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As a result of multiple reflections between an ultrasonic radiator and a solid, flat or concave reflector - which is adjusted concentrically at a distance of some multiple half wavelengths - a standing wave with equally spaced nodes and antinodes of the sound pressure and velocity amplitude will be generated (see Figure 2-1). Solid or liquid samples with effective diameters of less than half a wavelength will be levitated without contact below the pressure nodes as a result of axial r adiation pressure and radial Bernoulli stress. Minimal acoustic power is required for this levitation, when the sound wavelength is about three times the sample d iameter (see Table 2-1).