sound lens

 
Sound travels slower through carbon dioxide than it does through air. This fact can be used to make a lens to concentrate sound just as a glass lens can concentrate light.
 
Put about 8 ounces of vinegar or lemon juice in a soda bottle. Add about a tablespoon of baking soda. Quickly slip the mouth of a balloon over the bottle top. When the balloon is full tie it closed. Hold the balloon between your ear and some source of low volume sound such as a ticking watch. Move the balloon nearer and farther away until the sound is loudest.
 
The diagram shows short sections of sound waves traveling toward the balloon. When the end of a section enters the balloon, that part is slowed while the part outside the balloon continues at normal speed. The effect is similar to a line of people walking hand in hand toward the water at the beach. The line of people is traveling at an angle to the shoreline. As each person enters the water they are slowed down and the line is bent. The people in the water are then facing in a different direction and therefore travel in that new direction. Similarily the direction of travel of the sound waves changes as they enter the CO2.
 
Sound striking the balloon straight on is bent slightly. Sound striking the edges of the balloon is bent most. The sound is bent again as it leaves the balloon. The overall effect is to aim the sound toward one area.
 
The room must be very quiet. Although the shape of the balloon is not ideal, it will noticably increase the apparent loudness of the sound. If you need more time to put the balloon on the bottle, first put a tablespoon of water in the bottle. Then add the baking soda and put the bottle in a freezer until the water freezes. If you are using a 2 liter or larger bottle, do the following. After adding the vinegar, pour in enough water to nearly fill the bottle. That causes more of the CO2 to go into the balloon.