Abstract: An electro-magnet (11) is spatially coupled with respect to a support armature (15) by an operational gap (16) and their relative movement is automatically compensated by controlling a current (14) supplied to the electro-magnet (11). The current (14) and hence a magnetic field (F) are controlled by a current controller (13) which incorporates a transfer function and is operated in a feedforward path (19) by a signal (18) from a sensor (17) which detects variations in the gap (16) between the electro-magnet (11) and the support armature (12). The current controller (13) is also operated in a feedback path (22) by a signal (21) from a sensor (20) which detects variations in the flux intensity of the magnetic field (F).

Abstract: A sound wave propagated along a duct through a fluid contained in the duct is attenuated by generating sound waves from an array of sound sources spaced along the duct. Each source generates two waves travelling in opposite directions; those travelling in the same direction as the unwanted wave sum to give a resultant which interferes destructively with the unwanted wave, while those travelling in the opposite direction sum to give a negligible resultant. The source array may be operated in response to detection of the unwanted wave.