Abstract: A method of controlling a centrifugal compressor having a variable width diffuser section, the compressor being part of a chiller having a condenser and an evaporator, the method including the steps of defining the width of the diffuser which results in an optimum noise and performance level at given values of an operational parameter of the compressor, periodically sensing the operational parameter of the compressor, and periodically varying the diffuser width according to the sensed operational parameter to create a diffuser width that results in the optimum noise and performance levels at the sensed value of the operational parameter. The step of defining preferably defines the width based at least on the capacity at which the compressor is operating. The capacity is preferably determined by measuring the flow and temperature difference of the fluid passing through the evaporator. A control system for controlling the chiller system to minimize noise is also disclosed.