1. A system for actively attenuating primary vibrations, the system including: at least one actuator, responsive to drive signals applied thereto, for generating anti-vibrations for attenuating the primary vibrations by superposition of the anti-vibrations with the primary vibrations; at least one first sensing device for generating an error signal indicating the residual vibrations resulting from the superposition of the anti-vibrations and primary vibrations in the vicinity of a specific location; at least one second sensing device for generating an input signal indicating the primary vibrations; a controller, responsive to the input signal for generating the drive signals to the actuator, the drive signals being related to the input signal by a transfer function determined in accordance with a multichannel block adaptive algorithm in response to the error signals, the algorithm having a convergence factor which varies in accordance with a predetermined parameter.