*3.3. Composites*

Variants of the composites obtained from LI/PLA and PLA nonwovens are presented in Table 3.

Composites obtained according to variants 1, 2, and 3 were made of a nonwoven under conditions differing in system temperature, and this factor influenced the structure of the composite. By changing the temperature of the pressing process in the range from 165 to 180 ◦C, we changed the structure of the composite from fibrous to plastic. Differences in the sound absorption of the composites produced resulted from their structure. The values of the sound absorption coefficient for the individual sound frequencies are presented in Figures 6–8.


**Table 3.** Variants and structure characterization of the composites.

**Figure 6.** Sound absorption coefficient of composite 1.

**Figure 7.** Sound absorption coefficient of composite 2.

**Figure 8.** Sound absorption coefficient of composite 3.

For each sample, two different curves, depending on the receptor surface of the sound waves (two composite sides), are presented. For composite 1, Figure 6, the sound absorption coefficient is higher for middle and low frequencies than for the composites 2 and 3. It is more advantageous if the sound wave strikes the composite from the plastic side than the fibrous side. The highest values of sound absorption coefficient are above 0.9 at 6000–6400 Hz. The results for composite 2, Figure 7, show a clear difference between the sound absorption of surface 1 and 2. Surface 1, more plastic, presents a better structure for sound absorption at higher frequencies than surface 2, porous plastic. In low frequencies the difference is not so big. This sample is interesting because the maximum coefficient value of the plastic surface receptor is very high at 6400 Hz, between 0.9 and 1. For composite 3, Figure 8, the values of the sound absorption coefficient are similar, regardless of which the side of the composite faces the sound wave. This fact results from the similar structure of both surfaces of the composite. The maxim coefficient is lower than for sample 2's maxim coefficient. Generally, composites with a different structure for both surfaces give better sound absorption, and if the plastic surface is directed to the sound, the values of the sound absorption coefficient are higher.

Another important parameter of the composite is the profiling. Composite 4 was made on the basis of a blended nonwoven, but located between layers of PLA nonwoven, which, after compression, gives rigid, smooth plastic surfaces. Then, the modifications were used to profile the composite 4 plate, as shown in Figure 9. The results below show the influence of the surface profiling on the sound-absorption properties. Four samples were analyzed. The schematic view of the samples is shown below. The dependence of the sound absorption coefficient on the sound frequency for the profiled composite is shown in the diagrams in Figures 10–13. The composite plates were tested with the left side and right side facing the sound wave, respectively. From assessing the effects of profiling the composite plate, it can be seen that the proposed modifications have a beneficial effect on sound absorption. A two-sided flat plate shows the lowest values of the sound absorption coefficient in the entire tested frequency range, Figure 10. However, profiling a concave in the plate and directing it with its front face to the sound wave causes an increase in sound absorption, Figure 11.

**Figure 10.** Sound absorption coefficient of composite 4a.

**Figure 11.** Sound absorption coefficient of composite 4b.

**Figure 12.** Sound absorption coefficient of composite 4c.

**Figure 13.** Sound absorption coefficient of composite 4d.

Taking into account the published results of previous studies [21], which indicated the validity of the use of convexity on the back of the composite plate, which was confirmed in Figure 12, the absorption of the plate with concavity was measured comparatively. The results showed that convexity on the plate is more advantageous than concavity, Figures 11 and 12. In the case of a plate with convexity, the flat side facing the sound wave achieves the greatest sound absorption among the tested profiled plates, and the value of the absorption coefficient is equal to 1 for sounds with a frequency above 5500 Hz, Figure 12, Table 4. Among the tested profiled samples, a plate with convexity on one side has the greatest thickness over the entire area. This favors an increase in sound absorption. In addition, when such a sample is positioned with the flat side to the sound wave, and there is a convexity on the other side, an air space is created between the sample and the wall of the measuring tube in which resonance may occur. Under the conditions of use, such an orientation of the sample with a convexity in relation to the wall will be most advantageous. In the case of a concave–convex plate (Figure 13), where it does not matter

which side it faces the sound wave, the values of the sound absorption coefficient are higher than for a concave–flat, flat–concave plate (Figure 11), or convex–flat plate (Figure 12), and lower than for a flat–convex plate (Figure 12).


