Ultrasonic standing waves (USWs) at MHz-range frequencies enable particle manipulation by trapping particles in nodal- and antinodal planes. Achieving precise particle control necessitates a uniform USW field; however, near-field pressure inhomogeneities introduced by finite-sized transducers hinder manipulation close to the emitter surface. Due to the non-linear scaling of the near-field length—with the square of the aperture and linearly with frequency—large-volume manipulation becomes impractical. We propose a layered structure integrated with the transducer to suppress near-field pressure fluctuations by spatially filtering the angular spectrum of the emitted field. Using analytical and numerical methods, we evaluate the effect of key parameters in the angular spectrum filter performance. The results suggest improved particle trapping uniformity with the proposed filter.
