Determination of the near-field-acoustics of primary vehicle sound sources in relation to indoor pass-by noise testing for the verification of a virtual acoustic vehicle model

The exterior sound emission of a vehicle is an increasingly important criterion for the homologation of road vehicles. The latest limit value of 74 dB(A) is already a major challenge for car manufacturers and the EU is planning an even lower limit of 68 dB(A) in the near future. According to DIN ISO 362, the vehicle exterior noise is determined by a standardized procedure – the „Measurement of noise emitted by accelerating road vehicles“, also referred to as outdoor vehicle pass-by. In this contribution, a new approach for determining the vehicle exterior noise in an early stage of product development is presented. The aim is to transfer the measurements from outdoor testing facilities to acoustic roller test benches and furthermore into an entirely virtual environment. Therefore, a virtual acoustic vehicle prototype is verified and validated by measurements on a roller test bench and on a real test track. In a first step, a pass-by test series, using a linear microphone array and considering the vehicle as a point source is performed on the acoustic roller test bench of IPEK – Institute of Product Engineering at Karlsruhe Institute of Technology. In order to substantiate the virtual acoustic vehicle model, the contributions of the primary sound sources of the vehicle to the exterior noise have to be investigated, with the objective of a modular approach for the virtual acoustic prototype. For this purpose, the airborne transfer paths between the primary sources and the far-field of the vehicle are determined by reciprocal measurements. Therefore, a dodecahedral speaker applied with white noise is used. Furthermore, the sound pressure levels are measured in the near-field of the primary sound sources – the engine, all four wheels, the exhaust pipe as well as the frontal intake. Using the reciprocally determined airborne transfer paths, the contribution of the sound sources to the sound pressure in far field can be calculated. Based on the reciprocally determined transfer functions digital filters are computed. The digital filters are convolved with the audio signals which result from the near-field component measurements. Thus the acoustical properties of the vehicle can be reproduced. The processed audio data can be represented in real time by a 5.1 or a wave-field-synthesis framework which is part of a flexible audio-visual stereo projection system (FASP) at the Competence Centre Virtual Reality of Technische Universität Ilmenau. Index Terms – Near Field Acoustics, Simulated Pass-by Noise, Virtual Acoustic Vehicle Model URN (Paper): urn:nbn:de:gbv:ilm1-2014iwk-070:2