Assessment of the Acoustic Absorption Characteristics of Layered Composite Structures Obtained from Plates with Lignocellulosic Coatings (I)

The paper aims to present research on the determination of the coefficient of acoustic absorption as a function of frequency, for composite monolayer plates (made of: MDF wooden dried agglomerated fibres with medium density, RWB – rebated wood from dried beech, PS extruded polystyrene) and multilayer (with outer coatings of: MDF, RWB and polymeric core of extruded polystyrene). The sound-absorbing behavior of such plates is highlighted in the cabins made of multilayer structures.


1.Introduction
Acoustic pollution represents an important component of environment pollution due to its noxious and disturbing character for the entire humanity and also for other beings in the nature.
Therefore, the primary objectives of current scientific research on combating noise pollution are focused on the following strategic directions of action [1]: -identifying potential sources of noise and of causes of noise pollution; -development of new composite materials with high sound-absorbing properties; -implementation of innovative technical solutions to prevent and reduce the effects of acoustic pollution (for example: sound insulation, acoustic correction, individual protection measures etc).
Revaluation of wastes by obtaining composite materials with good sound-absorption characteristics is important from a technical, economic and ecological point of view [2].
In the specialized literature there are numerous studies regarding the determination of the acoustic characteristics of composite materials, using different research and investigation methods [3 -12].
Considering as the priority direction of research of the development of new composite materials with high sound-absorbing properties, the present paper addresses the comparative determination of the sound-absorbing capacity of two composite structures stratified with outer coatings of different ligno-cellulose nature and extruded polymeric core. For this purpose, the experimental determination of the acoustic absorption coefficient for the materials used is considered.

Materials and methods
The laminated composite plates (sandwich type) are made of materials with outer coatings of lignocellulose nature and polymeric core (  The sound absorption coefficient is used to express the capacity of the tasted material to absorb the acoustic waves of different frequency [13]. The absorption coefficient of a material varies according to frequency and the angle from where the sound or the sound wave reaches the material [14]. In order to determine the absorption coefficient, five types of circular shaped samples with a diameter of 63 mm were taken from the composite plates ( These five types of structural samples are used to determine the medium sound absorption coefficient med  , in order to make the research on acoustic absorption properties, using the sound interferometer, and the method used is in accordance with the legal standards [15,16]. https://doi.org/10.37358/Mat. Plast.1964 For characterization of sound absorption behavior of multilayered plates, there were made two experimental models (cabins/boxes) of parallelepiped shape, M1 and M2, with four lateral faces (620x620x60 mm) and a cover (500x500x60 mm) ( Figure 3): The method of analytical calculation of the sound absorption constant for sound insulation, includes the following calculation relationships [8,10]: -the equivalent sourface with sound absorption treatment A, is calculated analitically with the relationship: where, Si is the surface with sound absortion treatment i; i  -is the coefficient of sound absortion of the surface Si ; -the equivalent surface without sound absorption treatment, A0: where, Sj is the surface without sound absorption treatment j; -the medium coefficient of absorption of the sound insulating cabin αmed, is characterised through relationship: -the sound absorption constant inside the sound insulating cabin, Rα: According to the UE directives and the national legislations, the materials are classified in classes of sound absorption (

Apparatus used
The sound absorption medium coefficient αmed of the composite materials with sound absorption properties was determined through the method of stationary waves, in 1/3 octave, using an impedance tube (Figure 4), in the experimental installation ( Figure 5), according to a standardized method [16]. The measurement device of the sound absorption coefficient is made of a tube which has, at one end, a movable stand on which the sample is attached, and at the other end, the acoustic source. Two identical microphones for measuring the sound pression are placed in three positions along the tube ( Figure 4). The frequency range of the measured results is from 0 to 3200 Mhz. Data acquisition and process method uses hardware-software Bruël & Kjaer PULSE Platform, type 7758 [18]. https://doi.org/10.37358/Mat. Plast.1964  The equipment for signal processing contains an amplifier and a system of Fourier analysis. To determine the sound absorption constant R, the transfer function method is used [16]. This method is based on determining the transfer function between the two microphones, and then calculating the average acoustic absorption coefficient αmed, according to frequency.

Results and discussions
Generally, the sound absorbing capacity of a composite material depends on several parameters, including the nature of the reinforcement material and the mode of placement.
In the case of these tested materials, it is considered that it was obtained a good sound absorption capacity if the value of the medium sound absorption coefficient tends to 1   , and the absorption percentage at this value should be mentained on a large frequency interval.
All data obtained through measurements were processed and then it was made the graphic of the dependency of the medium absorption coefficient according to frequency, for the frequency band 0 -3200 Hz, for all five analysed samples ( Figure 6). Analysing this diagram, we get the following results: -at the frequency : the medium sound absorption coefficient increases only for sample P5, for the others it decreases.
The plates with sound insulating treatment (samples P1 and P2) have got the medium coefficient of maximum absorption in the field of low frequencies and, therefore, they can be assigned to class E for materials with sound absorption.
Using the calculation relationships (1) -(4) is determined the analytical values of the average sound absorption coefficient and the sound absorption constant Rα, for the sound insulated cabins in Figure 3. These values are presented in Table 2, compared to the acoustic characteristics values calculated for the cabins made of monolayer lingo-cellulosic composite materials, without sound insulating treatment (MDF and RWB).

Conclusions
Layered composite materials based on ligno-cellulosic waste, with good sound absorption characteristics, may be successfully promoted, due to their low price and the minimum impact upon the environment.
The absorption coefficient of sound waves represents a very important parameter for characterisation of any type of material which is used for building or fitting out the enclosed or open spaces.
In this study it was experimentally determined the sound absorption coefficient according to frequency for monolayered composite plates (MDF, RWB and PS) and multilayered composite plates (with external coating made of MDF, RWB and polymer extruded core made of polystyrene). The values obtained through measurements for absorption coefficients at multilayered plates are not big,