Bestimmung akustischer Kenngrößen an einem fahrzeugunabhängigen Prüfstand zur Modellierung der Schallentstehung und Schallausbreitung in Fahrzeugklimaanlagen

While in the past the noise of the engine and other ancillary components of vehicles decreased, other components like the HVAC system have become more and more important. In upper class vehicles the aerodynamic noise of the air-conditioning system at high level might be annoying to the passengers. Modifications made at the late stage of the development phase are often very expensive. Therefore, it is more profitable to carry out tests and acoustic measurements related to both the whole HVAC system and single components at the early stage of the conception phase. This doctoral thesis presents the theoretical background of sound propagation in duct systems. Methods for determining the transfer matrices of HVAC elements, termination impedances, reflection coefficients and sound source characteristics are introduced. In addition to that, empirical approaches for calculating sound source characteristics of induct elements are presented. The experimental determination of acoustic characteristics is a main aim of this work. Therefore, different test benches are needed. An important aspect of these test benches is the examination of test objects independent of any vehicle. With this requirement test benches for the application of whole HVAC systems as well as several elements (e. g. ducts, bends etc.) were developed and validated. For a plane wave propagation the sound field in a duct system can be derived using transfer matrices. The value of these matrices of various sections and built-in components were determined by using a transfer matrix test bench. Another important parameter of HVAC systems is the termination impedance of air openings inside the vehicle. It is fundamental to describe the proportion of sound being emitted into the interior of a vehicle. Another test bench was developed, accounting for the necessary source characteristics and providing the following features: a defined air flow of “silent” air and sound excitation by using a noise source. The emission characteristic values obtained during these measurements can be implemented into a calculation model in relation to parameters, such as flow rate or position of the valves. In addition to the fan, which is regarded as a primary sound source, flow-induced noise also occurs at valves, bends and air openings, and thus it also has to be taken into consideration for the description of the overall noise characteristics. Based on the results obtained by the measurements, a sound propagation model is derived. If this sound propagation is mainly one-dimensional, the acoustic transfer matrix model can be applied to simulate the transmission characteristics. Due to the research results obtained in this doctoral thesis, it is possible to implement the sound source approach into the transfer matrix description of duct systems. With the measurements results of the terminating impedances the sound propagation within an HVAC system from the fan through the flow ducts, including valves, bends and other elements, to the drivers position outside the system is possible. The characteristics of the several components are described by transfer matrices, terminating impedances and sound source properties that result from measurements.