Chapter 1 Introduction
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 industrywill 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 arenot 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 vehiclewill 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
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 describedin section 1.3, practical application of active control to power train noise has shown very good results, and has been applied in the industry. Control of noise induced by the interaction of the tires and the road surface also shows promise for practical research in active techniques in the future. The present research investigates the potential of a broad band multi-reference control system, whichimplements state-of-the-art lightweight, compact piezo-electric acoustic sources for the reduction of interior noise in a sport utility vehicle. Details concerning the mechanics of the source will be given in chapter 5. As it will be explained in section 1.3, most of the previous research in this area has been performed on light Japanese or European cars. The research discussed in this thesis wasapplied to a Ford Explorer, which already utilizes extensive passive noise control techniques unlike light Japanese and European cars. Thus, the initial acoustic field inside the cabin is heavily damped, even at low frequencies.
1.2 Active Noise Control
Before starting any discussion on active noise control strategy, it is important to define the application of the technology in terms ofsound frequency. In general, noise problems exist in the entire human audible range of frequencies (20 Hz to 20 kHz). For disturbances above 1000 Hz, the noise control strategy is typically to use passive techniques. Passive control techniques are based on the relationship between the wavelength of the noise and the thickness of the passive material. This means that control of low frequency noise,having longer wavelengths, would require very thick absorption materials. This is not feasible in an automobile due to the dimensions of the cabin. Active noise control, on the other hand, uses acoustic sources to generate a secondary sound field that destructively interferes with the original undesired sound field. The performance of this technique is also wavelength dependant. The lower the
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