**6. Conclusions**

**Figure 16.** The sound absorption coefficient in the function of sound frequency for the composites with SOF sub-micro-

**Figure 17.** The sound absorption coefficient in the function of sound frequency for the composites with HS sub-microfi-

The silane modification of sub-microfibres obtained from straws gives the best effect for SRFF straw because of the highest increase in tensile stress and in sound absorption coefficient. The sound absorption coefficient of the SRFF-1 composite was the lowest among all composites with sub-microfibres without silane modification but absorption coefficient of SRFF-2 composite is the highest in the whole range of investigated frequencies. The increase in stress at maximum load of the SRFF-2 composite is the highest among other composites with sub-

microfibres from straws and is 38.8% higher than that of SRFF-1 composite.

fibres: 1, SOF-1; 2; SOF-2.

234 Composites from Renewable and Sustainable Materials

bres: 1, HS-1; 2, HS-2.

The study included not only development of a method for manufacturing sound-absorbing composites, but also determination of the requirements for the characteristics of reinforcing materials, optimal from the point of view of sound absorption. A comparison was made to the raw material used, and its treatment, with respect to the sound absorption of the composite as well as its strength. Silane modification does not significantly affect change fibre dimensions, and the manner of its implementation and the origin of the fibres show no effect on the sound absorption of the composite. The use of fibres coated by silane preparation is preferred from the viewpoint of mechanical properties. Tensile strength of composites containing fibres coated with the silane preparation is about twice higher than that of the composites with the unmodified fibres.

Composites could be prepared from the layers of needled nonwoven formed from the matrix fibres or a mixture of matrix fibres and reinforcing standard fibres and layers of straw or ultrashort/ultra-fine fibres alternately arranged. In the case of composite reinforced by straw, the presence of the standard fibres is inadvisible. The addition of the straw, independent of straw type, increases the values of the sound absorption coefficient because of the additional voids caused by the particles of straw. Both grinded sunflower straw, grinded corn straw and random cut wheat straw reduce the tensile stress at maximum load of the composites, but increase the sound absorption even to 0.8–0.9 in some frequency ranges.

Studies have shown that the composite structure produced by compressing the fibrous layer system is most advantageous from the point of view of its sound absorption when the standard fibres are as a reinforcement with ultra-short/ultra-fine fibres. The addition of sub-microfibres in an amount of 10–20% by weight of this system gives the significant effect on the composite sound absorption in the entire tested range of sound frequencies, i.e., up to 6400 Hz. The rapid increase of the absorption coefficient to the value of approx. 0.8 is in the range of 500–4000 Hz, followed by a further but slower increase is observed. If, however, as ultra-short/ultra-fine fibres will be used sub-micro/nanofibres, obtained by additional mechanical treatment of submicrofibres, giving further fragmentation, then this relationship may adopt a different character. For a composite containing approx. 50% of the standard fibres and 20% of sub-micro/ nanofibres sound absorption coefficient reaches a high value of 0.75–0.80 even at lower frequencies, i.e., from 2500 Hz and maintains at such a high level at higher frequencies.

Studies have shown that the best effect on the sound absorption of the composite, i.e., high sound absorption coefficient over a wide frequency range, may be obtained using next the standard reinforcing fibres the ultra-short/ultra-thin fibres with a minimum dimensions, because the larger the surface of the fibres, the greater the impact of the sound wave with fibres, and therefore greater its attenuation.

The developed composites are thin (several mm thick), lightweight, rigid, washable, high sound-absorbing of middle and high frequencies, which can be produced from waste fibres and straw, and may be designed to eliminate sound from various sources, for example in devices, vehicles and other means of transport, and for sound absorption in the rooms.

These ecological composites have characteristics in line with European trends and can be used as a stand-alone screen or to fill sound-absorbing panels, as a sound-absorbing contribution under the plaster in the construction of walls and ceilings suspended or in silenced car door, hatch, wheel arches, headliner and trunk.
