# Investigation on the Acoustical Transmission Path of Additively Printed Metals
## 2 State of the Art  
Using the freedom of design which comes with additive manufacturing methods, it is possible to implement geometries that cannot be manufactured with conventional manufacturing methods. For the production of these 3D-printed parts, a powder-based laser manufacturing system can be used. The possibilities offered by 3D printing are widely known and described, for example, in the article by [PERSON] et al., where the use of 3D\(-\)printing technologies in terms of freedom of design, processability, and parameters is examined [4]. Moreover, it has to be assumed that different manufacturing methods will have a direct impact on the damping behaviour of materials. In addition to the manufacturing method, the impact of inner geometries in a metal bar is investigated. A selection of investigated samples with different damping behaviours are shown below. While the shape of the inner structure is changed in different samples, the ratio between powder chambers and solid material is constant over the different samples with inner geometries. The general principle of powder-based dampers using additive methods has been investigated in other studies. Several parameters for the design of powder dampers have already been developed. The damper itself, consisting of powder and the surrounding structure, is defined by their characteristics. [PERSON] and [PERSON] investigated the influence of particle size, the density of particle-filled space, the location of the particles and further parameters concerning the powder [1]. Other studies have shown the impact of the height of varying excitation forces [5] or the ratio of masses between the powder and rigid body. [PERSON] et al. provided a review of the development of particle dampers so far and about the theoretical configurations. Moreover, an overview of the existing design concepts for particle-based dampers is given in [6]. A selection of design concepts is shown in Figure 1.  
[PERSON] et al. summarised the methods for simulating and measuring the dynamic impact of powder-based dampers [7]. In their study, they showed the differences in measured outer geometries, and analysed frequencies for transient excitation and the used powder/particle size and classes of material. Most of the mentioned studies employed a transient excitation of a beam and the effect of the excitation was measured with accelerometers. The study by [PERSON] et al. [8], as an example of this measurement setup, used a shaker and accelerometer to analyse the damping capacity. The measurement setup is shown in Figure 2.  
In their study, they used simulation methods to analyse the frequency behaviour of particle dampers with longitudinally and transversally oriented inner geometries in beam-shaped samples. For the simulation of modal characteristics, they used a numerical approach to calculate the stiffness and damping of the bar. They evaluated this approach by comparing it with measurement data, while the variables in the mentioned expressions were unknown until then. They are investigating the amplitude spectrum of the acceleration of a beam sample with and without particles inside a cavity. They showed that the frequencies of a high acceleration amplitude are almost constant while the magnitude of amplitudes are significantly lower with particles inside the cavity. A few studies also used forces in the shape of the Dirac impulse for excitation by using an impulse hammer, [7]. The use of transient excitation is often limited in terms of amplitude because high-bandwidth and high-amplitude equipment is expensive and complicated to handle for small specimens. For this approach, the shape of the inner geometries will be investigated. This is highly relevant for mechanical construction because workplace requirements for employees lead to the necessity for machines and parts which avoid the development of unwanted vibrations and sound propagation. Moreover, the presented work investigated the bending vibration, because it is relevant in a variety of technical applications.