NOISE REDUCTION OF THE ENGINE HOOD OF A VEHICLE

摘要

Road traffic noise amounts to roughly half of the overall ambient noise. Usual emission andrninmission noise control techniques have not been enough to decrease significantly the noisernannoyance by road traffic along the last three decades. Therefore, a new traffic noisernassessment is proposed able to measure the contribution of each individual vehicle to thernoverall radiated noise. The system is based on two on board microphones, one for the enginernnoise and other for the rolling noise. The engine microphone is located under the hood, nearrnthe air intake. In order to relate the noise levels provided by these microphones in the vehiclernnear field with the usual levels radiated to the far field, it is necessary to characterize the noisernreduction produced by the engine hood. Here, preliminary results on such characterization arernpresented. Insertion Loss (IL) and Transmission Loss (TL) have been measured in severalrnlocations around the vehicle. In both measurements, a loudspeaker (close to the engine of thernvehicle) was used as a noise source. Besides the microphone inside the engine compartment,rnanother microphone is needed outside the vehicle. The responses between the loudspeakerrnand both microphones were measured using the Maximum Length Sequence (MLS) method.rnThe difference between the log-spectra in the outside microphone, with the engine hood openrnand closed, provides the IL. On the other hand, the difference between the log-spectra of therninside and outside microphones yields the TL. Both IL and TL spectra in third-octave bandsrnbetween 20 Hz and 5 kHz and overall noise reduction levels, were measured. Preliminaryrnresults show that noise reduction provided by the engine hood depends strongly on thernmicrophone location, varying from 16 dB at the vertical position to 27 dB at the lateralrnposition. Therefore, a more accurate measurement of the engine hood noise reduction isrnrequired, including the angle dependence (a directivity function).