Purpose:Demonstrate focusing of sound by refraction in a sound lens
Description:A balloon filled with carbon dioxide acts as a focusing sound lens, due to its convex shape and the smaller velocity of sound in the carbon dioxide. When the lens is inserted between the loudspeaker and the microphone, the sound wave is focused, increasing the sound level at the microphone, as seen on the oscilloscope. The source is either a small chunk of dry ice in a flask or a cylinder of carbon dioxide.
For comparison, air (very little focusing) and helium (defocusing) balloons can also be provided upon request.
For good results, position the microphone and the loudspeaker about 40 cm apart, inflate the balloon to about 20 cm diameter, and use a frequency of about 2-4 kilohertz.
If you use additional balloons of different gases, as mentioned above, have students make predictions about what effect density will have before showing the result.