Science | Science Experiment of the Week 12 | 325 - Hearing Directions

Sound Waves, Vibrations, Ear Drums, Signal - 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.

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This Week's Experiment - #325 Hearing Directions

These experiments are from Robert Krampf - The Happy Scientist

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.

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