PURPOSE: To 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 used.

SUGGESTIONS: 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.

See Question of the Week #141 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 3B35.10)

EQUIPMENT: Oscilloscope, oscillator, loudspeaker, microphone, and balloon with carbon dioxide.

SETUP TIME: 10 min.