Solid materials conduct sound waves well, so the use of an ultrasonic method to assess the quality of metals, concrete, and bricks has become one of the most accurate and popular methods. Another important point is that the sound wave is reflected not only from the surface, but also from internal defects – cracks, voids, and various inclusions. Depending on the decrease length of the sound’s wave and the increase oscillation frequency, the effect of the interaction of ultrasonic waves with the material increases.
f – Frequency (MHz)
с – Velocity (m/sec)
λ – Wavelength (mm)
These calculations lead to the conclusion that the optimal range used is 0.5-25 MHz.
With lower frequencies the index of the effect of the interaction of waves with defects inside the material decreases. So, in metal constructions, finding of defects becomes difficult (waves of greater length just bend around the cracks).