Which Controller?

Feedback controllers use a pulse output (the power is switched on and off). While the power is off the motor is still turning and acts as a generator producing a voltage proportional to its speed (BEMF). A feed back controller measures this voltage and adjusts the pulse duty cycle (ratio of on to off time) to maintain a constant speed.
In analog systems the pulse frequency is quite low (and audible) as the signal has to travel some distance from the controller to the loco. In a digital system a BEMF controller is inside the loco, resulting in quite short wire runs, which allows a much higher frequency to be used. This frequency can be outside the audible range.
Mark Thornton