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Caries, one of the most important oral diseases, is associated to a progressively demineralization process. The aim of the present work is the study of the dynamics of the artificial demineralization process of human dentin by means of a non destructive ultrasonic pulse-echo technique. Human dentin samples were sliced and artificially demineralized with phosphoric acid at 10%. The demineralization process was monitored using a 15-Mhz pulsed-echo transducer. The reduction of the stiffness of the dentin, and suggests that the acid demineralization, lasting about one hour, develops at a non-constant penetration rate. A model to study the diffusion of the acid into the dentine tissue is proposed to evaluate demineralization lesions produced by natural caries in-vivo.

Anthropogenic impulsive sound sources with high intensity are a threat to marine life and it is crucial to keep them under control to preserve the biodiversity of marine ecosystems. Underwater explosions are one of the representatives of these impulsive sound sources, and existing detection techniques are generally based on monitoring the pressure level as well as some frequency-related features. In this paper, we propose a complementary approach to the underwater explosion detection problem through assessing the arrow of time. The arrow of time of the pressure waves coming from underwater explosions conveys information about the complex characteristics of the nonlinear physical processes taking place as a consequence of the explosion to some extent. We present a thorough review of the characterization of arrows of time in time-series, and then provide specific details regarding their applications in passive acoustic monitoring. Visibility graph-based metrics, specifically the direct horizontal visibility graph of the instantaneous phase, have the best performance when assessing the arrow of time in real explosions compared to similar acoustic events of different kinds. The proposed technique has been validated in both simulations and real underwater explosions.

[EN] The microstructural features of heterogeneous and porous materials give rise to unique non-linear dynamic behaviour. The purpose of this work is to investigate the dynamic response of thermally damaged concrete specimens measured by two different techniques: Non-linear Impact Resonance Acoustic Spectroscopy (NIRAS) and new Flipped Accumulative Non-linear Single Impact Acoustic Spectroscopy (FANSIRAS). Specimens were characterised in two different dynamic condition states of the material: relaxed and conditioned. The specimen¿s relaxed state indicates that no previous dynamic excitation event ocurred. The conditioned state denotes that the specimen has been dynamically tested before. The NIRAS results show that the non-linear material parameters, af and aQ , are affected by their previous dynamic history (its conditioning). The recently proposed algorithm, FANSIRAS, extracts from a single resonant signal equivalent results to NIRAS when the specimen is conditioned. In this situation, both parameters af and aQ were equivalent. The results suggest that new NDT parameters based on non-linear hysteretic parameters can quantify the damage level of thermally treated mortar specimens., This work has been supported by the Spanish Administration under grants BIA2014-55311-C2, BIA2017-87573-C2, BES2015-071469, PAID-10-19 and FEDER funds.

[EN] Higher education, as other social and economic sectors in Spain, was disrupted on 15th March 2020 following health emergency Laws enacted related to COVID-19. Non-presential lecturing was stablished country-wide until the end of the academic year 2019-20, as society was set in lockdown. In this context, it is necessary to evaluate the impact of the mandatory changes implemented in the education paradigm in order to assess the degree of acquisition of general and specific competencies altogether the acquisition of transversal competencies, as an important factor for the alumni career. This paper reports a comprehensive study on the competencies degree of acquisition considering the lockdown scenario in Spain. The results from the last four academic years have been comparatively evaluated in the core subject `Teoría de la Comunicación¿, lectured in the fourth semester of the Telecommunications Engineering Integrated Program (Bachelor and Master) in the Universitat Politècnica de València, Spain, comprising data from 745 alumni. The results indicate that the degree of acquisition of technical competencies in this scenario has been adequate, being marginally better compared to previous academic year, probably due to new lecturing materials prepared. Nevertheless, the acquisition of the transversal competency `analysis and problem solving¿ exhibits degraded results, indicating inhomogeneous acquisition probably due to limitations in the group-based problem-solving practice. The results suggest that specific materials and remote lecturing strategies should be developed and implemented to guarantee adequate acquisition levels., The support by the 2020 Science Parks program from the Consellería de Innovación, Universidades, Ciencia y Sociedad Digital, Generalitat Valenciana, Spain, is acknowledged.

