3E30.20 - Reciprocity - Speaker as a Microphone / Microphone as a Speaker

Connect one speaker to the 4 ohm side of the transformer, and connect the other side of the transformer to the aux. input of the amplifier.  Connect the other speaker to the output of the amplifier.  Use the speaker connected to the transformer as a microphone and speak into it.  You should hear yourself over the output of the amplifier.
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:
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:
  • Chris Chiaverina, "Hair Dryers, Electric Drills, and Battery-Powered Toy Cars as Microphones and Loudspeakers", TPT, Vol. 51, # 7, Oct. 2013, p. 438.
  • 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.


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