Undirectional JMC actuators in the active attenuation of noise in ducts

In feedforward active noise control systems in ducts, a natural solution to avoid the acoustic feedback from the cancelling actuator to the reference detector is to use a underectional actuato. New realizable ideal undirectional three- and two-element source configurations, based on the JMC method, are introducted. In the two-element actuator the elements act as a dipole with the monopole effect included in the same elements. The inter-channel delay in the control structure of the two-element actuator is optimized with three different ways. In one of them the delay element is omitted altogether. The output volume velocities of all the three variations of two-element Ideal Source are mutually identical and independent of the inter-channel delay opitmization, and also identical with those of Swinbanks' two-element actuator, although the procedure of attaining them is quite different. It turns ou to be advantageous in the practical implementation of digital control systems that the new three- and two-element solutions can be realized with no interchannel delays. Thus, it is possible to achieve simplified control algorithms and reduce the computing time or improve the performance.