[EN] In this paper, the study of frequency-dependent ultrasonic attenuation in strongly heterogeneous cementitious materials is addressed. To accurately determine the attenuation over a wide frequency range, it is necessary to have suitable excitation techniques. We have analysed two kinds of ultrasound techniques: contact ultrasound and airborne non-contact ultrasound. The mathematical formulation for frequency-dependent attenuation has been established and it has been revealed that each technique may achieve similar results but requires specific different calibration processes. In particular, the airborne non-contact technique suffers high attenuation due to energy losses at the air-material interfaces. Thus, its bandwidth is limited to low frequencies but it does not require physical contact between transducer and specimen. In contrast, the classical contact technique can manage higher frequencies but the measurement depends on the pressure between the transducer and the specimen. Cement specimens have been tested with both techniques and frequency attenuation dependence has been estimated. Similar results were achieved at overlapping bandwidth and it has been demonstrated that the airborne non-contact ultrasound technique could be a viable alternative to the classical contact technique., The authors acknowledge the support from University College Cork (Ireland), Universidad Politecnica de Valencia and the Spanish Administration under grant BIA2014-55311-C2-2-P and Salvador Madariaga's Programme (PR2016-00344/PR2017-00658).

Nonlinear hysteretic techniques have demonstrated their suitability for damage detection in mesoscopic materials. These methods are mainly based in the evaluation of the frequency and damping shifts with the increasing strain amplitude under dynamic conditions. A deep understanding of the impact reverberation phenomena has been achieved in order to introduce a novel signal processing approach called FANSIRAS (Flipped Accumulative Non-linear Single Impact Resonance Acoustic Spectroscopy). This brand new procedure respects the underlying phenomena, composed of both an exponentially decaying signal and a time varying phase signal, which results in the feasibility of using a single impact to evaluate the non-linear features of damaged materials. The traditional acoustic spectroscopy technique, NIRAS (Non-linear Impact Resonance Acoustic Spectroscopy), and FANSIRAS have been analyzed and compared when providing quantitative information related to the degree of damage. In this work, both techniques were applied...

[EN] High amplitude non-linear acoustic methods have shown great potential for the identification of micro-damage in inhomogeneous materials such as concrete. Usually, these methods evaluate non-linearity parameters related to the hysteretic behaviour from the dependence of the shifts in both frequency and damping on the amplitude of the strain. A deep understanding of the reverberation phenomena has been obtained in order to introduce a novel signal processing approach called FANSIRAS (Flipped Accumulative Nonlinear Single Impact Resonance Acoustic Spectroscopy). Traditional acoustic spectroscopy techniques, NIRAS (Non-linear Impact Resonance Acoustic Spectroscopy) and NSIRAS (Non-linear Single Impact Acoustic Spectroscopy), have been analyzed and compared with the brand new approach when providing quantitative information related to the degree of micro-cracking in thermal damaged concrete based materials. The new resonance-based algorithm demonstrates that the non-linear non-classical parameters can be determined through a single resonance frequency measurement, obtaining the expected sensitivity to internal damage. Its simplicity and robustness may be important in industrial applications. (C) 2018 Elsevier Ltd. All rights reserved., This work has been supported by the Spanish Administration under grants TEC2011-23403, BES2015-071469, BIA2014-55311-C2-1-P and BIA2014-55311-C2-2-P. The authors are also grateful for FEDER funds.

Extracting frequency-derived parameters allows for the identification and characterization of acoustic events, such as those obtained in passive acoustic monitoring applications. Situations where it is difficult to achieve the desired frequency resolution to distinguish between similar events occur, for example, in short time oscillating events. One feasible approach to make discrimination among such events is by measuring the complexity or the presence of non-linearities in a time series. Available techniques include the delay vector variance (DVV) and recurrence plot (RP) analysis, which have been used independently for statistical testing, however, the similarities between these two techniques have so far been overlooked. This work suggests a method that combines the DVV method with the recurrence quantification analysis parameters of the RP graphs for the characterization of short oscillating events. In order to establish the confidence intervals, a variant of the pseudo-periodic surrogate algorithm is proposed. This allows one to eliminate the fine details that may indicate the presence of non-linear dynamics, without having to add a large amount of noise, while preserving more efficiently the phase-space shape. The algorithm is verified on both synthetic and real world time series., This work has been supported by the Spanish Administration under Grant No. TEC2011-23403.