**Table 4.** Comparison between the sound absorption values for variants of composite 4.

The homogeneous porous material is not a good and practical low frequency sound absorber, as it would have to be extremely thick or very far from the back boundary surface. For example, at 500 Hz, the total wavelength is 0.688 m, so the porous material would need to be approximately 0.172 m from the back boundary surface to meet the 1/4 wavelength requirement for significant sound absorption. The lower the sound frequencies, the greater the material thickness/distance should be. In order to improve the sound absorption at low frequencies, the material thickness can be compensated for by air space in the rear. In studied porous composites, periodic inclusions have been employed to significantly enhance the sound wave manipulation abilities, Figures 2 and 9. The 4c and 4d profiled composites having the convexities at the rear provide just such an air space at the rear on the stiff wall side, and therefore exhibit better sound absorption towards lower frequencies than composites that are flat on both sides or flat–concave. Taking into account the front surface of the composite, when the back surface is flat, the best sound absorption is provided by the surface with convexities, then with concavities, and the worst by a flat surface. Sound absorption, apart from the thickness of the material, is also influenced by the surface area of interaction with the sound wave, the largest surface area is provided by convexity, then concavity, and the smallest one by a flat surface. The porosity of the material is also an important factor, and the convexity promotes increased porosity. A detailed presentation of the influence of individual factors on the obtained effect of sound absorption requires model considerations, which, in the case of fiber-based materials, and especially natural waste fibers with wide variation in thinness and length, and the sophisticated profiling of the composite plate, is an extremely complex issue [38,45,46]. It is known that resonant inclusions embedded in the soft matrix are conducive to the effective conversion of long longitudinal sound waves into short shear waves, which are subsequently absorbed. The literature reports that in the case of a periodically voided viscoelastic coating made of soft rubber embedded with a layer of cylindrical voids of infinite length, uncertainty in the geometric parameters has greater impact on the resonance frequency of the voids and sound transmission through the coating than the uncertainty in the material properties [47].

#### *3.4. Pre-Pressed Nonwovens and Composites*

In order to verify the possibility of obtaining an increase in sound absorption, systems consisting of pre-pressed nonwovens and composites in various combinations were tested. Composite 3 (described in Table 2 as "8xLI/PLA") was joined in a two-layer system successively with individual pre-pressed nonwovens. Successive material samples were stacked on top of each other without space between them, and put together into the measuring tube. The scheme of the arrangemen<sup>t</sup> of layers, where a nonwoven is situated on the side of the sound wave, and the composite on the back, is shown in Figure 14, and the sound absorption of such a system is shown in Figure 15. The scheme of the

arrangemen<sup>t</sup> of layers, where the composite is situated on the side of the sound wave, and the pre-pressed nonwoven on the back, is shown in Figure 16, and the sound absorption of such a system is shown in Figure 17.

**Figure 14.** Schematic view of the sample's position inside the tube. From the left: one pre-pressed nonwoven—one composite. Both layers are together without space between them inside the tube.

**Figure 15.** Sound absorption of composite 3 with one pre-pressed nonwoven layer in front.

**Figure 16.** Schematic view of the sample's position inside the tube. From the left: one composite—one pre-pressed nonwoven. Both layers are together without space between them inside the tube.

**Figure 17.** Sound absorption of composite 3 with one pre-pressed nonwoven layer behind.

The graphs of the dependence of the sound absorption coefficient on the sound frequency, Figures 4, 15 and 17, show that the combined systems show greater absorption than the individual layers separately, which results from the increase in the thickness of the final absorbing material and depends on the structure and, in consequence, on the absorption of the component layers. In the "pre-pressed nonwoven + composite" system, the 1N nonwoven and the 2N nonwoven provide the best effects. In such a system, the layer on the side of the sound wave is more fibrous, and on the other side, the layer is a porous composite. In the case of the "composite + pre-pressed nonwoven" system, the 2N nonwoven also provides the best effect. As shown in Figure 4, among the nonwovens tested, this nonwoven, next to the 3N nonwoven, shows the highest sound absorption in the sound frequency range up to 4800 Hz. The composite layer forms a better soundabsorbing system with a more fibrous and compact layer than with a more plastic layer. The comparison of the sound absorption coefficient for both systems, with the example of composite 3 and 2N pre-pressed nonwoven, i.e., 3-2N and 2N-3, is shown in Table 5. The results show that it is more advantageous to position the composite layer from the sound wave side, and the nonwoven as the back layer. Then, the absorptive surface of the system is plastic and the back is fibrous, and as shown in the earlier Figures 6 and 7, this structure, with more plastic surface, promotes sound absorption [48].



