Can someone unconfuse me? I am unsure of whether these fans are 120VAC or 12VDC motors. If they are AC motors, you want the same type of switch used to control ceiling fans: triac based. If they are DC motors, then you would want a slider controlled rheostat.
If I understand them correctly, triac based speed controllers do not change the frequency; they control the amount of time the AC signal is turned on. They chop out part of the sine wave so that the frequency stays the same. In order to change the frequency, you would have to have a fairly complicated circuit that regenerated the AC sine-wave.
The rheostats are high powered variable resistors that change the voltage supplied to the fan motor. That sounds more like what is being talked about, since many motors have a much higher voltage required to start versus the voltage at which they will stall. If it is DC, you might consider PWM to achieve a wider range of speeds, but it might add a bit of buzz to your motor noise.
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