Motor Speaker

Permitted Materials

DC electric motor (permanent magnet field)

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.

1. The speaker will be placed facing upward on a table. A box (30 cm. X 30 cm. X 30cm.), with an attached microphone and decibel meter, 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.  To qualify, the dominant frequency, as seen on the spectrum analyzer, must be 400 Hz. 

3. The amplitude of the recorded signal (decibels) will determine the score.  The loudest speaker wins.  In the event of a tie, a run off test using 1000 Hz will be held.

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 (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.