Advanced measurement and modelling methods for noise source analysis

Sammanfattning: In daily life sound is a vital dimension for the perception of the world around us but can in some cases be considered as annoying, irritating, or even harmful. In these cases the sound is labelled as noise and can be addressed in two ways, either by insulating the noise problem from the receiver (person) or by altering the source. The work performed in this thesis has been focusing on the latter approach. To be able to alter the source, knowledge about the physical properties governing the sound generation process is essential. Advanced measurement and modelling methods have been applied in the study of two different cases of noise generation.Milling machine noise: The first study addressed sound generating vibrations in milling machine operations which are a common operation in e.g. the automotive and aerospace industry. Large metal work pieces are reduced to a fraction of their original weight when creating complex thin structures. During these operations it is important that unwanted behaviours such as excessive tool vibrations (chatter) can be avoided. Chatter causes poor surface finish and/or material damage and can expose machine operators to annoying and/or harmful noise levels. In order to predict process parameters for a chatter-free milling operation, knowledge of the properties of the dynamic system are essential. To improve the possibility of measuring milling machine tool vibrations or any other rotor vibration with a high accuracy a method for single beam Laser Doppler Vibrometry (LDV) measurements on rotating spindles was developed. The method solved two major problems using LDV on rotating targets, speckle noise and cross talk. To analyse the dynamic response of a rotor and its speed dependency, a method based on inductive displacement measurement, electromagnetic excitation, and FEM was developed. The measured dynamic response and the simulations of the studied milling machine, revealed the ball bearings as the weakest link capable of causing chatter vibrations and noise at high rotational speeds.Friction induced noise: The second study addressed problems regarding annoying sound generating vibrations in car door sealing systems and how these could be predicted and simulated for future car models. In the design process of a car door weather strip seals different conditions and demands must be considered. The primary goal of the seal is to act like a flexible barrier and protect the door/frame joint. The seal should prevent e.g. water and dust from entering the compartment and insulate the compartment from temperature and sound pressure differences between the two sides. Different types of seal geometries and rubber material can be used to achieve this. The seal stiffness affects the dynamic behaviour of the door and could result in squeak and rattle problems if not designed correctly. Relative displacement between the door and the seals can also be a source to squeaking noise. A common problem in winter conditions at sub-zero temperatures is when humidity on the seal surface freezes and cause the seal to stick to the door, with the risk of ripping the seal when the door is opened. In order to avoid this, different kinds of consumer lubrication products can be applied to the seal surface. By doing this the contact conditions between the door and the seal will change. To analyse the affect of different contact conditions and relative displacement, measurements and simulations of a seal segment compressed by a metal plate have been performed. The study showed that different contact conditions can affect the desired seal function by altering the resulting seal shape during compression, and that the energy relaxation is an important aspect when establishing the long term stiffness. A stick-slip phenomenon generating audible sound was generated by translating a metal plate along a car door seal. A difference, governed by the surface properties of the rough metal plate, between the static and the kinetic friction allowed the seal to stick to the translating surface and release its charged elastic energy in a sliding repelling motion creating an audible sound pulse. The frequency of this stick-slip oscillation was found to be linearly dependent on the plate speed and the static friction was found to be dependent on the plate speed. FEM simulations confirmed that a difference between the static and the kinetic friction could result in a stick-slip vibration.