applied sciences  
A Feasibility Study of an ESG to Suppress Road Noise of a Car [PERSON],*, [PERSON] 1, [PERSON] 2 and [PERSON] 2  
Although the FxLMS-based feedforward ARNC using built-in door loudspeakers achieved noticeable outcomes in various studies as summarized above, the commercialization of this approach was not easily adopted in the car industry, because it was almost impossible to implement such an ARNC logic into an existing audio processing board as the board was designed optimally, not considering such a new logic. As a result, it was required to re-design a new board completely to accommodate the necessary computation and memory to run an ARNC logic in real-time, as well as existing audio logics. Thus, the application of an ARNC system using a car audio unit in a car can cause a cost problem to the car industry. In addition, this can lead the car price to increase, which weakens the competitiveness of the car with the new board. As an alternative to the built-in audio system, some vibration actuators as secondary actuators have been considered to reduce road noise by suppressing vibration levels of car structures [10; 11; 12], where the shakers were operated with their own control board and power amplifiers. [PERSON] and [PERSON] adopted six electro-dynamic inertia shakers mounted on a mini-sized wagon car body to attenuate road noise in the car cabin and presented a reduction of 6.1 dBA in the frequency range of 75-105 Hz on a rough asphalt road at 90 kph using an FxLMS-based active structural acoustical control algorithm [10; 11]. [PERSON] et al. [12] used two inertia shakers installed at a suspension system of a quarter-car and showed an averaged vibration level reduction of 4.6 dB in the frequency range of 50-250 Hz after a bench road noise test using an FxLMS-based active vibration control. However, the electro-dynamic inertia shakers used were not adopted in the production of cars, because of practical issues as well as the superiority of built-in loudspeakers in terms of noise reduction performance and stability. In this study, an electronic sound generator (ESG) [13] as a secondary actuator was studied in-depth for ARNC. The ESG is a kind of the voice-coil motor that can be combined with a structure such as a thin plate so that it can generate vibro-acoustic sound by vibrating the plate [13]. The ESG can be operated stand-alone with its own control and power unit that is completely separated from the other control systems of a car. Moreover, the ESG can easily be installed in a small space as it is smaller than a typical door loudspeaker or electro-dynamic inertia shaker. Nevertheless, it can generate low-frequency structure-borne sound for the use of ARNC by vibrating a panel structure. In addition, the sound generated from a panel with an ESG is more car-borne, which is comparable to the artificial sound from a typical door loudspeaker. With these advantages, the ESG has been applied to sound control in cars [14; 15]. Therefore, an ESG was attached to a cowl panel of a car to generate structure-borne anti-noise, and its performance and stability were analyzed with an FxLMS-based multiple-reference single-input single-output (MR-SISO) ARNC system. However, there are some considerations in using this ESG-cowl pair arrangement due to the structural response of the pair. The sound pressure generated by the pair depends on its characteristics such as the location of the ESG and the size of the cowl panel, and the pair can have a longer impulse response in the secondary path model with more propagation delay than those of the loudspeakers. This comes from the nature of the mechanical properties such as damping of the metal panel. These features are described in detail in this paper. The real-time ARNC experiments were carried out on a paved road with a test car, and their results are discussed. Contributions of reference signals and the effects of road surface conditions and vehicle speeds were also investigated.