door. What did it sound like? Probably not
much.
Now put your hands over the openings
of your ears. (Don't put your fingers in your ears!) Hold your hands tight
to the sides of your head. Lean over and gently bump the hanger again.
Wow! Now what does it sound like? Church bells?
Chimes?
Want to hear what a spoon sounds like? Unwrap
your fingers, then pull on the loop end of the string. The whole string
will come off the hanger, and you can reloop it around the spoon.
