Loudspeaker and Microphone Lab

LOUDSPEAKER AND MICROPHONE

Property of LS&A Physics Department Demonstration Lab

UM Physics Demo Lab 07/2013

Property of LS&A Physics Department Demonstration Lab

Pre-Lab Question

How are the vibrations produced by sound converted to electrical signals to be recorded?

EXPLORATION

Materials

Property of LS&A Physics Department Demonstration Lab

1 plastic spool

1 32 gauge wire on metal spool

1 Petri dish (top and bottom)

1 MP3 Player

1 amplifier

1 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

Property of LS&A Physics Department Demonstration Lab

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 the steel scraper, used on the wooden block (NOT THE TABLE TOP).
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.

6. Include an amplifier in the arrangement. Connect the 1/8th 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.

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?
Everyday Applications

Telephones
Sound recordings
PA systems
Mp3 players, such as iPods
Stereo systems

APPLICATION

Materials

Property of LS&A Physics Department Demonstration Lab

1 assembled Petri speaker

1 amplifier

2 alligator leads

1 1/8th inch pin plug-double bare ends wire

Property of LS&A Physics Department Demonstration Lab

1. Now remove theMP3 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?

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:

Microphones use electromagnetic induction to convert vibrations into electrical signals. Microphones are therefore a type of electric “generator” based on Faraday Induction.
Speakers use electromagnetic induction to convert electrical signals into mechanical vibrations. Speakers are therefore another type of electric “motor”.
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.

Loudspeakers and Microphones

In the motors and generators lab you observed that when you place a current-carrying 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 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.

Property of LS&A Physics Department Demonstration Lab