**4. Conclusions**

The combination of successive pre-pressed nonwovens with each other causes the thickness of the resulting sound-absorbing systems to be greater than that of the individual layers. 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.

Thermoplastic composites with different structures of both surfaces give better sound absorption than composites with the same surfaces. If the more plastic surface of the composite is directed to the sound, the values of the sound absorption coefficient are higher.

Shaping the composite plate has a positive effect on its sound absorption. In the case of a one-sided concave, it is better to position the plate with the concave side facing the sound and the flat side on the back. In the case of a one-sided convexity, it is better to place the plate with its flat side facing the sound, and the convex side at the back, then the absorption is the highest among the tested variants of profiled composites. For a composite plate with opposite profiles, i.e., concavity and convexity, no difference in absorption is related to the orientation of the plate relative to the sound.

The addition of a pre-pressed nonwoven to the composite, regardless of whether it is on the side of the incident sound wave or on the back, increases the sound absorption coefficient in the entire tested frequency range. However, it is preferable to have a composite on the sound side and a nonwoven on the back. Increasing the number of differentiated nonwoven layers in terms of structure means increasing the value of the sound absorption coefficient and broadening the frequency range of sounds most absorbed. The presented research is part of the work on sound absorbing composites on the basis of natural materials, and will be continued towards identifying the optimal composite structure. The proposed materials containing natural waste fibers will be an environmentally friendly, cheaper, and more sustainable alternative to traditional sound-absorbing materials. They could be used in the walls and partitions of vehicles, in buildings, or in door panels.

**Author Contributions:** Conceptualization, E.G.; formal analysis, J.P.d.A.; project administration, I.K.; supervision, E.G.; writing–original draft, J.P.d.A.; writing-review and editing, E.G. and M.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This work was partially supported by statutory research fund of the Institute of Material Science of Textiles and Polymer Composites, Lodz University of Technology, Poland, no. I42/501/4-42-1-1.

**Conflicts of Interest:** The authors declare no conflict of interest.
