GRADE LEVELS:  9-10

SUMMARY:

Audio speakers are an everyday item in today's world found in televisions, computers, stereos, etc. This project is designed to teach a student about how sound is generated using electrical currents and magnetic fields to produce pressure differentials perceived by humans as sound. Students construct a speaker from raw materials such as magnets and wire while learning vital concepts that describe the theory behind sound generation. A speaker is a very tangible object that most students are familiar with and have interest in developing their own version. The finished product is a speaker through which a student can listen to their favorite song or whatever they please.

LEVEL OF DIFFICULTY [1 = Least Difficult : 5 = Most Difficult]

5, most difficult

TIME REQUIRED

12-15 50 min. class periods

COST

$240 per class of 12-15 working in groups of 2-3 students (magnets are a major cost, having these items donated can drastically reduce price)

2.4 Identify and explain the engineering properties of materials used in structures, e.g., elasticity, plasticity, thermal conductivity, density.
5.1 Describe the different instruments that can be used to measure voltage, e.g., voltmeter, multimeter.
5.2 Identify and explain the components of a circuit including a source, conductor, load and controllers (controllers are switches, relays, diodes, transistors, integrated circuits.)
5.3 Explain the relationship between resistance, voltage, and current (Ohm’s Law).
5.4 Determine the voltages and currents in a series circuit and a parallel circuit.

Electromagnetic Fields
Construction techniques
Pressure and vibrations
Electricity (current, voltage, resistance)
Energy transformation
Design

This project should coincide with or follow discussions / lectures on electricity, waves, and magnetism. To complete this project, those topics must be covered in reasonable detail since designing and building a speaker is based on those subject areas.

A speaker is an electromechanical transducer. A transducer is anything that changes energy from one form to another. So a speaker must change electric energy (voltage and current) into mechanical energy (moving mass) in this case, vibrating air. The change of energy is done by the interaction of a current with a permanent magnetic field, created using stationary magnets.

The force on a current-carrying wire is equal to the strength of the magnetic field times the current times the length of the wire:
F = Bli
(Assuming everything to be at right angles)

Before the mechanism of the transducer is explained, a few key terms must first be outlined.

Cone - The cone is the piece of the speaker that actually has to do the work and push the air. It will be attached to the coil and pushed by the coil.
Coil - The coil must carry the current to interact with the magnetic field, and it must be attached to the cone to push the cone up and down. The coil must be the correct electric resistance. The coil needs to be put very close to the magnet poles so that the maximum force can be applied.
Magnetic Field Apparatus - The motion of the cone depends upon the magnetic force that can be placed upon it, so it is important to create a strong and concentrated magnetic field that is pointing in the right direction.
Suspension - The suspension connects the moving cone to the non-moving frame.
(Please see figure 1 for a diagram of speaker components)

A speaker receives an input current from the source that is fed through the coil. The shape and resistance of the coil produce an electric field that interacts with a magnetic field already in place. The interaction between the two fields forces the coil to move vertically which pushes the cone. The cone, mounted to the speaker box with the suspension system, moves up and down, vibrating the air to create pressure differentials that are interpreted as sounds by the human ear.

Circular Magnets
36-gauge voice coil wire (5-7 meters per speaker)
Styrofoam cups and plates
Plastic cups, plates, and straws
Paper cups and plates
Index cards
Various adhesives (glue guns, super glue, masking tape, duct tape, Loctite 411, etc.)
Latex rubber sheeting
Flat metal stock, ½ “ (iron or steel)
5/8 “ Iron or Steel rod (6 cm. per speaker)
Binding posts for speaker leads
Speaker Wire, 16-gauge
Foam Board
Scissors
Utility knives
Rulers

It would be beneficial to produce a prototype of certain parts of the speaker for the students to use as a model. Specifically, the Magnetic Field Apparatus should be constructed so that students know what the final product should look like. This is a crucial component in the speaker and accuracy has a significant impact on the quality of sound produced by the speaker. Other speaker components can be explained as students get to a point where they’re ready to build them. Cone design and speaker box design should be left up to the students; this is where design freedom comes into the picture. Suggestions for box size should be made and materials should be limited so that the projects do not become monstrosities, but good judgment has to be used here based on material costs and supplies available.

DIRECTIONS:

Please see attached worksheets: General Assembly Guidelines, Magnetic Field Apparatus.

Cone Design
1. What do we need to think about in designing a cone?
2. Should it be big or small? Heavy or light? Stiff or floppy?
3. Does shape matter?

Coil Design
1. Why does coil resistance make a difference?

Magnetic Field Apparatus
1. What is the best way to orient the magnetic field? How can we do this?

Suspension
1. Should if be stiff or compliant? How elastic should it be?
2. Heavy or light?
3. Wide or narrow?

Box Design
1. Why must the box be as close to airtight as possible?
2. What will loose construction of the box produce? Loose construction meaning weak joints and loose connections.

WORKSHEETS:

See Associated Download.

SAMPLE RUBRIC:

See Associated Download.