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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.

Abstract-Cellular ferroelectrets exhibit interesting electromechanical-acoustical characteristics. Their recent appearance and remarkable properties open up new possibilities for the design and development of ultrasonic transducers. In particular, the feasibility of fabricating ultrasonic vortex generators using ferroelectret films is demonstrated in this work. To this end, a transducer prototype was built by gluing the material onto a tangential-helical surface (outer diameter: 40 mm, pitch: 3.45 mm). Experimental results agree well with the theoretical estimation of the pressure and phase of the acoustic field in the near field and far field, which corroborates the potential of ferroelectrets to customize special acoustic fields. Furthermore, the proposed fabrication procedure is inexpensive and represents a new alternative for exploring and analyzing the special characteristics of acoustical helical wavefronts.

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.

This paper reports preliminary work with an RFID with a growing importance, Radio Frequency (RF) signals [1], based local positioning system (LPS) designed for location and [2]. Belonging to this last category, many different possibilities guidance of people and autonomous vehicles in indoor environ-have been explored: WiFi, Bluetooth, Ultra-WideBand (UWB), ments. The system consists of an RF reader carried by the Radio Frequency Identification (RFID), etc. It should be noted mobile user, and a number of active RFID tags, disseminated at known positions in the displacement region, which regularly that all these technologies (with the obvious exception of GPS emit RF signals with an identification code. Upon reception and UWB, which permit direct measurement of the signal of a signal, the range of the user to the corresponding tag is propagation times) were designed for digital communication estimated indirectly from the received signal strength (RSSI), using rather than for localization purposes. a previously obtained statistical model. A computationally efficient RFID systems, which provide easy multiple reading of the Bayesian localization method (particle filter) is used to process the RFID systion codes and an indirec eatin of the measurements and produce an estimation of the user's position. tag's identification codes and an indirect estimation of theirThe RFID-LPS is tested empirically in a displacement region range to the receiver through the perceived signal strength comprised of three adjacent rooms, with a total area of 250 m2, (RSSI), are becoming very popular for many applications. Its in which there are placed 21 tags. Our first results show a typical advantages include not requiring wiring or line-of-sight to the mean positioning error of 3.25 m, which compares favorably with reader, a relatively long read range (with active tags), an easy other systems reported in the literature. deployment of tags, with minimum infrastructure modification, KeVwords -Local positioning systems, RFID localization, particle a capability to remain operative in harsh environments, and an filters overall low cost. Besides, RFID has quickly become a mature technology, with a high degree of penetration in the market.

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Noncontact ultrasonic spectroscopy (NCUS) is used to excite and sense thickness resonances in films of polypropylene ferroelectrets. From the comparison of these measurements with theoretical calculations it is possible to extract some material properties: film thickness and density, velocity, and attenuation of ultrasounds and variation in these two magnitudes with the frequency. Hence elastic compliance and acoustic impedance are worked out. Observed variation in the attenuation with the frequency exhibits classical viscoelastic behavior which can be used to investigate the underlaying physical mechanism. In addition, the influence of the metallization on the film response is studied. A modification of the NCUS method is proposed on the basis of the piezoelectric response of these films, which give rise to an alternative characterization method. Consistency of both methods is verified.

Acoustic vortices have attracted a great deal of attention in recent years due to their numerous applications. We introduce a highly efficient method for the generation of acoustic Bessel vortices in air, using spiral-shaped active diffraction gratings, which can be operated within a broad spectral range of ultrasonic frequencies. Using a single-arm active spiral source, we achieve the simultaneous generation of vortices of different topological charges, well separated among each other along the propagation axis. With a theoretical analysis, numerical simulations, and experiments, we demonstrate some features about spiral diffraction gratings, such as the equivalence between specific diffraction orders of an m-armed spiral and a single-arm spiral, and the annihilation of prescribed diffraction orders by tuning the width to pitch ratio.