Instructor Outline:
Loudspeaker and Microphone
UM Physics Demo Lab 07/2013
Lab length: 70 minutes
Lab objective: Instruct the students about speakers, microphones, induction,
amplification, recording and transmission of sound and Faraday Induction.
Materials
1 plastic spool
1 32 gauge wire on metal spool
1 Petri dish (top and bottom)
1 amplifier
1 1/8th inch pin plug-double bare ends wire
1 1/8th inch pin plug-1/8th inch pin plug wire
2 alligator leads
1 MP3 Player
1 mini-magnet
Gluing template
1 band steel wire scraper
1 wooden block (scraping backstop)
(optional) 2 – 25’ wires for telephones
Exploration stage: 30 minutes - Group Lab Work
The students work in groups to build a speaker using Petri dishes and a
cassette player. They introduce an amplifier and observe how the signal is magnified.
Analysis stages: 15 minutes – Lecture
Magnetic forces on current-carrying wires and Faraday induction are
reviewed. Next speakers are shown to be another type of “motor”: a device that
produces mechanical motion due to the magnetic force on a current-carrying wire.
Microphones are then shown to be another type of generator: a device that
produces voltage due to changing magnetic flux by Faraday Induction.
Application stage: 25 minutes – Group Lab Work
The students use their Petri speakers as microphones.
Concepts developed:
1. Microphones use electromagnetic induction to convert vibrations into electrical
signals. Microphones are therefore a type of electric “generator” based on
Faraday Induction.
2. Speakers use electromagnetic induction to convert electrical signals into
mechanical vibrations.
Speakers are therefore another type of electric
“motor”.
3. A speaker can function as a microphone and a microphone can function as a
speaker. Specialized speakers and microphones are made so as to be
optimized for their chosen function.
Suggested Demos:
Prototype
Show and Tell Speakers
Show and Tell Cassette Tape Player
3B70.10 - Sympathetic Oscillation between Two Tuning Forks
5K10.48 - Current-Coupled Oscillators
5K10.21 - Electromagnetic Induction
5K40.u1 - Moving Magnet Generator
Simple Radio Control Systems with Magnetic Actuators
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109