**4. Methods**

The thickness of composites was determined according to the standard procedure ISO 9073-2:1995 and their apparent density was determined as the ratio of mass to volume of samples. The mechanical properties of composites were estimated in unidirectional tensile test by means of testing machine type 3119-410 (Instron, UK) according to the standard ISO 527-4:1997. The sound absorption coefficient was determined according to the standard procedure ISO 10534-2:1998 within the frequency range of 500–6400 Hz. A small size impedance tube (Kundt tube) type 4206 (Brüel&Kjaer, Denmark) was used (**Figure 6**). The diameter of the investigated samples was equal to 29 mm.

**Figure 6.** The Kund device for acoustic measurements.
