wave distribution in the case of the disk shaped resonator having a diameter of 20 mm within the liquid medium the sound wave caused by the vibrations of the disk shaped resonator generates the sound field as illustrated in fig9 b when the frequency of the vibration of the disk shaped resonator is approximately 1 mhz or more the sound field exhibits a half value angle of about 2 through 3 degrees or less and is formed like a beam thus the sound field can be approximated to a plane progressive wave in practical use on the other hand when the frequency of the vibration of the resonator is high there arises the damping or less thus when the frequency of the vibration of the resonator is not less than 10 mhz the damping distance is several tens of centimeters or less in liquid as shown in fig1 which results in that it is preferable to operate or vibrate the resonator at a frequency ranging from 1 through 10 mhz in order to constitute an energy converter of a few centimeters through several tens of centimeters in size or scale moreover when a liquid is the radiation medium for the sound wave it is possible to input a sound power which is larger by three digits than the case where.