Electromagnet Speaker


To build a speaker with electromagnet field and voice coils.

Permitted Materials
  1. Magnetic coils wound from copper wire (any type of insulation)
  2. Cones and frames may be made from paper, cardboard, wood, plastic, tape or glue (any type)
    • No metals other than the copper wire coils and iron core(s) may be used in any part of the construction.
    • Permanent magnets may not be used.
    • Parts taken from commercial speakers may not be used.
Other Constraints
  1. Coils must be hand wound - not commercial coils. Iron cores may be incorporated in the design.
  2. The speaker must fit completely in a box 30 cm. X 30 cm. X 30 cm.
  3. The speaker must be free standing facing upward.
  4. The field magnet and voice coil must be visible or easily accessible for inspection.
  5. Wires that connect to the voice coil and field coil must be at least 20 cm. long.
Testing Procedure
  1. The speaker will be placed facing upward on a table. A box (30 cm. X 30 cm. X 30cm.) with an attached microphone, will be placed over the speaker.
  2. A 400 Hz sine wave produced by a function generator will be amplified by a standard power operational amplifier (18 watts rms, 4-ohm output) and applied through a one-ohm resistor to the speaker. The signal produced by the microphone will be viewed on a digital oscilloscope/spectrum analyzer connected to a computer.
  3. The decibel level of the 400 Hz signal, as read from the spectrum analyzer, will determine the score. The highest level (loudest) wins. In the event of a tie, a run-off test using 1000 Hz will be held.
  4. Contestants wishing to do their own preliminary testing can closely duplicate the testing procedure by using one channel of a typical automobile tape or CD deck. A series resistor connected to the speaker voice coil (at least 1-ohm, 10 watts) is recommended to protect the deck's internal amplifier.
  5. A battery eliminator ( 0-12 volts, 10 amps) will be made available to contestants during the competition, or they may furnish their own power source for the field coil.
  6. None of the testing equipment will be available for practice runs prior to the actual competition.
Testing Equipment – Decibel Measurement
College of Saint Benedict
Saint John’s University

Jim Crumley
Chair, Physics Department
SJU PEngl 107
Physics Department