Have one person stand in place and hold the balloon up to his or her ear. Have a second person stand on the other side of the balloon, about 3 feet (1 meter) away from the center of the balloon, and make noise: encourage him or her to talk, sing, click the clicker, or use some other noisemaker to produce sounds. The first person should then move the balloon around to find the position in which the sound is loudest.
The point where the sound is the loudest should be on a straight line from the sound source through the center of the balloon to about 18 in (46 cm) away from the center. The exact distance will depend on the size of your balloon and the position of the sound source. When you find the loudest location, remove the balloon and listen to the noise again. Notice that the sound is now quieter.
What happens if you change where you’re standing? Have the noisy person move to another position and have the first person find the loudest point again. Notice that the loud point moves in a direction opposite to the motion of the sound source. When the sound moves up, the loud point moves down. When the sound moves closer, the loud point moves farther away. (Note that if you move the sound source too close, there will be no loud point.)
Try moving farther and farther away. Find the loudest point for a distant sound (one that’s more than 10 feet [3 meters] away). The distance from the center of the balloon to this point is the focal length of the balloon.