At the beginning of the project the task was to develop and construct a certain torsional pendulum as a matter for detecting rotational mechanical effects of sound. As a result, a pendulum was created in the form of a disk on a wire, which is able to rotate around an axis passing along the wire. The next task was to develop and assemble an optical system for detecting rotation, since these are angles of rotation not visible to the naked eye. As a result, a system was assembled that allowed the camera to record the shift of the laser beam reflected from the pendulum when the pendulum itself was turning at a small angle. The next task was to assemble a complete experimental setup for detecting rotational mechanical effects of sound and conduct experiments in the amount necessary for the set of statistics. As a result, an experimental setup was assembled, consisting of a sound source, namely source of an ultrasonic vortex, a torsional pendulum, system for installing and moving the pendulum in the ultrasound area, and an optical system for rotation detection. A series of experiments were conducted with two types of acoustic vortex. The first results were obtained on the acoustic torque acting on the pendulum.
The results were actively exploited by our internship students in January (internship on «Visualising temperature distribution and sound waves with Schlieren optics») and in the summer of 2023 (internship on « Acoustic drone» and internship on « Waves in the deep water regime»)
The intermediate results were disseminated in a form accessible to the general public at the annual public event «la Nuit Européenne des Chercheur.e.s 2023» on September 29. The intermediate results were also presented to the scientific society, namely to «Société Française d'Acoustique», at the event «Les journées jeunes chercheurs d’Acoustique Physiques (JJCAP)» on October 26-27, 2023.
Then the other type of a torsional pendulum was developped and constructed. It is the so-called wireless pendulum where instead of the wire there were used the magnetic carpet and the disk made of a pyrolytic carbon, material that is capable of levitating in a magnetic field. A spiral phase plate, which generates an acoustic vortex when reflecting plane acoustic waves, was attached to the disk. An acoustic torque on this system was detected too by the rotation of the pendulum relative to the axis of rotation when exposed to acoustic waves.
Very useful exploitation of the final results was made by internship students in the spring and summer of 2024 during internships on «Study of the orbital motion of objects under the influence of acoustic vortex waves» and «Optical visualization of the phase and amplitude structure of an acoustic vortex».
To summarize, rotational mechanical effects of sound were detected and measured in two different independent experimental ways by using completely different equipment and processing of the data. The sound with a vortex structure was produced directly by the ultrasound transducer with 8 channels in the first experimental setup and not directly by the reflection of the ultrasound plane waves by the spiral phase plate generating an acoustic vortex beam in the second experimental setup. The rotational effect was detected on the pendulums in both cases but in the first case it was a disk on a wire and in the second case it was a wireless pendulum with a magnetic carpet.
The final results were disseminated at the event «Laboratoire Ondes et Matière d’Aquitaine day 2024» on July 7, 2024.
