09NVC-0096 In situ PU surface impedance measurements for quality control at the end of an assembly line Emiel Tijs Microflown Technologies, the Netherlands Hans-Elias de Bree Microflown Technologies, the Netherlands HAN University, dpt. Vehicle Acoustics, the Netherlands Eric Brandão Carneiro LVA/UFSC (Federal University of Santa Catarina - Brazil Copyright © 2009 SAE International ABSTRACT With PU probes the sound pressure and acoustic particle velocity can be measured directly. Over recent years, the in situ surface impedance method, making use of such a probe, has proven to be an alternative to Kundt’s tube measurements for product development type of work. The in situ method can also be used for the purpose of quality control on the acoustic material, be it during manufacturing or assembly, ensuring the best possible way to monitor the practical effectiveness of the acoustic package designed earlier on. In order to assess the variance of the acoustic package material leaving the assembly line, a relevant number of samples were taken over time. The quality of both the headliners, and the passenger seats were measured, of 25 cars of the same type. The robustness of the measurement method will be discussed, and the results will be presented. INTRODUCTION The design of acoustic trim packages is an ongoing trade off between reducing noise levels, saving weight and saving costs. Therefore, the accurate acoustic assessment of the properties of trim materials as they are applied in real conditions in a cabin interior is a desirable matter. The effectiveness of acoustic materials might change during installation in a car, due to inconsistency of the supplied materials, or due to variations in the way the materials are mounted, and should preferably be measured after they are installed. Although Kundt’s tube and alpha cabin measurements are widely used in industry, they do not allow the complete and proper characterization of materials as applied in situ. Some in-situ methods are available, but none of them are suitable for usage inside the small and reverberant environment of a car. Up to now it is only possible to measure materials in a laboratory environment while there is no control of the actual acoustic performance after installation. The Microflown is an acoustic particle velocity sensor that is invented at the University of Twente in 1994 [1], [2]. In 1997 the sensor was commercialized. Over years the particle velocity sensor is used to determine the acoustic impedance. The first application was the measurement of the acoustic impedance of a horn loudspeaker with a PU- method. In the throat of a horn loudspeaker a pressure microphone (p) and a particle velocity sensor (u) was placed and the impedance was successfully measured [3]. Afterwards, two particle velocity sensors (in stead of microphones) where used in a standard Kundt’s tube. This technique showed also that particle velocity sensors could be used to determine the acoustic properties of materials in a Kundt’s tube [4]. At the University of Leuven, Belgium, the PU probe was used to determine the specific acoustic impedance at the surface of an absorbing material [5]. With this free field method several disadvantages of the Kundt’s tube are avoided. Disadvantages of the tube are the upper frequency limit, determined by the tube diameter, the fact that only the normal reflection coefficient can be obtained and that not all materials can be easily put in the tube. Material properties can also