In the probe systems, ultrasound is directly applied by a vibrating‘‘horn’’. Depending on the geometry of the probe, it could be used simply to transmit the ultrasonic energy or to concentrate it on a lower surface in order to amplify the intensity and, therefore,
their effects (Mason, 1998). In applications with this type of systems,the distance between the sound tip and the treated sample is an important parameter to be controlled due to the attenuation of the ultrasonic field with the distance.