**3. Results**

Comparatively, the pre-pressed nonwovens, the composites made of nonwovens, and the layered systems composed of pre-pressed nonwovens and composites were used for the acoustic measurements.

## *3.1. Nonwovens*

Nonwovens were obtained with an area weight of 180 g/m2, differing in fiber composition: fiber composition I—20% LI fibers and 80% PLA fibers, mixed together—"LI/PLA", fiber composition II—100% PLA fibers—"PLA".

#### *3.2. Pre-Pressed Nonwovens*

The obtained LI/PLA nonwoven was pre-pressed on the nonwoven press machine at different conditions. The parameters of the pre-pressing process and the characteristics of the nonwoven sound-absorbing materials obtained after the pre-pressing process are presented in Table 1.


The dependence of the sound absorption coefficient on the sound frequency for individual variants of the pre-pressed nonwovens is shown in Figure 4. For single, very thin layers of pre-pressed nonwovens, 0.8 to 1.75 mm thick, an increase in sound absorption is observed with increasing sound frequency. In the case of sounds with frequencies up to 4800 Hz, the highest similar values of sound absorption coefficient were obtained for the 2N and 3N pre-pressed nonwovens. Higher frequency sounds are better absorbed by the 1.08 mm thick 3N pre-pressed nonwoven, presenting an almost plastic structure, than by the 1.75 mm thick 2N pre-pressed nonwoven with a compact fibrous and rigid structure. Nonwoven 3N shows the highest value of the sound absorption coefficient, i.e., 0.44 at the sound frequency of 6400 Hz. The other three pre-pressed nonwovens show a lower absorption. In all frequency ranges, the dependence of the sound absorption coefficient on the sound frequency is similar for these three nonwovens. The maximum value of the sound absorption coefficient, obtained at 6400 Hz, is 0.23 for 1.55 mm thick 1N nonwoven with a fibrous structure, 0.20 for 1.24 mm thick 5N nonwoven with a compact/fibrous structure, and 0.17 for 0.8 mm thick 4N nonwoven with an almost plastic structure but with a mesh surface.

**Figure 4.** Sound absorption coefficient of individual pre-pressed nonwovens.

The combination of successive nonwovens with each other causes the thickness of the resulting absorbent systems to be greater than that of the individual layers, but does not mean adding up their individual sound absorption. It can be seen from Figure 5 that adding three nonwovens, i.e., 2N, 3N, and 4N, to the nonwoven 1N successively increases the value of the absorption coefficient by a value corresponding to the individual nonwovens in a given frequency range. The addition of another layer, i.e., a 5N nonwoven, no longer increases the sound absorption by a value corresponding to this nonwoven, but rather only slightly. The contribution of the next added layer with specific sound absorption characteristics to the increase of a system's sound absorption coefficient depends on the resulting system structure and sound frequency. The same layer can show a different absorption as a separate layer, and a different one to the arrangemen<sup>t</sup> with another layer, because then, a new structure is created, which constitutes different conditions for attenuating the energy of the acoustic wave. As a consequence, the sound absorption of the system is different from that resulting from adding up the absorption of both layers. Table 2 shows that the tested pre-pressed nonwovens, very thin and with low absorption, can be combined into multilayer systems in order to increase sound absorption in the frequency range of 2500–5500 Hz in relation to the total absorption of the individual layers. The share of each next added nonwoven layer characterized by a specific sound absorption in the increase in the system's sound absorption coefficient depends on the sound frequency and on the sound absorption of the system without this layer.

If both previous graphs, Figures 4 and 5, are compared, it is possible to observe that the shapes of the curves for individual nonwovens are completely different when the nonwovens are together. For individual nonwovens, the curves are practically flat, with an important increase for high frequencies. However, for the nonwovens together, the curves are concave, more similar than a composite curve, which have higher values for a wider range of frequencies.

In the case of sound-absorbing porous materials, a low-frequency sound absorption is higher if the material is thicker. A homogeneous material of high thickness or a layered material with layers of different structure can be used. Homogeneous material can be used with a thick material layer or a different layer structure.

**Figure 5.** Sound absorption coefficient of nonwoven layered systems.

**Table 2.** Comparison between sound absorption values for layered systems of pre-pressed nonwovens—sound absorption coefficient calculated (in black color) and measured (in blue color).


However, joining the layers of nonwovens, the thickness of the material can increase up to several cm, which is why, in this work, pre-pressed nonwovens were used. However, combining layers of nonwovens causes an increase in the material thickness of up to several cm, so, in this research work, the pre-pressed nonwovens were proposed.

The multilayer structures consist of several pre-pressed nonwoven layers with different acoustic characteristics, and are a promising material for noise reduction. Thanks to such acoustic systems, it is possible to increase the level of absorption and extend the frequency range of high absorption.
