## 1 Introduction
In fact, measurements made on logs and standing trees in the forest can segregate the timber resource, diverting the higher quality material to construction timber and the poorer material to other markets, and reducing wastage [24]. In the literature, there is a certain number of researches aimed at pre-grading timber starting from the standing trees or the logs. In particular, papers aimed at timber pre-grading starting from logs [25; 26; 27] or from wet boards or specimens [27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38] have provided interesting results, showing good correlations between timber physical-mechanical parameters in the wet condition and those in the dry one, and highlighting the possibility of evaluating dry timber quality starting from the measurement of its characteristics in the wet condition. The parameter generally used in these studies for timber pre-grading is the dynamic modulus of elasticity MoEdyn, which nowadays is the most used feature in timber grading, evaluated through the measurement of the parameters associated with the propagation of acoustic waves in the material (velocity, frequency). As reported in [39], wood has a structured hierarchy, dominated by a cellular and layered structure, and is composed of three constituents: lignin, hemicellulose, and cellulose. Cellulose and hemicellulose are hydrophilic, whereas lignin is less hydrophilic than hemicellulose. Cellulose crystalline regions and hemicellulose structure are responsible for the elastic behavior of wood under tension, and the presence of both hydrophobic and hydrophilic molecules explains why the mechanical properties of wood are strongly dependent on its water content. This behavior highlights the importance of understanding the mechanical properties of wood also in the wet condition. This paper illustrates an experimental investigation aimed at determining the MoEdyn in the wet and dry condition of Sardinian maritime pine boards. The MoEdyn was determined by measuring the velocity of propagation of acoustic waves in the boards before and after the drying process. Sardinian maritime pine is a wood species very common in Sardinia, currently under study as regards its possible structural use as glue-laminated timber (GLT) and cross-laminated timber (CLT). It is generally characterized by medium-low quality due to the presence of defects (knots, clusters of knots, resin pockets, grain deviation, etc.). Previous research about its yield for structural use showed that about 50% of the boards had to be rejected due to their defects [40; 41; 42], so that the possibility of defining the structural quality of the boards already in the wet state is a fundamental step to ensure the sustainability of the entire production process. In fact, the ability to provide a pre-grading on wet boards could significantly increase the profitability by saving the cost of drying and subsequent processing for non-structural grade boards sold at the same price whether wet or dry. This may lead to increased efficiency, energy savings, and lower costs. The primary objectives of this study are to investigate the influence of moisture content on the MoEdyn of Sardinian maritime pine, to quantify the variation of MoEdyn from wet to dry condition and to examine the relationship between wet and dry MoEdyn in Sardinian maritime pine, and to verify the feasibility of wet pre-grading of Sardinian maritime pine by means of the MoEdyn determination from the velocity of propagation of acoustic waves in wet boards.