Noise reduction systems designed for use on vessels of 1 000 tonnes displacement or more, as follows: (a) Systems that attenuate underwater noise at frequencies below 500 Hz and consist of compound acoustic mounts for the acoustic isolation of diesel engines, diesel generator sets, gas turbines, gas turbine generator sets, propulsion motors or propulsion reduction gears, specially designed for sound or vibration isolation and having an intermediate mass exceeding 30 % of the equipment to be mounted; (b) ‘Active noise reduction or cancellation systems’ or magnetic bearings, specially designed for power transmission systems. Technical note: ‘Active noise reduction or cancellation systems’ incorporate electronic control systems capable of actively reducing equipment vibration by the generation of anti-noise or anti-vibration signals directly to the source.