To demonstrate focusing of sound by refraction in a sound lens.
Oscilloscope, oscillator, loudspeaker, microphone, and balloon with carbon dioxide.
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 used.
Haym Kruglak and Charles C. Kruse, A Visual Method For Demonstrating Refraction of Sound, AJP 8, 260-261, (1940).