Emitting long-distance spiral airborne sound using low-profile planar acoustic antenna

Recent years have witnessed a rapidly growing interest in exploring the use of spiral sound carrying artificial orbital angular momentum (OAM), toward establishing a spiral-wave-based technology that is significantly more efficient in energy or information delivering than the ordinary plane wave technology. A major bottleneck of advancing this technology is the efficient excitation of far-field spiral waves in free space, which is a must in exploring the use of spiral waves for long-distance information transmission and particle manipulation. Here, we report a low-profile planar acoustic antenna to modulate wavefronts emitted from a near-field point source and achieve far-field spiral airborne sound carrying OAM. Using the holographic interferogram as a 2D modulated artificial acoustic impedance metasurface, we show the efficient conversion from the surface wave into the propagating spiral shape beam both numerically and experimentally. The vortex fields with spiral phases originate from the complex inter-modal interactions between cylindrical surface waves and a spatially-modulated impedance boundary condition. This antenna can open new routes to highly integrated spiral sound emitters that are critical for practical acoustic functional devices.

DETERMINATION OF THE ACTUAL AREA OF DRY ACOUSTIC CONTACT IN THE SYSTEM “TRANSDUCER–PRODUCT” IN LOW-FREQUENCY DEFECTOSCOPY

The conditions for the emission of acoustic energy into the pipeline environment and the reception of reflected signals from inconsistencies in dry acoustic contact cause certain dimensions of the actual contact area between the transducers and the pipe surface. The basic approaches to the determination of the actual area of ​​dry acoustic contact between the surfaces of the piezoelectric transducer and the pipe are formulated under the influence of constant static force of pressing the surfaces in low-frequency flaw detection using ultrasonic directional waves. Expressions have been proposed to determine the area of ​​actual acoustic contact for single and numerical micro projections of the pipe surface. The principle of quality control of balancing of acoustic antenna piezoelectric transducers in modern systems of low-frequency diagnostics of the technical state of longitudinal pipelines by ultrasonic directed waves is described. It is revealed that after correct balancing of all the acoustic antenna piezoelectric transducers, the column image does not appear on the display screen and the mathematical support of the system will automatically collect the technical status of the diagnosed section of the pipeline, the results of which are displayed on the display screen. It is established that the actual area of ​​dry acoustic contact in the "piezoelectric product" system in low-frequency defectoscopy depends on the magnitude of the static force of pressing the surface of the piezoelectric transducer to the surface of the product. It is revealed that the deformation of the micro protrusions of the surface of the product under the action of static clamping force is uneven, which does not allow to fully calculate the actual area of ​​dry contact by mathematical methods. It is shown that in modern systems of low-frequency ultrasonic diagnostics of extended pipelines, directional waves control the quality of dry contact of the surface of the piezoelectric transducer with the surface of the pipe by balancing acoustic antennas with the use of special test programs.

Acoustic Signature Analysis for Localization Estimation of Unmanned Aerial Vehicles Using Few Number of Microphones

In recent years, the current technological improvements of Unmanned Aerial Vehicles (UAV) have allowed more and more efficient use for applications ranging from simple amateur shooting to more professional tasks. Being handy, drones can easily fly near sensitive sites, such as power plants, airports or ministries. It is therefore necessary to develop systems able to keep watch such sites. However, the size and the composition of these devices make optical or electromagnetic systems inefficient for their detection. This study proposes an alternative exploiting the sound wave emitted by their motorization and / or their aerodynamic whistling. For this, an acoustic antenna with few microphones was sized to be sensitive to frequencies emitted by a drone. Firstly, an experimental analysis on the noise made by a drone was carried out. The results allowed us to settle the geometry of the antenna in order to process localization. Two methods of location are used. The first is based on an energy approach providing the acoustic field reconstruction in all directions of space. The second estimates the position of the source by inverting a system exploiting the arrival time differences of the acoustic wave between different pairs of microphones. Numerical simulations, supported by a measurement campaign, make it possible to highlight the performance of the methods used.