Sound Localization: Judging Sound Direction

Materials

• Grassy field
• Lots of string
• Protractor
• Nails or wire stakes
• Plastic freezer bag
• Flour
• Blindfold
• Jingle bell
• Tape measure
• 2 or more friends

Procedure

1. Can your friends determine the direction of a sound? How well can they pinpoint the direction that a sound is coming from? Create a hypothesis, your best guess about what is going to happen. What directions are the easiest to identify? Which ones are the hardest?
2. First, you’ll create a semicircle so that you can make sounds in different places. To build your circle, put a nail in the ground and tie one end of a spool of string to the nail. Measure out 30 meters of string and pull the string taut. Put another nail in the ground and tie the other end of your string to the nail. This length of string will act as the diameter of your semicircle.
3. Walk along the string until you get to the middle. Put a nail at that point, and tie more string to that nail. The new string should be 15 meters long. Walk to one end of the diameter, holding the new string. Start walking in a curved path, holding the string taut. As you walk, you or a friend can hold a plastic freezer bag of flour with the corner clipped off and use it to mark a semicircle on the grass.

4. Now it’s time to make some noise! Ask one of your friends to stand at the midpoint, the place where you put the second nail. Put a blindfold on your friend so that he will have to use only his ears to pinpoint the direction of the sound.
5. Measure out another 15-meter length of string for yourself, and give your 2nd friend his own 15-meter length of string and a jingle bell. Tie the ends of these strings to the nail close to where the blindfolded subject is standing. You and your friend should stand at the edge of the circle, holding your strings to either side of your blindfolded friend.

6. Have your friend with the jingle bell quietly move along the edge of the circle, stop, and jingle the bell. Ask your blindfolded test subject to point to the place where he heard the bell. Walk around the edge of the circle to move your string so that it lines up with the place your blindfolded friend is pointing. Ask your blindfolded friend to take his blindfold off and measure the angle that each string makes with the semicircle’s diameter. Place the results into a table:

7. Do five trials for each blindfolded test subject, then ask your friends to switch jobs.
8. How accurate were your friends’ guesses? Was one person more accurate than the others, or were they all similar? What directions were most difficult for your friends to guess?

Results

Surprisingly, you and your friends probably will have had a harder time determining the precise direction of sounds that happened in front of them!

Why?

How do people tell where a sound is coming from? Different animals have heads and ears that are capable of capturing sound in different ways. Owls have flat facial disks that act like satellite dishes, capturing sound. Many bats have large pinnae (“ears”) that collect sounds.

People have pinnae too—but we call them ears. The part of the ear that processes sound is actually on the inside of your skull, and the things we call ears are our sound-collectors, or pinnae.

Our pinnae sit on the sides of the head. This means that it’s easy to hear sounds that are coming directly from the left or the right. When a sound is above us, below us, or behind us, it can be harder to pinpoint exactly where the sound is coming from. However, we make up for this weakness with our eyes! Humans usually use our eyes to detect things that are right in front of us, because our eyes point forward.

We can still accomplish sound localization and determine where a sound is coming from even when that sound isn’t directly to the left or the right of us. A sound that comes from behind us and to the right moves into our ear pinnae and gets amplified in a certain way—meaning the sound is louder in one of our ears than it is in the other. Then, the brain takes that information and decodes it. Over time, we will have heard many sounds from this direction, and we get trained to realize that the particular amplification pattern this direction produces means that this sound comes from behind us and to the right.

A sound that comes directly from the back of your head is hard to figure out because it doesn’t move as easily into your ear pinnae. It might echo off something in front of you, and that could help. Usually people turn their head slightly when they hear a sound behind them, and this helps their ears capture the sound and helps their brain decode it.