The Use of Nearfield Acoustical Holography (NAH) and Partial Field Decomposition to Identify and Quantify the Sources of Exterior Noise Radiated from a Vehicle

Since powertrain noise sources are usually “hidden” within the engine compartment, it is difficult to use NAH to identify those sources and the associated partial radiation fields that together create the exterior noise field of a motor vehicle. Integrated Nearfield Acoustical Holography (INAH) has been developed to address this concern. INAH represents a combination of NAH, reference microphone selection procedures, and coherence techniques. The procedure entails sensing the sources inside the engine compartment by using an array of reference microphones, and then calculating the associated partial radiation fields by using NAH. A key factor in the success of this procedure is the selection of a good reference microphone sub-set. A selection procedure has been developed by combining condition number and coherence analyses. The partial field determination problem has been approached by using both partial coherence and Singular Value Decomposition (SVD) procedures. It has been found that the partial coherence technique maintains a closer relationship with the physical partial fields than does the SVD procedure. In the work discussed in this paper, INAH has been demonstrated through the identification of the hidden sources and the partial fields that contribute to the exterior noise field of a vehicle.