A muffler for muffling noise in a fluid flow, such as exhaust gases from an internal combustion engine comprises a chamber, loudspeakers for injecting anti-noise into the chamber under the control of processing circuitry in dependence upon residual noise detected by a microphone. The gas flows from an inflow pipe to an outflow pipe past a radial aperture that produces an acoustic coupling between the chamber and the gas flow, to allow the anti-noise to mix with noise in the gas flow and produce noise cancellation. The coupling is so arranged that the chamber becomes pressurized by the gas flow, in use, so as to restrict hot gas flow through the chamber and thereby provide an essentially benign environment for the loudspeaker.