An "open loop" control methodology for sound reduction inside a car

In this paper we consider the problem of reducing the sound generated by the engine inside a car enclosure by using piezoelectric actuators located on the car's roof and microphones placed in correspondence of the passengers. A preliminary analysis shows that the frequency responses from the actuators to the microphones are characterized by a very large band and exhibit a high modal density with many resonance peaks. Based on these facts, the design of a feedback control law could originate spillover phenomena, which will manifest through the generation of high frequency sounds even more unpleasent than the original noise. Therefore we propose the design of a controller based on an open loop control philosophy; that is the control action is designed in such a way that the spectrum of the control signals is contained into the disturbance band. Since the noise we deal with is not of a classical type (pure sinusoidal or multi-tonal) but is a wide band disturbance, classical open loop control techniques, based on model inversion, cannot be implemented; to this end the concept of Generalized Frequency Response, which allows us to deal with wide band signals, is introduced in the paper. Numerical simulations show the benefits of the proposed control technique.