frequency gaertner w frequency dependence of ultrasonic cavitation j acoust soc am 26 977 80 1984 therefore 2 5 mhz is considered a reasonable estimate of the upper frequency limit for the occurrence of cavitation in fluids at therapeutic ultrasound intensities hence since cavitation plays an important role in transcutaneous permeation the synergistic effect of sonophoresis and iontophoresis shall be nearly absent with frequencies higher than 2 5 mhz as far as the use of ultrasound for compound delivery is known results have been largely disappointing in that enhancement of permeability has been relatively lower than expected causing no consensus on the efficacy of ultrasound for increasing compound flux across the skin suggesting that other driving forces must also be used when ultrasound energy is applied into the body using for example a circular plane metallic transducer two fields are created the near field known as fresnel field and the far field known as fraunhofer field as shown in fig1 in fresnel field ultrasound energy radiated from different parts of the element travels as spherical waves that interfere both constructively and destructively thus there are zones of maxima and minima of mechanical pressure along and across the beam this field is characterized by a length l which depends on the radius of the radiant surface and the wavelenght of the ultrasound in the medium in front of it i e the skin and soft tissues.