Demos: 4C-02 Doppler Effect II

A loudspeaker suspended from a long rod is swung back and forth near a flat reflecting surface. By reflection from the wall, one has effectively a second (virtual) source moving in a direction opposite to that of the speaker (source). The resulting beat frequency is twice that which one would observe with one stationary and one moving source.

Directions: Turn on the amplifier to get the sound source started. Pull the speaker back and let it go.

Suggestions for Presentation: Suggest to the students that, like a mirror in optics, the reflected sound appears to be coming from a virtual source equidistant on the other side of the wall. If the real source is moving toward the wall, the virtual source is also. Thus we have the equivalence of two sources moving toward each other at a relative speed twice that of the source speed.

Applications: This is an usual set-up for the Doppler effect, so most applications won’t involve this version.

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