Observation of Polarization Singularities and Topological Textures in Sound Waves.

Polarization singularities and topological polarization structures are generic features of inhomogeneous vector wave fields of any nature. However, their experimental studies mostly remain restricted to optical waves. Here, we report the observation of polarization singularities, topological Möbius-strip structures, and skyrmionic textures in 3D polarization fields of inhomogeneous sound waves. Our experiments are made in the ultrasonic domain using nonparaxial propagating fields generated by space-variant 2D acoustic sources. We also retrieve distributions of the 3D spin density in these fields. Our results open the avenue to investigations and applications of topological features and nontrivial 3D vector properties of structured sound waves.

Acoustic analysis of a broadband spiral source for the simultaneous generation of multiple Bessel vortices in air.

A detailed characterization and analysis of the acoustic field radiated by an active diffraction grating is presented. This is a spiral-shaped source of constant width of the spiral paths and spacing among them, built with flexible ferroelectrets, which is suitable to operate within a broad range of ultrasonic frequencies. The simultaneous airborne generation of multiple Bessel vortex beams with different topological charges along the propagation axis is thoroughly discussed in terms of the structure of the spiral source and the driving frequency, along with some technical details of the prototype. Finally, the quality of the experimentally generated Bessel vortices is discussed, including the phase structure and the amplitude of the acoustic pressure.

Characterization of phased array-steered acoustic vortex beams.

Acoustic vortex (AV) beams generation is a subject of current interest. Even though different applications have been proposed using AV, their potential of use is still to be explored. Recent research works on particle manipulation use phased array systems for AV generation because it allows a flexible beam configuration, i.e., the beam can be easily focalized and modified in its shape. However, little attention has been paid to the fact that the AV can also be electronically steered. In view of this, this work presents a study of the steering capability of an AV. In particular, this paper gives an analysis of the effect of the applied delay law on the structure of AV beams steered at different angles using an array transducer of 32 equidistant elements, deployed on a triangular lattice, operating at 40 kHz. Special attention is paid to the appearance of grating vortices. The effect of the individual element directivity on the resultant beam is also studied. Experimental measurements were carried out in order to validate numerical estimations. Obtained results paves the way for the use of electronically steered vortices in different applications. Also, the potential of use of acoustic grating vortices is discussed.

Broadband EMFi-based transducers for ultrasonic air applications.

In this work, we explore the possibilities of electromechanical film (EMFi) as a new material for developing broadband transducers for ultrasonic air applications. The advantages of the EMFi film are its wide usable frequency range and easiness to use, making it highly suitable for self made, customizable ultrasonic sensors. This paper presents theoretical and experimental information focused on the needs of the sensor's end user, namely, frequency response, actual dynamic mass and Young's modulus, bandwidth, sensitivity, electromechanical dynamical model, acoustic response, and directivity. It is found empirically that the behavior of the film as an almost ideal piston-like acoustic source permits accurate prediction of the characteristics of transducers built on a developable surface. The results obtained represent the first step to more complex geometries, and, ultimately, to completely customizable field ultrasonic transducers.

Applications of airborne ultrasound in human-computer interaction.

Airborne ultrasound is a rapidly developing subfield within human-computer interaction (HCI). Touchless ultrasonic interfaces and pen tracking systems are part of recent trends in HCI and are gaining industry momentum. This paper aims to provide the background and overview necessary to understand the capabilities of ultrasound and its potential future in human-computer interaction. The latest developments on the ultrasound transducer side are presented, focusing on capacitive micro-machined ultrasonic transducers, or CMUTs. Their introduction is an important step toward providing real, low-cost multi-sensor array and beam-forming options. We also provide a unified mathematical framework for understanding and analyzing algorithms used for ultrasound detection and tracking for some of the most relevant applications.

Airborne ultrasonic phased arrays using ferroelectrets: a new fabrication approach.

In this work, a novel procedure that considerably simplifies the fabrication process of ferroelectret-based multielement array transducers is proposed and evaluated. Also, the potential of ferroelectrets being used as active material for air-coupled ultrasonic transducer design is demonstrated. The new construction method of multi-element transducers introduces 2 distinctive improvements. First, active ferroelectret material is not discretized into elements, and second, the need of structuring upper and/or lower electrodes in advance of the permanent polarization of the film is removed. The aperture discretization and the mechanical connection are achieved in one step using a through-thickness conductive tape. To validate the procedure, 2 linear array prototypes of 32 elements, with a pitch of 3.43 mm and a wide usable frequency range from 30 to 300 kHz, were built and evaluated using a commercial phased-array system. A low crosstalk among elements, below -30 dB, was measured by interferometry. Likewise, a homogeneous response of the array elements, with a maximum deviation of +/-1.8 dB, was obtained. Acoustic beam steering measurements were accomplished at different deflection angles using a calibrated microphone. The ultrasonic beam parameters, namely, lateral resolution, side lobe level, grating lobes, and focus depth, were congruent with theory. Acoustic images of a single reflector were obtained using one of the array elements as the receiver. Resulting images are also in accordance with numerical simulation, demonstrating the feasibility of using these arrays in pulse-echo mode. The proposed procedure simplifies the manufacturing of multidimensional arrays with arbitrary shape elements and not uniformly distributed. Furthermore, this concept can be extended to nonflat arrays as long as the transducer substrate conforms to a developable surface.