Since 1989, the forestry department of the CIRAD develops a transverse vibration technique (TVT) for evaluating stiffness of wooden beams. A special integrated device, named BING system, is designed to obtain mechanical properties. After a percussion on a simply supported beam, a digital recorder piloted by a PC performs a Fast Fourier Transform of the sound wave. As a linear expression (approximate solution of the Timoshenko beam equation) links every eigenfrequencies to the mechanical parameters, the MOE and the shear modulus are quickly computed. A strong correlation with a four static bending test in a wide range of specimen size has been observed (structural timber or specimen with a length-to-depth ratio as low as 16). In comparison with other non destructive techniques methods (ultrasounds, visual assessment... ), the TVT gives the best correlation with strength of structural timbers. This technique allowed to determine MOE with a very good repeatability and accuracy and can be expected as an efficient grading tool. Furthermore, for enhance the prediction of the strength it is possible to take into account some defaults that affects strength by others techniques (microwave, X-ray, image analysis... ) but also to study the eigenfrequencies spectrum. Indeed, the latter is disturbed by the presence of big defaults such as knots, large grain angle, compression failure. it can be observed some eigenfrequencies shifts and peak spreading. The possibilities to exploit these perturbations are discussed.