**1. Introduction**

Composites are more and more popular products present in our lives. An extensive use of composites results from the diversity of their functional properties, what is connected with the combination of different components, diversed in terms of materials and forms, and with diversity of the final structures. Recently, the composites have been produced from renewable and sustainable materials. Renewable materials are natural, environmentally friendly and usually cheap. The composites produced from such materials are called 'green composites', but this concept is much broader and should concern their production and usage. Now the filling components in composites should be waste and the matrix material should be recyclable such as, for example, thermoplastics. Sustainability concerns three aspects: environmental, economic and social. The use of biomass, paper, fibres, wood as a waste filling material and bio-based thermoplastic polymers as a matrix can be the optimal material's solution [1].

The production of environmentally friendly composites on the basis of materials obtained from renewable resources is important for the economy and the environment. Recently, new materials on the basis of different plants are more and more popular and used in many industrial and life fields. Materials on the basis of straw, reeds, cattails and bent grass stalks are used in the ecological building sector [2]. The natural fibres can be used as a reinforcement or as a filling in composites, but also can give some new functions to them. Moreover, there is observed a growing trend towards replacing high-modulus reinforcing fibres with natural fibres [3–5]. Many technologies of composites based on different plants, natural fibres and even fibres isolated from plants are developed. The cellulose micro- or nanofibres, or micro-fibrils can be obtained through the chemical, mechanical, ultrasonic and enzymatic treatment of plants, such as jute [6, 7], soya bean source [8], wheat straw [9], soy hulls [10], rice straw [11], regenerated wood fibres [12] and canola straw [13].

Due to the great variety of plants, the properties of composites made of natural components are much diversified what favors different applications. Natural fibre-reinforced composites are used not only for construction but also for new uses, e.g., attenuation of sounds. Soundabsorbing composites can show a high degree of sound absorption, especially at high frequencies [14–19].

The commercially available porous sound-absorbing materials are usually fibrous. The fibres used are mostly synthetic, but recently, natural fibres are more and more popular as a raw material of the sound-absorbing products. Natural fibres are biodegradable and safer for human health than most mineral or polymer synthetic fibres. Introduction of the cellulose ultra-short/ultra-fine fibres prepared from different kinds of biomasses into functional composite structures causes increase in their sound absorption. Conversion of biomass to ultrashort/ultra-fine fibres perfectly fits into the current trends of cellulose nanostructures receiving by a top-down method. Depending on the size of the structures the obtained cellulose is suitably named, for example: microcrystalline cellulose, nanocrystalline cellulose or nanowhiskers.

Ultra-short/ultra-fine fibres, i.e., fibres not only extremely thin, but also extremely short, can be obtained by the enzymatic treatment of flax fibres and different kinds of straw. This form of fibres provides a greater surface area but causes application problems different from the case of electrospun filament fibres. A dust form requires direct screening of fibres onto the substrate. The use of ultra-short/ultra-fine fibres means larger area than in the case of longer fibres, in particular standard fibres. This form of fibres is advantageous from sound absorption point of view. The energy of the sound wave propagating in the material is reduced, and the internal energy of the material increases. Sound waves cause vibration of the fibres in the material and as a result of friction the created energy is converted into heat. A larger fibre surface promotes greater energy loss of the sound wave [16].

The virgin straw can also be used in sound-absorbing composites as an absorption enhancer. Independent of straw type, the values of the sound absorption coefficient increase because of the additional voids caused by the particles of straw and internal channels [20].

The aim of this work is to develop acoustic thermoplastic composites and the method for their production with the participation of the above-mentioned bio-based components. The effect of different kinds of natural materials on the acoustic composites was studied. Both waste natural fibres, straw and cellulose ultra-short/ultra-fine fibres, obtained from biomass can be used as an acoustic component increasing sound absorption of the composites.

In this work, the thermoplastic composites were obtained on the basis of textiles, i.e., nonwovens and above-mentioned natural materials in a thermal pressing process.
