A fan assembly includes a fan having an outlet side, a fan duct positioned at the fan outlet side, and a hollow cylindrical chamber positioned within the fan duct, wherein the chamber has two closed ends, a cylindrical side wall, and an inlet port. The dimensions of the chamber are determined to suppress one or more noise frequencies emitted by the fan. In one example, one or more dimensions of a chamber disposed in an outlet duct of an axial fan is automatically adjusted to suppress a noise frequency that is a function of a rotational speed of the axial fan.