# A Feasibility Study of the Use of PZT Actuators for Active Control to Enrich Engine Sound
###### Abstract  
This study examines the feasibility of a novel active sound enrichment (ASE) system using piezoelectric actuators as sound generators and an inverse control filter to supplement poor engine sound at the driver's ear location in a passenger car instead of using an interior audio system. The proposed ASE algorithm is developed as a purely feedforward control strategy to track the pre-defined target engine sound (three engine orders). Theoretical and experimental analyses are investigated in-depth on the vibro-acoustic characteristics of a PZT (lead zirconate titanate) actuator bonded on a steel plate and a dedicated control filter to supplement sound using an inverse method to compensate for the secondary path. The location of the PZT-plate actuator was carefully chosen to satisfy the causality condition and robustly stable control of the ASE algorithm. The experimental ASE system was set up in a test car, and the ASE algorithm was implemented in a real-time controller. The real-time ASE experiment results showed that the measured sound of the three orders was well supplemented as the tracking of the target sound was achieved robustly with small errors without any divergent instability. Thus, this study suggests that the proposed ASE system using the PZT  
mass-produced audio boards are designed optimally to run conventional audio algorithms not considering such an unexpectedly large ASE algorithm. It may not be easy for car manufacturers to accept redesigning the audio board purely to accommodate the ASE algorithm. As an affordable alternative, this study proposes a novel ASE system that is independent of the existing audio system. The proposed ASE system includes a piezoceramic PZT (lead zirconate titanate) actuator bonded on a structural plate of a car body as the secondary actuator to radiate sound and a new ASE algorithm suitable for such a PZT-plate actuator which has a longer impulse response than that of a typical loudspeaker. As these actuators can generate structure-borne sound by being integrated into car structures and vibrating them [15; 16], their acoustical characteristics differ from those of a typical door loudspeaker. This is because the characteristics of the sound from these actuators are governed by the vibro-acoustical properties of the structure [1]. However, a well-designed PZT-plate actuator can be capable of producing sound pressure equivalent to that of a door loudspeaker, even with both a smaller volume and lighter weight [17]. Moreover, it can be made in various geometries. Hence, the piezocatuor can be integrated into some car structural plates although the installation spaces are small. Additionally, the sound generated by the PZT-plate actuator can be more car-like compared with loudspeaker sound in terms of sound characteristics. Such sound could be more effective in masking the mechanical noises produced in ICEV and EV. There have been some studies using such piezoelectric actuators to generate sound instead of interior door loudspeakers in the car sound system. For example, [PERSON] in his patent proposed attaching a number of piezoelectric actuators to the headliner of a car for use as sound radiators [18]. [PERSON] et al. designed a structure-integrated loudspeaker system with MFCs (macro fiber composites) attached to a door of a car [19]. In this study, the feasibility of the application of the PZT-plate actuator is examined in-depth for active sound enrichment to supplement the poor engine sound of an ICEV. The design of the control filter in the ASE algorithm based on the vibro-acoustic properties of the PZT-plate actuator to satisfy the causality condition and robust control stability is also discussed. The tracking performance to follow a given target sound using the actuator is an important factor in assessing the feasibility. Section 2 describes the theoretical and practical considerations of the design of the PZT actuator in a car structure and the proposed ASE algorithm. The experiment set-up is presented in Section 3.