The ultrasonic measuring system used for the experiment consists of an ultrasonic timer, which shows the time between the emission and the reception of the ultrasonic wave and an oscilloscope, which measures the amplitude of the ultrasonic wave. The frequency used for the experiment was 45 kHz. The coupling between the measuring apparatus (See fig. 1) and the wood was attained through a piece of sandpaper. The contours of the specimens for shear test were drawn on the side of the glued panels. The amplitude and the time where measured at the location of each designed specimen in two points at both sides of the panel. The means of the four values were considered as results of the ultrasonic measurements. Figure 2 shows the amplitude of a defected glue-line and figure 3 shows a perfect glue-line. The difference between the two specimens is evident.

After the ultrasonic testing the specimens were cut for the shear strength measurement. The shearing surface was measured and the shear force parallel to the grain was measured and recorded with a testing apparatus for small wood specimens, using a constant cross-head speed of 2mm/min. The shear strength was calculated using the general formula: t = F/A,
where F - maximum shear force (N), A- shear surface (mm²).

The correlation coefficient between the measured time and the shear strength was 0,64.

Conclusion: the measuring system can detect the gluing defects at the surface of the solid wood panel. In the nearest future an industrial method to find gluing defects in solid wood panels (which can be included for quality control in the solid wood panels mass-producing technology) can be developed with more extended experiments with a larger number of more exactly prepared specimens, including other wood species.