In manufacturing and fabricating highly precise products, a non- contact suspension or transporting system is indispensable for preventing any tiny defect or minute scratch in the transported subjects. It is suggested that the levitation by using the acoustic radiation force can be used for this purpose. In conventional acoustic levitation, levitated objects are positioned at the nodal planes in the standing wave sound field formed between a vibration source plate and a reflection surface. If the specimen is a planar object, the specimen itself acts as a reflector and the method is referred to the near-field acoustic levitation (NFAL). In this study, in order to understand the causes of the discrepancy between the conventional one-dimensional theory and the experimental results, experiments and theoretical investigation about NFAL were carried out. The one-dimensional theory is not consistent with experimental data in the levitation height and it cannot also explain the difference in levitation height for different sizes of levitated objects. In one-dimensional theory, the radiation surface is assumed to be a perfectly rigid piston. However, the vibration amplitude at the edge is smaller than that at the center in actual condition. It is revealed that the largest effect of source velocity distribution is about 6% in comparison with the one-dimensional theory. In addition, the visco-thermal effect of the medium in the gap is investigated by employing the space-averaged attenuation coefficient concept. And the effect is found out to be less than 1%. In order to explain the variation of levitation height for different sizes of levitated objects, the scattering correction factor (SCF) is introduced to consider the scattering and diffraction phenomena due to the finiteness of levitated object. The sound field is represented by using the spheroidal coordinates. From the calculation result, It is shown that one can determine the size of levitated object which is advantageous to its levitation height.
