We report a theoretical and experimental study of an array of Helmholtz resonators optimized to achieve both efficient sound absorption and diffusion. The analysis starts with a simplified 1D model where the plane wave approximation is used to design an array of resonators showing perfect absorption for a targeted range of frequencies. The absorption is optimized by tuning the geometry of the resonators, i.e., by tuning the viscothermal losses of each element. Experiments with the 1D array were performed in an impedance tube. The designed system is extended to 2D by periodically replicating the 1D array. The 2D system has been numerically modeled and experimentally tested in an anechoic chamber. It preserves the absorption properties of the 1D system and introduces efficient diffusion at higher frequencies due to the joint effect of resonances and multiple scattering inside the discrete 2D structure. The combined effect of sound absorption at low frequencies and sound diffusion at higher frequencies, may play a relevant role in the design of noise reduction systems for different applications.

The broadband reduction of the specular reflections by sonic crystals (SCs) is theoretically and experimentally reported in this work. The analysed system consists of a sound source radiating a SC made of acoustically rigid scatterers embedded in water partially covering an open cavity. By comparison with a reference flat reflector, we observe that reflected waves spread in space as a consequence of the spatially modulated properties of the SC. Moreover, due to the different working frequency ranges of the SC a significant noise reduction is produced in a broadband region. Therefore, due to the spreading of the reflected waves, the system produces a broadband noise reduction in the area of the source. In particular, the noise reduction is close to 2 dB for the two octaves emitted by our source, which represents a decrease of 37% of the acoustic energy. The results shown in this work constitute a proof of concept for the use of SCs as broadband-noise reduction systems at the launch pad. An approach to the geometry of the Vega launch vehicle the European Space Agency is proposed and the limitations of the study are discussed.

The localization of sound sources has received increasing interest over the last few decades, given its wide range of applications. The triangulation method using the Time of Arrival (ToA) of a signal has shown to be useful and easy-to-use and, at the same time, provides accurate results. In this work, the acoustic trilateration method is applied in experimental measures to study and demonstrate its precision in air. Firstly, the method is tested in an anechoic chamber (low reverberating environment) demonstrating its functionality and accuracy. The next step has been the application of the method by using a low-cost system to demonstrate how a non-anechoic environment affects the accuracy of the localization. The detection of the received signal is implemented using a cross-correlation method in the time domain for both cases. Furthermore, the influence of the number and positions of the receiver that are used for this process in the accuracy of the results is also studied.

Since remote sensing of ocean colour began in 1978, several ocean-colour sensors have been launched to measure ocean properties. These measures have been applied to study water quality, and they specifically can be used to study algal blooms. Blooms are a natural phenomenon that, due to anthropogenic activities, appear to have increased in frequency, intensity, and geographic distribution. This paper aims to provide a systematic analysis of research on remote sensing of algal blooms during 1999–2019 via bibliometric technique. This study aims to reveal the limitations of current studies to analyse climatic variability effect. A total of 1292 peer-reviewed articles published between January 1999 and December 2019 were collected. We read all the literature individually to build a database. The number of publications increased since 2004 and reached the maximum value of 128 in 2014. The publications originated from 47 countries, but the number of papers published from the top 10 countries accounted for 77% of the total publications. To be able to distinguish between climate variability and changes of anthropogenic origin for a specific variable is necessary to define the baseline. However, long-term monitoring programs of phytoplankton are very scarce; only 1% of the articles included in this study analysed at least three decades and most of the existing algal blooms studies are based on sporadic sampling and short-term research programs.

Propagation of mechanical waves in unidimensional systems is a fundamental part of physics, necessary for learning subjects such as acoustics and vibrations. The vibration of transverse waves in strings is the easiest case of elastic system. Usually, this is the first continuous elastic system in which students apply fundamental mathematical concepts as vibration mode, equation of motion and boundary condition. In this work the use of simulation methods is proposed to reinforce the understanding of vibratory and acoustic simple phenomena. This will be applied to the case of a string, a beam and a membrane of finite length with different physical characteristics and boundary conditions. La propagación de ondas mecánicas en sistemas unidimensionales es una parte fundamental de la física, necesaria para el aprendizaje de asignaturas como acústica y vibraciones. La vibración de ondas transversales en cuerdas es el caso más sencillo de sistema elástico. Habitualmente, este es el primer sistema elástico continuo en el cual los alumnos aplican conceptos matemáticos fundamentales como modo de vibración, ecuación de movimiento y condición de contorno. En este trabajo se propone el uso de los métodos de simulación para reforzar la comprensión de fenómenos simples en acústica y vibraciones. Lo aplicaremos a los casos de cuerda vibrante, barras y membranas de longitud finita con diferentes características físicas y condiciones de contorno.