Complaints of low frequency noise arose following installation of a CHP system in an existing boiler room. Control was by a multi-stage process, the main phases of which were: Reduction of LF noise in the boiler room: Detuning the stack: Reduction of breakout from the stack: Identifying the residual problem. The final phase enabled the correct exhaust silencer to be selected. The end result was that annoying excitation of room modes and irritating throbbing noises were reduced to below complaint level.