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This Week's Experiment - #325 Hearing Directions
In the past, we have done experiments to show how we use our two eyes to
judge distances. This week we will see how we use our two ears to judge the
direction a sound came from. To try this, you will need:
5 or 6 friends
Find a nice, large area to work in. Stand or sit in the center of the area.
Have your friends spread out in a circle around you, so that each is about
10 feet from you. Close your eyes. Then have your friends take turns
making sounds. Whenever you hear a sound, point in the direction it came
from. Pretty easy, right?
Now comes the tricky part. Try the same thing again, but this time use a
finger to plug one of your ears. It will not block all the sound, but it
should block enough for our purposes. This time you will find it much
harder to tell exactly where each sound came from.
Why? Two different things are working together to let you decide where the
sound came from, and they both require two ears. When one of your friends
makes a sound, the sound waves spread out in all directions. The vibrations
of these sound waves reach your ears and shake your ear drums. That
vibrates the tiny bones inside your ear. They shake your inner ear, which
converts the vibrations into a signal that your brain can understand. Your
brain gets a signal from each ear. Then the fun begins.
Your brain compares the signals from both ears. The ear that is closest to
the sound will hear the sound slightly louder than the other ear, and produce
a stronger signal. That is the first clue to the direction the sound came
from.
The other clue has to do with the speed of sound. Although it travels very
fast, it is not instant. The sound reaches the nearest ear a few millionths
of a second before it reaches the other ear. While this difference is tiny,
your brain is up to the task. It can recognize the difference in the
timing, getting its second clue for the sound's direction.
Putting these two things together, at speeds that would shame a super
computer, your brain tells you where the sound came from. When you have
only one ear to work with, your brain gets only one signal. There is
nothing to compare it with, so you can't tell where the sound came from,
unless you add in other information. That is why you close your eyes. If
you leave them open, then your brain adds in any clues that it gets from your
eyes to help decide where the sound came from. Right now, the sound I hear
is our cat demanding her bedtime snack, so I better finish this before she
decides to walk on the keyboard to get my attention.
Have a wonderful week.
These experiments are from Robert Krampf - The Happy Scientist
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