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Code Number: 3E30.20
Demo Title: Reciprocity - Speaker as a Microphone / Microphone as a Speaker
Condition: Excellent
Principle: Faraday's law of Induction, Coil in a magnetic field.
Area of Study: Oscillations & Acoustics
Equipment:
Two small speakers, amplifier, impedance matching transformer.
Procedure:
Connect the speaker to the output of the amplifier and then play the tape player to show the speaker works normally. Then connect that same speaker to the 4 ohm side of the impedance matching transformer, and connect the other side of the transformer to the aux. input of the amplifier. Now use the speaker as a microphone and speak into it. You should hear yourself over the output of the amplifier.
You can also try this with a piezo buzzer as the microphone.
References:
- Christopher J. Chiaverina, "A Survey of Reversiable Devices", TPT, Vol. 63, #3, March 2025, p.169.
- Chris Chiaverina, "Hair Dryers, Electric Drills, and Battery-Powered Toy Cars as Microphones and Loudspeakers", TPT, Vol. 51, # 7, Oct. 2013, p. 438.
- Eugene Ackerman, William Holak, "Reciprocity Calibrations", AJP, Vol. 25, #7, Oct. 1957, p. 454.
- Martin C. Sagendorf, "Sounds We Can't Hear (the Bug Box)", Physics Demonstration Apparatus, 2009, p. 59.
- Mark Davids, et al., ""Activity 4, Teaching about Lightwave Communications, 1994, p. 3-9.
- Martin Gardner, "78, Paper Cup Telephone", Smart Science Tricks, p. 116.