Anthropogenic underwater sound is now recognized as a world-wide problem. Shipping, seismic surveys and pile driving, among some other human activities, have shown a broad range of negative effects in a variety of species. It is thus important to control, register and characterize these activities in order to establish policies that mitigate its effects. For this purpose, automatic detectors of the major anthropogenic sound categories (impulsive and tonal sounds) are needed. An efficient algorithm capable of detecting impulsive and tonal sound categories is proposed. The detection algorithm is based on the Pulsed to Tonal Ratio (PTR) and, in this work, we show not only its simplicity but also how well it performs in a variety of situations. In contrast to what happens with some other techniques, the proposed detector generally succeeds in detecting pulsed and tonal events with additive white and pink noise. This makes the PTR detector particularly appropriate for passive acoustic monitoring.

This work addresses the study of sounds produced by odontocetes using a brand-new approach based on signal modality. Characterizing the mechanisms used in sound generation, as well as classifying the repertory of sounds, is a complicated task given the complexity of the marine environment and the huge number of different marine mammal species. Several works in the last years have been raised to clarify how these sounds are generated. Understanding the behaviour of the organs responsible for the production and the mechanisms used in the generation is essential for the proposal of new classification features related to the underlying physics. This complex panorama may gain leverage from the advances in signal processing to the design of new algorithms based on the nature of the signals. Reconstructing the phase space, obtaining Recurrence Plots (RPs) and quantifying the complexity of the resulting structures are processed used with the aim of discerning and characterizing the different sounds produced by the odontocetes. Specifically between sounds with strong (almost-)periodic components, where traditional algorithms of signal processing are unable to get information related to the corresponding underlying model.

[EN] In this paper, a new approach for characterizing material damage, using ultrasonic waves, is proposed. Two concrete series with two types of cement with different C3A content and similar mechanical properties were subjected to external sulphate attack (ESA) and evaluated using a novel Recurrence Plot Quantification Analysis (RQA) method. This brand new technique was compared with several methods, such as mechanical tests (compressive and flexural strength determination), dynamic test (dynamic modulus) measurements, and traditional ultrasonic measurements (propagation velocity and ultrasonic wave attenuation). In these experiments, RQA showed a high sensitivity to damage in spoiled series, improving the reliability of damage detection with ultrasonics in non-homogeneous materials compared to other non-destructive techniques. Interesting advantages of this new non-destructive technique are: a) the RQA parameter is normalized (range of 0 to 1); b) a calibration process is not required; c) the values of its standard deviation show the dispersion of the damage. It can contribute greatly to the diagnosis of the degree of damage to a material, when combined with other traditional measures such as the attenuation of the material., This work was supported by the Spanish Government under grants TEC2011-23403, BIA2014-55311-C2-1-P and BIA2014-55311-C2-2-P. This work is protected by the Spanish Patent and Trademark Office (SPTO) under reference P201630212.

[EN] In this study, complete ultrasonic monitoring of Glass-fiber Reinforced Cement plates under bending tests was addressed. In this kind of experiment, the mechanical properties of the specimen continuously change during the test, thus, the acquisition time of the ultrasonic signals is a critical variable. In order to overcome this drawback, a new ultrasonic procedure based on broadband signals (chirp) has been applied. Following this line of thought, the analysed ultrasonic parameters have been split into the parameters that only depend on time, and those that depend on both time and frequency. In particular,the frequency dependent attenuation parameter allows characterizing the evolution of the plate being damaged over a wide frequency range and significantly detecting the main two events happening during the experiment: the first crack and the maximum stress point. In short, this paper demonstrates the suitability of ultrasonic broadband signals for characterizing fiber-reinforced cementitious composites under bending stress., This work has been supported by the Spanish Administration under grants, BES-2015-071469, TEC2011-23403, BIA2014-55311-C2-1-P and BIA2014-55311-C2-2-P.

