Depending upon the conditions in which they are used, wood products are required to have been produced with the necessary moisture levels for the intended job. In many cases, this means that drying is required in order to achieve these moisture levels. Wood is also often thermally modified. During the required processing, stresses often occur in wood assortments. If such stresses exceed the limits of durability in the wood, cracks will appear. Similar cracks in wood can occur prior to the drying and/or heat treatment stage. These defects are usually internal and invisible, but they can significantly alter the mechanical properties of the product. This study has shown that wood defects can often be detected using the original method, which involved transverse resonant vibrations, invisible drying, and others. It has been found that a defect of at least 12.5 % of the specimen’s overall length will change the mechanical properties of that specimen. When the defect length makes 25 % of the specimen’s overall length, or even more, the assortment sometimes behaves as a system of several bodies. In addition, when the defect reaches half the total length of the specimen, the modulus of elasticity may decrease to 20 %, and the coefficient of damping may increase to 80 %.