In this work, non-linear ultrasonic wave techniques have been used to detect the onset of micro-cracking due to steel corrosion in model reinforced concrete elements. The specimens were of prismatic shape with a single steel rebar. The corrosion was forced by admixing an appropriate amount of sodium chloride at the moment of preparing the concrete mix, and by the application of an electric field, using a constant current density power source, and making the steel rebar work as the anode, and an external counter-electrode as the cathode. The preliminary results indicate that the onset of cracking seems to be accompanied by the appearance of higher-harmonic generation at the output signal (harmonic distortion), when the system is excited by the means of an ultrasound wave with a burst central frequency. Other phenomena related to the micro-cracks induced by corrosion, such is the parametric generation with respect to the fundamental amplitude, have not been observed until now.

ABSTRACT:In the present paper, a methodology is proposed for obtaining empirical equations describing the sound absorption characteristics of an absorbing material obtained from natural fibers, specifically from coconut. The method, which was previously applied to other materials, requires performing measurements of airflow resistivity and of acoustic impedance for samples of the material under study. The equations that govern the acoustic behavior of the material are then derived by means of a least-squares fit of the acoustic impedance and of the propagation constant. These results can be useful since they allow the empirically obtained analytical equations to be easily incorporated in prediction and simulation models of acoustic systems for noise control that incorporate the studied materials. RESUMEN:Un modelo para materiales absorbentes acústicos derivados de la fibra de coco. En este trabajo se describe el proceso seguido para obtener ecuaciones empíricas del comportamiento acústico de un material absorbente obtenido a partir de fibras naturales, concretamente el coco. El procedimiento, que ha sido ensayado con éxito en otros materiales, implica la realización de medidas de impedancia y resistencia al flujo de muestras del material bajo estudio. Las ecuaciones que gobiernan el comportamiento desde el punto de vista acústico del material se obtienen a partir del ajuste de ecuaciones de comportamiento de la impedancia acústica y la constante de propagación del material. Los resultados son útiles ya que, al disponer de ecuaciones analíticas obtenidas empíricamente, facilitan la incorporación de estos materiales en predicciones mediante métodos numéricos del comportamiento cuando son instalados formando parte de dispositivos para el control del ruido.

ABSTRACT:The use of sustainable materials is becoming a common practice for noise abatement in building and civil engineering industries. In this context, many applications have been found for porous concrete made from lightweight aggregates. This work investigates the acoustic properties of porous concrete made from arlite and vermiculite lightweight aggregates. These natural resources can still be regarded as sustainable since they can be recycled and do not generate environmentally hazardous waste. The experimental basis used consists of different type specimens whose acoustic performance is assessed in an impedance tube. Additionally, a simple theoretical model for granular porous media, based on parameters measurable with basic experimental procedures, is adopted to predict the acoustic properties of the prepared mixes. The theoretical predictions compare well with the absorption measurements. Preliminary results show the good absorption capability of these materials, making them a promising alternative to traditional porous concrete solutions. RESUMEN: Propiedades acústicas del hormigón poroso a base de áridos ligeros de arlita y vermiculita.El uso de materiales sostenibles se está convirtiendo en una práctica común para la reducción de ruido en las industrias de la edificación e ingeniería civil. Este trabajo investiga las propiedades acústicas de hormigón poroso fabricado a partir de áridos ligeros de arlita y vermiculita. Estos recursos naturales todavía pueden considerarse sostenibles ya que pueden ser reciclados y no generan residuos peligrosos para el medio ambiente. La base experimental utilizada se compone de especímenes de diferente tipo cuyas prestaciones acústicas se evalúan en un tubo de impedancia. Adicionalmente, se ha adoptado un modelo teórico simple para medios porosos granulares, basado en parámetros medibles con procedimientos experimentales básicos, con objeto de predecir las propiedades acústicas de las mezclas preparadas. Las predicciones teóricas muestran una buena concordancia con las medidas de absorción. Los resultados preliminares demuestran la buena capacidad absorbente de estos materiales, haciendo de ellos una alternativa prometedora a las soluciones de hormigón poroso tradicionales.

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ElsevierCarbajo San Martín, J.; Ramis Soriano, J.; Godinho, L.; Amado-Mendes, P.; Alba Fernández, J. (2015). A finite element model of perforated panel absorbers including viscothermal effects. Applied Acoustics. 90:1-8. doi:10.1016/j.apacoust.2014.10.013. A FINITE ELEMENT MODEL OF PERFORATED PANEL ABSORBERS INCLUDING VISCOTHERMAL EFFECTS

Low-frequency airborne noise reduction is an issue of major concern in most practical cases due to the limiting space constraints. The applicability of acoustic resonators that not only work in this frequency range but can also be tuned is of great interest in many noise control applications such as muffler devices, noise barriers, or building isolation walls. This Letter studies the acoustic behavior of perforated panel absorbers with oblique perforations. Unlike more complex devices, the proposed absorber uses a simple concept that relies on the increase in the effective length of the panel by using perforations aligned obliquely with respect to the panel surface. In doing so, a shift of the resonance frequency toward low frequencies along with an increase in the sound absorption can be achieved provided that the geometrical characteristics of the absorber are properly chosen. A simple predictive model that relies on the fluid-equivalent theory was developed to investigate the acoustic properties of these absorbers, measurements in an impedance tube over additive manufactured samples serving to confirm the previous assertions. Preliminary results show the potential of these absorbers and encourage their further development for practical purposes.

The use of porous concrete solutions with lightweight aggregates has become increasingly common in noise control due to their versatility in exterior and interior applications. In this work, samples of porous consolidated concrete with aggregates of expanded clay were produced, in order to study the influence of the grain size, thickness and water/aggregate/cement ratio on the sound absorption. Experimental techniques were used to obtain the surface impedance and sound absorption coefficient. In addition to experimental characterizations, an inverse method was used (based on a genetic algorithm) to obtain the macroscopic parameters capable of representing the materials studied through the theoretical model of Horoshenkov-Swift. Using the theoretical Horoshenkhov-Swift model it becomes possible to represent these materials in numerical models as equivalent fluids.

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