Noise and Vibration Control of Combustion Engine Vehicles
Sammanfattning: Noise and vibrations have over the last two decades been regarded as significant environmental health problems. Regulations regarding acoustic as well as vibration levels have therefore become more stringent. This thesis embraces two different techniques to reduce unwanted noise and vibrations, spectral subtraction and active noise and vibration control. The applications treated for noise and vibration problems are mainly means of transportation driven by combustion engines as for example, helicopters, boats, and cars. All these vehicles have low-frequency noise and vibration problems which are difficult to solve by means of passive isolation, hence alternative methods must be sought. Two different scenarios are studied. First, the high noise level in the interior of the vehicle is accepted and the humans inside are equipped with headsets utilizing both passive as well as active noise control. If the means of transportation employs some kind of communication equipment, such as for a cellular telephone or an intercom radio, the noisy speech signal picked up by the microphone is cleansed by spectral subtraction, which is a non-linear filtering method employed in the frequency domain. In the second scenario the entire interior of the vehicle is subjected to noise and vibration reduction by means of active noise and vibration control. Active noise and vibration control is the art of reducing a primary sound or vibration field by interference with a secondary anti-field. The thesis focus on real-life applications which implies that a lot of measurements and practical difficulties must be treated for both scenarios, especially in the area of active noise and vibration control. In this area, the basic idea seems straight-forward, but implementing it in large and complex structures, such as vehicles, is extremely difficult, in particular if high attenuation is required.
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