Motor Speaker


To build a speaker using a DC motor to produce the sound.

Permitted Materials
  1. DC electric motor (permanent magnet field)
  2. Frames and cones or sounding boards, which have been attached to the motor, may be made from any type of material.
Other Constraints
  1. Parts taken from commercial speakers may not be used.
  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. Wires that connect to the motor must be at least 20 cm. long.
  5. The motor may not be disassembled other than removing attached gears or pulleys.
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 DC motor. 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. None of the testing equipment will be available for practice runs prior to the actual competition.
Testing Equipment – Decibel Measurement

Contestants wishing to do their own preliminary testing can closely duplicate the testing procedure by using one channel of a typical automobile sound system (radio, tape or CD deck). A series resistor (at least 1-ohm, 10 watts) between the deck’s output and the speaker is recommended to protect the deck’s internal amplifier. If the voice or music reproduced is clearly recognizable, the speaker should qualify.

College of Saint Benedict
Saint John’s University

Jim Crumley
Chair, Physics Department
SJU PEngl 107
Physics Department