# Investigation on the Acoustical Transmission Path of Additively Printed Metals
## 1 Introduction  
When it comes to the machines being used in the everyday life of co-workers, the engineer has to consider many parameters to avoid any kind of injury to the user. Therefore, it is necessary to calculate not only how rigid the components have to be to withstand the emerging static forces, but also the dynamic behaviour.  
Rigid bodies are generally capable of transferring structure-borne sound and vibration, thus inducing vibration into the connected machine parts. At the boundaries of the machine connecting it with the environment, this results in radiation of sound and vibrations in the direct surroundings of the machine.  
A machine element is generally defined by the selection of material and its functional and geometrical shape. These parameters affect the damping properties due to both the stiffness of the material itself, influencing the frequency behaviour, and the outer shape of the element, acting as an impedance difference.  
So far, the impact of the shape of the inner geometry, filled with powder, has not been evaluated completely. Different studies show an influence of the degree of filling, but the assertion is not fully clear, and some are even contradictory. Whilst powder-based dampers are mostly considered as frictional dissipation elements, the influence of impedance differences in combination with frictional dissipation has not been evaluated completely.  
In order to construct and build more energy-efficient systems, lightweight constructions play an important role. To make machine parts such as transmission gears lighter, one strategy could be to reduce material usage by integrating cavities. Even if an engineer ensures that there are no static mechanical problems caused by these material savings,the savings could lead to a massive modification of the stiffness of machine elements. An integrated damping behaviour could be useful for lightweight systems if it acts as a sink for dynamically caused issues.  
The aim of this research approach was to implement mechanical damping behaviour in the transmission path of structures and constructions. To reach this goal, it was necessary to either increase the damping factor of the structure by using different material properties of alternative materials or by adding geometrical impedance elements. In this approach, different geometric shapes were investigated for their damping; and the samples were manufactured using SLM with 100% filling. The research of [PERSON] and [PERSON] shows that with a filling volume of 100% of a particle damper, there is hardly any system damping left [1]; [PERSON] shows the opposite [2]. The discrepancy is intended to prove the improved damping and to investigate the different geometric shapes. In addition, [PERSON] et al. highlight that the volume dimensions in the direction of the oscillation movement are particularly decisive for absorption [3].  
Information on the development of damping by changing the inner geometry was gathered by performing the following steps: First, the state of the art concerning powder-based dampers and particle dampers was illuminated briefly. In the next step, the design of measurements and test plan were evaluated, and the measurement setup was specified. In the following section, the calculation of the comparison parameters for the measurement setup is discussed. In Section 4 the calculated parameters, to qualify the damping of the samples, are composed for further comparison. After a short evaluation of the validity of the gathered data in the last section, a conclusion about the influence of inner shape is drawn.