Chapter 1 Introduction
1.1 Motivation
Due to the complexity and the number of sources, control of noise inside car cabins presents a challenging problem. The complexity continues to increase with the development of lighter more fuel-efficient automobiles, which are more susceptible to low frequency vibration and noise. The improvement of noise reduction technologies in the automobile industry will likely remain an important area of practical research, since passenger comfort is key to competition between manufacturers. Low frequency noise in automobiles is primarily caused by a combination of the disturbances due to the firing of the engine and the interaction between the tires and the road surface. Conventional passive noise absorption techniques work well at higher frequencies but are not effective at the low frequency range because of the long wavelengths associated with these frequencies. Fortunately, this is exactly the range in which active control technology demonstrates its best results [1-4]. Also, due to variations that may occur in the manufacturing process from vehicle to vehicle, there is no guarantee that a conventional passive solution designed for one vehicle will be as effective for another sample of the same vehicle type. This problem of variation is overcome with active techniques, since an appealing characteristic of the technology is that it is adaptive. Therefore, a good approach to controlling low frequency interior automotive noise would be to add the benefits of an active noise control (ANC) system

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to a preexisting, more classical, passive noise reduction technology in order to lower sound levels with little penalty in terms of space requirements. The implementation of such an active system is far from straightforward, since the structural-acoustic system is very complex. No clear description of the acoustic and vibration disturbance path in automobiles can be found in the literature today. Nevertheless, as it will be described