A rotating electrical machine such as a generator has a rotor disposed within a rotor housing and an air flow path passing from the rotor housing to and through a silencer to an exhaust. The silencer comprises at least one air flow path disposed adjacent at least one sound absorbing column, the column comprising a first portion having a first sound absorbing material therein and a second portion having a microperforated panel (MPP) and a resonator cavity. The second portion comprises a reactive sound absorbing unit which can be specifically tuned to suppress aero-acoustic noise generated by ventilation slots in the rotor, typically peaks at 100 Hz or 120 Hz.