Normally, split phase (motors with start and run windings), have start windings controlled centrifugally and drop out by means of a mechanically switch at about 75% of their rated RPM. This type of motor does not play well with electric speed controls. As the RPM is lowered it forces the start windings to re-engage and the motor quickly over heats.

If noise is an issue, you can connect fibre-flex ducting to the fan and distribute the intake to several areas thus reducing the noise and loftiness of the exhaust. The fibre-flex ducting in itself will lower the noise, but multiple inlets drops the noise to almost negligible. I have used this method with the fibre-flex to exhaust sound studios very successfully.