Electropneumatic transducers as secondary actuators for active noise control part III: Experimental control in ducts with the subsonic source

The possibility of performing active control of periodic noise propagating in ducts using a subsonic electropneumatic acoustic generator as secondary source is investigated. The subsonic generator has been studied both theoretically and experimentally in two companion papers, and this sound source was shown to be highly efficient, but non-linear. The non-linear behaviour of the source decreases as the acoustic pressure at its output is reduced, however, as in the case when the source is used as a secondary actuator in an efficient active control system and thus the source is well suited to such applications. Residual non-linearities of the subsonic source are shown to be due to the mechanical design of the actuator. An harmonic controller is discussed which accounts for the residual non-linear behaviour of the subsonic source. Experiments carried out with a manual version of this controller, that controls the first five harmonic components of the signal driving the subsonic source, reveal that it is efficient in controlling periodic primary sound fields. The implementation of a fully coupled harmonic controller is, however, shown to require large processing capacities. A theoretical analysis of a simplified version of the harmonic controller-the decentralized harmonic controller-is carried out, and simple conditions are established under which the decentralized controller offers similar performances to the fully coupled harmonic controller. These conditions are shown to be satisfied when using the subsonic source as a secondary actuator. The non-linear behaviour of the subsonic source can also cause a further problem, since the error surface experienced by the control system may exhibit local minima. It was found that the likelihood of this happening was much reduced is only the fundamental component of the harmonic controller was adapted initially, and then the other harmonics were changed in a second phase of adaptation. The implementation of a decentralized harmonic controller is considered, using a dual channel signal processing board. A purely linear model for the plant under control is shown to be accurate enough for modelling the system under control and to ensure convergence of the controller. Experiments with the automatic controller reveal that attenuations measured at the monitor microphone are around 25 dB, for sinusoidal primary sound fields. (C) 1999 Academic Press.