|
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 out two ears to judge
the direction a sound came from.
To try this, you will need:
-
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 if
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.
Back to Experiment List
|
