Patent ID: 7006639

Claim:
Active noise attenuating duct element, comprising one or more inflow-pipes, the lengths and widths of which are selected to achieve the acoustical behaviour of an air-mass or transmission-line in the frequency range to be attenuated, one or more duct-chambers, arranged such that air flows from said inflow-pipes into said duct-chambers, said duct-chambers being equipped with an outflow-port through which air leaves the chamber, one or more active acoustical impedance elements pneumatically connected to said duct-chambers for influencing the air pressure in these chambers, wherein the lengths and widths of said inflow-pipes and the dimensions of said duct-chambers and the transfer characteristics of said active acoustical impedance elements are adapted to each other to achieve the behaviour of an acoustical low-pass or band-pass filter of higher order, and wherein said active acoustical impedance element further comprises a housing, inner wall means for dividing the inner volume of said housing into at least two chambers, wherein the first of said chambers is pneumatically connected to said duct-chamber, at least one electromechanic transducer, being built into an opening of said inner wall means such that its membrane separates said first chamber from a second chamber, pressure sensing means being placed in said first chamber or in said duct-chamber, for measuring the air pressure and for producing first signals indicative of said air pressure, calculating means, to the input of which said first signals produced by said pressure sensing means are applied, for calculating output signals based on the values of said first signals and based on a methematical impedance function, which describes the dependance of the membranes movement on the measured air pressure, a control system, for controlling the movement of said membranes of said electrodynamic transducers, comprising one or more power amplifiers, the outputs of said amplifiers being connected to said electrodynamic transducers to drive said transducers, one or more electrical controllers, to the inputs of which said output signals produced by said calculating means are applied, the outputs of said electrical controllers being connected to the inputs of said power amplifiers, wherein said calculating means and electrical controllers are dimensioned to force said transducers' membranes to move with a speed which is determined by said impedance function.