Acoustic levitation in a cylindrical cavity; a modal superposition approach

Abstract

A method has been developed for calculating the acoustic radiation force acting on a small spherical bead in a sound field within a rigid boundary cylindrical cavity, using a modal approach. Modes are selected by a method comparing their relative strengths, within a specified dB range below the strongest mode. The total sound field is calculated as the superposition of fields from different excitation points, each composed of a summation of mode shapes within the specified range. The total sound field is then used to calculate the force, by use of a previously established model. Three different simulated acoustic traps with a sound field excited by a plane transducer array at the bottom of the cylinder are presented. The traps are constructed by numerical optimization of the transducer phases and amplitudes such that the total sound field gives rise to converging forces at a designated position. One of the three simulated traps was also tested experimentally, where an acoustic trap at the designated position was not found. However, three other traps, one about 1 cm above the designated position, one in the center of the cylinder, and one about 5 mm offset from the center radially and in height, were found. Two of these alternate traps are also predicted by the developed model, which points towards the existence of a trap at the designated position as well, although hard to find by manual bead placement. The third found trap is not predicted by the model. Further research is needed to understand why that is, and how the model can be improved