Study On The Phase Distribution Characteristics Of The Low Frequency Vortex Sound Field

As an analogy to the widely-studied optical vortices, acoustic vortex is characterized by a screw phase dislocation of the wave field around its propagation axis with phase singularity and pressure null at its core, which can be used for the purposes of alignment and manipulation for small object. The wave field is also known to carry orbital angular momentum associated with its screw phase dislocation, which can be transferred to object and hence exert torques to produce object rotation. At the same time, because the acoustic beam can propagate through media without any damage, it can be used in particle manipulation inside object. Compared with optical vortices, acoustical vortices have broader prospects in noninvasive medical application.Linear phase distribution of phase-coded acoustical vortices was theoretically investigated based on the radiation theory of point source, and was also confirmed by experimental measurements. With the proposed criterion of positive phase slope, the possibility of constructing linear circular phase distributions is demonstrated to be determined by source parameters. Improved phase linearity can be achieved at larger source number, lower frequency, smaller vortex radius and/or longer axial distance.In this study, a 16-source experimental system was setup by employing the phase-coded approach to verify the linear phase distribution of acoustical vortices. Square signals with controllable phase difference were sent out by a FPGA device. Filtered by low-pass filters and amplified by power amplifiers, sinusoidal waveforms were generated to excite the speakers to generate acoustical vortices. Radial and circular distributions of pressure and phase were measured. Good agreements were observed between numerical simulations and measurement results for circular phase distributions. The favorable results confirm the feasibility of precise phase control for acoustical vortices and suggest potential applications in particle manipulation.This study provides basis for further investigation on precise movement control of small objects with the rotational torque generated by angular momentum transfer from high-frequency acoustical vortices, which shows broad application prospects in the field of sonar, instrument calibration and nanoparticle manipulation.