LOUDSPEAKER AND MICROPHONE
Pre-Lab Question
UM Physics Demo Lab 07/2013
How are the vibrations produced by sound converted to electrical signals to be
recorded?
EXPLORATION
Materials
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plastic spool
32 gauge wire on metal spool
Petri dish (top and bottom)
MP3 Player
amplifier
1/8th inch pin plug-double bare wire
1 1/8th inch pin plug-1/8th inch pin
plug wire
1 alligator lead card
1 mini-magnet
1 gluing template
1 wood block backstop
1 band steel scraper
1. Build a microphone/speaker with the Petri dish as your vibrating surface. The thin
plastic is ideal for vibrations, and will be effective as a microphone surface.
Figure 1: Petri Dish and Template
Center the large top on the template, and glue the magnet to the inside center of the top
of the Petri dish with superglue according to the template with a magnet sized ring as a
guide for the center of the dish. The glue is at the back of the classroom.
2. Use all the provided wire to wind a coil on the plastic spool. The more
windings you add, the more voltage will be induced, providing a better
signal. Wrap neatly and leave 12 inches on both ends to allow for
connections. Strip about ½ inch of insulation off the ends of the wire with
steel scraper, used on the wooden block (NOT THE TABLE TOP).
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3. Glue the spool to the Petri bottom, using the template as a guide for where to glue the
spool so it is centered. Before you glue, make sure that the spool seats nicely with the
glued magnet in the Petri top.
Figure 2: Petri Dish with Glued Components
4. Route the wire ends through the hole in the bottom Petri dish. Close the Petri dish.
Connect the Petri dish to an MP3 player with the 1/8th inch pin plug-double bare wire and
alligator leads. Plug the 1/8th inch pin plug end into the “phones” slot in the MP3 player.
Be sure that none of the alligator leads touch each other!
Figure 3: Petri Speaker with MP3 Player
5. Press play on the MP3 player. What happens? Explain what you observe.
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6. Include an amplifier in the arrangement. Connect the 1/8 th inch pin plug-bare wire
between the Petri and the amp Ext SPKR plug. Connect the 1/8th inch pin plug-1/8th inch
pin plug wire between the MP3 player and the amp INPUT plug.
Figure 4: MP3 Player, Amplifier, and Petri Speaker
Turn on the amplifier and the MP3 player. What do you observe, how is this different than
without the amplifier? Explain why the amplifier is used.
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Challenge Work:
Describe two devices you use every day that reproduce sound in the same way as the
simple speaker you have just built.
Compare a speaker to an electric motor. How are they the same? What fundamental
aspect of magnetism and electric currents do both devices employ to operate?
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Everyday Applications
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Telephones
Sound recordings
PA systems
Mp3 players, such as iPods
Stereo systems
APPLICATION
Materials
1 assembled Petri speaker
1 amplifier
2 alligator leads
1 1/8th inch pin plug-double bare ends wire
1. Now remove the MP3 player and plug the speaker into the amplifier INPUT slot.
Figure 5: Petri Microphone and Amplifier
Turn on the amplifier by rotating the dial on the right side. Speak into the Petri dish,
and observe what happens. Do not put your thumb on the surface with the magnet
glued to it or you may “damp” the signal. Record your observations. What is the
name of a device that reacts to sound in this way?
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2. Now compare this new way of using the speaker to an electrical generator. How
are they similar? What fundamental magnetic principle do they both employ to
operate?
Summary:
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.
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Loudspeakers and Microphones
In the motors and generators lab you observed that when you place a currentcarrying wire in a magnetic field, it experiences a force that moves the wire. You also
saw that when a wire loop moves near a magnet or a magnet moves near a loop, a
voltage is induced. This reciprocal relationship is also how speakers and microphones
work.
Sound waves cause the air molecules near a detector, such as the membrane of an
ear drum, to vibrate. The vibrating air molecules in turn cause the ear drum
membrane to vibrate. If instead of an ear drum we substitute an artificial membrane
attached to a magnet and place the magnet near a coil, then the vibrations of the
membrane will be reproduced as tiny voltages induced in the coil by Faraday
induction due to the vibrating (moving) magnet. This is what a microphone does; it
converts the vibrations of a sound wave into an oscillating voltage that can be
connected to a circuit (with or without an amplifier) to drive current in the coil of a
speaker. The oscillating current in a speaker coil experiences an oscillating magnetic
force due to the nearby speaker magnet, which drives the speaker cone to oscillate
and again produce a copy of the sound. The oscillating voltage produced by a
microphone can also be recorded on a magnetic tape or as a digital music file to be
played back later or written to digital storage such as a CD. Speakers and
microphones are inverse devices in exactly the same way that motors and
generators are inverse devices. Speakers (motors) produce mechanical motion due
to magnetic forces on current carrying wires. Microphones (generators) produce
voltage due to the changing magnetic flux through a coil of wire. For both
generators and microphones the changing magnetic flux is due to the relative motion
of the coil and a magnet.
Transmitting Signals
With the relay and buzzer lab, you observed that connecting and disconnecting a
circuit can relay a message. This is how an old style “wild west” telegraph worked, a
pattern of on and off signals transmitted over wires was translated into an alphabetic
Morse code.
Sound transmission is more complex. Instead of two options (on or off) there are
many signals related to the magnitude of the voltage generated by the magnet and
coil. These signals vary in both amplitude (loudness) and frequency (tone). A
microphone can encode all this information and a loudspeaker can reproduce a copy
of the original sound detected by the microphone.
Microphones
Alexander Graham Bell was instrumental in the transition from telegraph technology
to the transmission of sound that we use today for telephones and sound recordings.
The microphone that you’ve constructed is precisely the technology he refined.
Sound sources produce vibrations in the air. When those vibrations reach surfaces,
they cause those surfaces to vibrate. The Petri dish is a decent surface for recording
because it is rigid and thin. When the dish surface vibrates, the magnet vibrates with
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it. The vibrations are tiny, but they are enough to induce a voltage in the coil. That
voltage can be increased to produce a louder sound by means of an amplifier.
Speakers
The amplifier you used with your microphone also contains a speaker and it produces
sound from the microphone signal by reversing the microphone process. A current is
produced when the microphone signal voltage (usually amplified) is applied across
the speaker coil, which is attached to the paper or plastic speaker cone. The coil is
placed in or near a permanent magnet built into the back of the speaker. This
alternating current, an electric copy of the original sound, creates a magnetic field
(the phantom bar magnet of a loop) that alternatively attracts and repels the
speaker coil to/from the speaker magnet. The resulting magnetic forces on the
speaker coil drive the speaker cone to oscillate (vibrate) producing a copy of the
original vibration that produced the oscillating microphone voltage (signal). The
vibrating cone drives the nearby air and a copy of the original sound wave that was
detected by the microphone is reproduced in the air by the speaker.
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