Abstract
Road traffic noise amounts to roughly half of the overall ambient noise. Usual emission and inmission noise control techniques have not been enough to decrease significantly the noise annoyance by road traffic along the last three decades. Therefore, a new traffic noise assessment is proposed able to measure the contribution of each individual vehicle to the overall radiated noise. The system is based on two on board microphones, one for the engine noise and other for the rolling noise. The engine microphone is located under the hood, near the air intake. In order to relate the noise levels provided by these microphones in the vehicle near field with the usual levels radiated to the far field, it is necessary to characterize the noise reduction produced by the engine hood. Here, preliminary results on such characterization are presented. Insertion Loss (IL) and Transmission Loss (TL) have been measured in several locations around the vehicle. In both measurements, a loudspeaker (close to the engine of the vehicle) was used as a noise source. Besides the microphone inside the engine compartment, another microphone is needed outside the vehicle. The responses between the loudspeaker and both microphones were measured using the Maximum Length Sequence (MLS) method. The difference between the log-spectra in the outside microphone, with the engine hood open and closed, provides the IL. On the other hand, the difference between the log-spectra of the inside and outside microphones yields the TL. Both IL and TL spectra in third-octave bands between 20 Hz and 5 kHz and overall noise reduction levels, were measured. Preliminary results show that noise reduction provided by the engine hood depends strongly on the microphone location, varying from 16 dB at the vertical position to 27 dB at the lateral position. Therefore, a more accurate measurement of the engine hood noise reduction is required, including the angle dependence (a directivity function).
Keywords: engine noise, vehicle noise, engine hood, Transmission Loss, Insertion Loss