[EN] An exhaustive monitoring of an internal sulphate attack of Portland cement-based materials is addressed. Four series of Portland cement mortars with different amounts of gypsum (0%¿2% SO3 respect to the cement by mass) were monitored by means of physical, microstructural and non-destructive tests, studying specimens with a low expansion rate to examine the sensitivity of the applied techniques. The expansion analysis has shown the suitability of a fitting model, allowing the examination of two characteristic parameters: the characteristic time of the expansion reaction and the maximum amplitude of the expansion. In the mechanical analysis, higher values of Rc and Rf were attained as the gypsum content decreased. A microstructural analysis (thermogravimetry and FESEM) supported ettringite formation and expansion process. These results have been correlated with non-destructive tests: impact resonance acoustic spectroscopy and ultrasonic measures. The dynamic modulus and ultrasonic pulse velocity have closely predicted the stiffness of the specimens. The total material attenuation (absorbed energy of the chirp signal ultrasonic wave) presented different trends, showing clear differences for the most damaged series (2% SO3). Attenuation supplied interesting information about the hardening process and the microcracking effect due to a mortar expansion higher than 0.04%. The novelties of this study are the exhaustive monitoring of an internal sulphate attack, as well as the examination of the sensitivity of brand new NDT techniques when this damage process overlaps with the curing process., This work has been supported by the Spanish Administration under grants TEC2011-23403, BIA2014-55311-C2-1-P and BIA2014-55311-C2-2-P.

[EN] In this paper, we present a novel and completely different approach to the problem of scattering material characterization: measuring the degree of predictability of the time series. Measuring predictability can provide information of the signal strength of the deterministic component of the time series in relation to the whole time series acquired. This relationship can provide information about coherent reflections in material grains with respect to the rest of incoherent noises that typically appear in non-destructive testing using ultrasonics. This is a non-parametric technique commonly used in chaos theory that does not require making any kind of assumptions about attenuation profiles. In highly scattering media (low SNR), it has been shown theoretically that the degree of predictability allows material characterization. The experimental results obtained in this work with 32 cement probes of 4 different porosities demonstrate the ability of this technique to do classification. It has also been shown that, in this particular application, the measurement of predictability can be used as an indicator of the percentages of porosity of the test samples with great accuracy., This work has been supported by the Spanish Administration under grant TEC2011-23403. We also wish to acknowledge Dr. Jorge Gosalbez for providing the ultrasound data of the cement paste probes.

[EN] Background Previous research has studied the effects of games in pediatric wards, but none of it has focused on the impact of the hospital¿s school staff on the psychosocial state of the children nor on the gameplay itself. Objectives To present the Tangibot application and evaluate its impact on the children¿s psychosocial state in the short term and the impact of the teacher on their psychosocial state, communications and coordination during the activity. Methods A study was conducted in a hospital classroom with 20 participants, who participated twice in the game: one with the teacher playing along and another without her. An observational scale was used by two evaluators in order to assess the impact on the children. Results and conclusions The study revealed that the teacher has an impact on the children¿s communication and coordination procedures but has no impact on the psychosocial state of the participants. The teacher¿s impact was found to be positive about communications. Dialogue management significantly improves when the communication includes the teacher, which means speaking turns are observed more consistently. Information pooling also improves, and the participants ask the teacher more questions. Consensus is also reached more often and more easily, but this does not reflect on the performance, as the time management is evidently worse when the teacher is present, as is also the joint task orientation. On the other hand, it was found that the teacher does not have an impact on the psychosocial state of the participants during the game, and that it is the game itself which changes their state over time. In the case of affection, which reflects the participants¿ emotions of joy or boredom, their state improved significantly after a few minutes of play. The same thing occurred for physical activity, interest in the activity and interaction between peers, which increased in value in the first part of the game, although physical activity and interaction were reduced towards the end. No changes were found throughout the game in the number of complaints, nervousness or satisfied comments, which remained very low for all these aspects, showing that the game distracted them from their various symptoms. Based on these results, future work will explore the effects of gamification on the overall hospitalization perception, with special focus on the social opportunities during the hospital stay, to provide ways for the children to meet others during their treatment, to make the experience less painful and reduce their feelings of isolation. Some game strategies should also be evaluated to determine the ones that provide the best opportunities to improve the children¿s hospital experience., This work is funded by the European Development Regional Fund (EDRF-FEDER) and supported by Spanish MINECO with Project TIN2014-60077-R-AR. The work of Jorge Montaner is supported by a national grant from the Spanish Ministry for Education (FPU17/03333). Special thanks to the staff of La Fe Hospital in Valencia who have collaborated in the experiment.

