Sound field inside acoustically levitated spherical drop

The sound field inside an acoustically levitated small spherical water drop (radius of 1 mm) is studied under different incident sound pressures (amplitude p0 =2735-5643 Pa). The transmitted pressure ptr in the drop shows a plane standing wave, which varies mainly in the vertical direction, and distributes almost uniformly in the horizontal direction. The maximum of ptr is always located at the lowermost point of the levitated drop. Whereas the secondary maximum appears at the uppermost point if the incident pressure amplitude p0 is higher than an intermediate value (3044 Pa), in which there exists a pressure nodal surface in the drop interior. The value of the maximum ptr lies in a narrow range of 2489-3173 Pa, which has a lower limit of 2489 Pa when p0 =3044 Pa. The secondary maximum of ptr is rather small and only remarkable at high incident pressures.