Composite 3, with a thickness of 5.5 mm, shows a sound absorption very similar to that presented by Hao et.al. [49] for kenaf/polypropylene nonwoven composites with a thickness of 6 mm. The values of the sound absorption coefficient of these composites at high frequencies are high, about 0.9 at 6400 Hz. The addition of a pre-pressed nonwoven, with a thickness in the range from 0.8 mm to 1.75 mm, to the tested composite has the effect of increasing the sound absorption in a wide sound frequency range. For both two-layer systems, the values of the sound absorption coefficient increase with increasing sound frequency, and begin to stabilize at about 6000 Hz. The results of absorption are very good (above 0.7) at higher values of the tested frequency range, i.e., from 3750–4230 Hz to 6400 Hz for the "pre-pressed nonwoven + composite" systems, and from 3340–4000 Hz to 6400 Hz for the "composite + pre-pressed nonwoven" systems. Separate composite 3 reaches a value of sound absorption coefficient of 0.7 at frequencies from 4600 Hz to 6400 Hz. At the highest sound frequencies, the sound absorption coefficient for both kinds of systems is around 0.9 or even higher. At the lower sound frequency range, the absorption of the system increases by a value approximate to the absorption of the added layer; the higher the frequency, the smaller the increase in absorption.

An increase in the sound absorption level can be obtained by combining a fibrous material, e.g., a pre-pressed nonwoven, with a composite plate acting as a rigid membrane, which can also extend the range of high absorption. The absorber then consists of two types of material, and there are probably two mechanisms for damping the sound wave in different frequency ranges. A composite layering sequence in a multilayer structure also plays an important role, the most absorptive are the systems where the composite as a rigid material is on the side of the incident sound wave, and the more porous is on the back. All proposed layers differing only in pressing conditions are produced on the basis of the same LI/PLA nonwoven fabric, which simplifies the production process and makes it more economical.

In order to improve the results for low frequencies, the next tests were made with a combination of composite and pre-pressed nonwoven layers. The scheme of the arrangement of the layers, where the composite is located on the side of the sound wave and the individual nonwovens are placed on the back, is shown in Figure 18, and the sound absorption of such a system is shown in Figure 19. The values of the sound absorption coefficient increase as the frequency increases, and then stabilize. As more nonwoven layers are added, the results of absorption increase, but less and less. This is because the total thickness increases, and the values of the absorption coefficient of the final multilayer system result from the structure, and consequently from the absorption of the individual layers. Sound absorption of 0.9 for the system (3-1N-2N-3N-4N-5N) with a total thickness of only 11.92 mm occurs in the frequency range from 2700 Hz to 6400 Hz, and 0.7 from 1850 Hz to 6400 Hz, which is a very good result. The literature [50] states that, for bilayered nonwoven composite with the thickness of 12.02 mm, the absorption is 0.92 at a peak at 1500 Hz, and 0.7 in the range of about 1000–3500 Hz. The more layers the system has, the more its absorption is close to 1.0 and the range of highest absorption is extended towards lower frequencies. This means that this combination will have a good absorption for almost all frequencies. Then, this material could be used in a wide variety of applications. Adding the next layers to the system increases the maximum sound absorption and extends its frequency range. A nonwoven, more fibrous, and more open structure absorbs better low frequency sound, and a composite, more plastic, and rigid structure absorbs better high frequency sound, [51]. Combining such structures into one system extends the range of high sound absorption.

**Figure 18.** Schematic view of the sample's position inside the tube. From the left: one composite— pre-pressed nonwovens. All layers are together without space between them inside the tube.

**Figure 19.** Sound absorption of composite 3 with the pre-pressed nonwoven layers behind.
