An exhaust system comprising: an exhaust chamber having a longitudinal axis and an outer wall defining alternating longitudinally-extending ribs and grooves; and an insert within the exhaust chamber, the insert including a plurality of fins extending generally perpendicular to the longitudinal axis, each fin including a distal edge extending substantially close to the plurality of ribs, the insert defining expansion chambers between adjacent fins. Pressurized gas flowing through the exhaust chamber flows along the grooves and expands within the expansion chambers to reduce the pressure of the pressurized gas prior to the gas exiting the exhaust chamber. The fins may in some embodiments be sufficiently stiff to resist substantial deflection under the influence of the gas.