## 4 Preliminary Investigations on Textile-Based Ccld  
While the investigations so far focussed on foam-based compressible viscoelastic layers, other material groups are also considered. The already described criteria (Section 2) were applied for a set of alternative materials. It has been found that polyester needle-punched nonwovens (PNPN) in particular shall meet these requirements and also exhibit strong compression-dependent shear properties. Thus, first experiments to determine the shear properties were conducted on cylindrical specimens (diameter: 25 mm, height: 3.4 mm) of a commercially available PNPN material using a rotatory rheometer (MCR 502, Co. [PERSON][PERSON]_) in plate plate mode with the same set-up as used for the characterisation of the foam materials [35]. The average diameter of the polyester fibres was determined experimentally at 25 \(\upmu\)m (Figure 7), the density of the sample at 160 kg/m\({}^{3}\). Figure 8 shows a comparison of the shear stress versus shear deformation for the PNPN material (blue curves) and the previously introduced foam D (red curves). To compare the materials with each other, three pressure levels were selected: 5 kPa (Figure 8A), 13 kPa (Figure 8B) and 49 kPa (Figure 8C). This resulted in a foam compression of 19%, 80% and 89% and a PNPN compression of 5%, 10% and 20% respectively. Thus, the same pressures generate significantly less compression of the PNPN. When examining the results obtained, some interesting aspects become apparent. First, the dissipated energy per vibration cycle--represented by the enclosed area--is for PNPN much higher than for foam D. In order to illustrate this, the damping ratio (dissipated energy of PNPN divided by dissipated energy of foam D) is given in Table 3. Especially with low compression (5 kPa) and small shear deformation (1%), the difference is particularly huge (factor 21.5). At high compression  
Figure 8: Shear stress versus shear deformation for PNPN in comparison with foam D for three different shear deformation amplitudes at a compression pressure levels of (**A**) 5 kPa, (**B**) 13 kPa and (**C**) 49 kPa. The area enclosed by the curves represents the dissipated energy per vibration cycle and the slope of major axis the shear stiffness. The curves represent the raw data obtained using the rotatory rheometer configuration mentioned in Section 2. This result presentation was selected instead of listing storage and loss shear moduli due to the noticeable non-linear behaviour of the tested nonwovens. Figure 7: Optical microscope image (bright-field) of the analysed polyester needle punched nonwovens (PNPN) material with clearly visible randomly arranged polyester fibres. Focus stacking has been used for a image with a greater depth of field. (49 kPa) and large shear deformation (10%) the dissipated energy of PNPN is still factor 1.6 higher than that of foam D, characterised by the highest storage and loss moduli among the tested foams. Secondly, both materials reveal a strong dependence of shear stiffness from compression, expressed by the increasing slopes of the main axes of the ellipses, which can be an advantageous feature for system detuning in resonance-like conditions. As for the damping ratio, a stiffness ratio is shown in Table 3. Since the non-linear behaviour of the PNPN entails that the use of the storage module as a measure of stiffness is no longer valid, the slope of the major axes was chosen as a measure of the average stiffness and these values were divided by each other. While considering the deviation from ideally viscoelastic behaviour taking into account the stress-deformation curves (Figure 8) of the PNPN, following relations can be preliminary identified:  
* the decreasing compression level causes that the PNPN has an increasing frictional character. This can be explained by the reduction of the normal forces and through it by the reduction of the stiction forces,
* with increasing shear deformation also the influence of the frictional damping component on the overall damping behaviour increases.