[Otros] Lamb waves have emerged as a valuable tool to examine long plate-like structures in a faster way compared to conventional bulk wave techniques, which make them attractive in non-destructive testing. However, they present a multimodal and dispersive nature, which hinders signal identification. Oblique incidence is one of the most known methods to generate and receive Lamb waves and it is applied in different experimental arrangements with different types of sensors. In this work, several setups were conducted and compared to determine the optimal ones to launch and detect ultrasonic Lamb waves, especially in non-homogeneous specimens. The chosen arrangements were contact with angle beam transducers, immersion in a water tank, localised water coupling using conical containers and air coupling. Plates of two different materials were used, stainless steel and Portland cement mortar. Theoretical and experimental dispersion curves were compared to verify the existence of Lamb modes and good correspondence was achieved., This research was funded by the Spanish Administration, grants number BES-2015-071958 and BIA2017-87573-C2.

Financial markets typically undergo periods of prosperity followed by periods of stagnation, and this undulation makes it challenging to maintain market efficiency. The efficient market hypothesis (EMH) states that there exist differences in structural complexity in security prices between regular and abnormal situations. Yet, despite a clear link between market acceleration (cf. recession in security prices) and stress in physical systems, indices of financial stress still have significant scope for further development. The overarching aim of this work is therefore to determine the characteristics of financial indices related to financial stress, and to establish a robust metric for the extent of such 'stress'. This is achieved based on intrinsic multiscale analysis which quantifies the so called complexity-loss hypothesis in the context of financial stress. The multiscale sample entropy and our proposed Assessment of Latent Index of Stress methods have successfully assessed financial stress, and have served as a measure to establish an analogy between transitions from 'normal' (relaxed) to 'abnormal' (stressed) financial periods with the sympatho-vagal balance in humans. Four major stock indices of the US economy over the past 25 years are considered: (i) Dow Jones Industrial Average, (ii) NASDAQ Composite, (iii) Standard & Poor's 500, and (iv) Russell 2000, together with FTSE 100, CAC 40 and exchange rates. Our findings support the EMH theory and reveal high stress for both the periods of Internet bubble burst and sub-prime mortgage crisis., This work was supported by the Financial Signal Processing Laboratory (http://www.fsplab.com/) at Imperial College London. The guest editor coordinating the review of this manuscript and approving it for publication was Prof. Ali N. Akansu.

In this paper the study of frequency-dependent ultrasonic attenuation in strongly heterogeneous materials is addressed. To determine the attenuation accurately over a wide frequency range, it is necessary to have suitable excitation techniques. Three kinds of transmitted signals have been analysed, grouped according to their bandwidth: narrowband and broadband signals. The mathematical formulation has revealed the relation between the distribution of energy in their spectra and their immunity to noise. Sinusoidal and burst signals have higher signal-to-noise ratios (SNRs) but need many measurements to cover their frequency range. However, linear swept-frequency signals (chirp) improve the effective bandwidth covering a wide frequency range with a single measurement and equivalent accuracy, at the expense of a lower SNR. In the case of highly attenuating materials, it is proposed to use different configurations of chirp signals, enabling injecting more energy, and therefore, improving the sensitivity of the technique without a high time cost. Thus, if the attenuation of the material and the sensitivity of the measuring equipment allows the use of broadband signals, the combination of this kind of signal and suitable signal processing results in an optimal estimate of frequency-dependent attenuation with a minimum measurement time., This work has been supported by the Spanish Administration under grant BIA2014-55311-C2-2-P.

The detection and characterization of non-linearities in temporal series is a hot topic in some disciplines such as nondestructive testing of materials, bioacoustics and biomedical research domains. This is a complex interdisciplinary field where many different researchers are striving to achieve better and more sophisticated techniques. In this scenario, the search for new perspectives that can explain and unify some of the theories is of key importance. Recurrence Plots (RPs) and Recurrence Quantification Analysis (RQA) can play such a role. In this work, we show how RPs can be used to design tests for non-linear detection and characterization of complexity . The proposed tests are less parameter dependent and more robust than some of the traditional discriminating measures. We also illustrate the applicability of the proposed algorithms in simulations and real-world signals such as the analysis of anomalies in the voice production of mammals.