Rosa Martínez-Sala

Acoustic barriers based on periodic arrays of scatterers

It is well known that certain periodic structures built by repetition of elements produce sound attenuation effects as a consequence of the destructive interference of the scattered waves by these elements. The sound attenuation results that we got from transmission experiments with these kind of structures, so-called sonic crystals (SCs), led us to think that SCs could be used as an acoustic barrier. Until now, most of the transmission experiments with these periodic arrays of scatterers have been performed under controlled conditions, so how they would behave outdoors is still not well known. In this letter we present outdoor-experimental results for two-dimensional SCs and from these it can be concluded that periodic arrays of scatterers are a suitable device to reduce noise in free-field conditions.

Reflectance properties of two-dimensional sonic band-gap crystals

An analysis of the reflectance of sonic band-gap crystals consisting of square arrays of rigid cylinders in air is presented. The standing wave formed in front of the structures is studied both experimentally and theoretically. Experiments have been performed with a mobile robotized microphone that obtains pressure maps on the plane perpendicular to the axes of the cylinders. Enhancements of the standing wave ratio (SWR) are observed in frequency regions where attenuation bands appear in zero-order transmission experiments. Also, the SWR presents oscillations that can be related to the finite dimension of the structure (Fabry-Perot effect). Both features are well described by calculations based on a double-scattering approach.

The existence of full gaps and deaf bands in two-dimensional sonic crystals

Theoretical and experimental determination of sonic band structures of two-dimensional (2-D) arrays of rigid cylinders in air is reported. We present measurements for square and triangular lattices. A variational method is employed to calculate the acoustic dispersion relation. Experimentally, a transmission technique and the analysis of the phase delay between the incident and scattered waves by the structure are used to construct the acoustic bands. The comparison between theory and experiments allows to fully characterize the band gaps and it has also demonstrated the existence of deaf bands; i.e., bands which cannot be excited due to symmetry reasons. For the case of square lattice we show that the structure with a filling fraction of 0.41 has a full acoustic gap.

Design of lightweight multilayer partitions based on sonic crystals

The sound transmission coefficient of different multilayer partitions commonly encountered in buildings has been measured as a function of frequency. Most of the samples studied showed an increase in the sound transmission coefficient over a specific frequency, called the critical frequency, depending on the layer material. However, for partitions built with the same materials, but built with a periodic arrangement of layers, this behavior has not been observed. This kind of periodic multilayer partition can be considered as a sonic crystal, because the stopband corresponding to a one-dimensional sonic crystal with a constant lattice equal to the modulation of the partition is in the same range as the critical frequency of the panel.

Sound attenuation by a two dimensional array of cylinders

Here, it is shown, for the first time, a systematic analysis (experiment and theory) of the acoustic transmission of a two‐dimensional periodic array of rigid cylinders with two different configurations: square and triangular. The influence of the filling fractions, ranging from 0.06 up to 0.41, on the appearance of the pseudogaps and full band gaps is studied. Above a certain filling fraction, an overlap is observed between the attenuation peaks measured along the two high symmetry directions of the Brillouin zone. This effect is considered as the fingerprint of the existence of a full acoustic gap. Nevertheless, the comparison with our calculation of band structures shows that the lattice has band states in that frequency range. These bands are called deaf bands (i.e., they cannot be excited by experiments of sound transmission) [Sanchez‐Perez et al., Phys. Rev. Lett. (to be published)].

Waterfront Depth Analysis in Hardened Concrete by Means of the Nondestructive Ground-Penetrating Radar Technique

Durability of concrete structures depends mainly on the ease whereby water and any aggressive chemical agents dissolved therein can penetrate. Therefore, measuring water penetrability in concrete structures is crucial mostly when structures are in service. In this context, nondestructive techniques play an important role. In particular, the electromagnetic waves emitted by ground-penetrating radar (GPR) are very sensitive to the water content of the medium through which they propagate. This fact provides an interesting opportunity to analyze if the GPR technique allows the assessment of water penetrability in concrete with enough accuracy. In line with this, this paper describes the laboratory experiments and relevant analysis carried out to study the capability of GPR to assess water penetrability in hardened concrete. For this purpose, concrete specimens were fabricated and dried in an oven after 90 days of curing. They were then dipped into water and GPR measurements were taken at different intervals, based on coupling a 2.0 GHz antenna. The results showed that the agreement between velocity increments and the waterfront advance was excellent. In addition, a specific processing of the data acquired was developed. This process included the isolation of the reflection due to the waterfront, produced just before the reflection of the bottom of the samples. As a result of this processing, the in-depth waterfront location at different times was determined with high reliability.

Study of the waterfront advance in hardened concrete by means of energy level increment analysis

The durability of concrete structures depends mainly on the ease with which the water and aggressive chemical agents dissolved therein penetrate it. Moreover, the electromagnetic waves emitted by Ground-penetrating radar (GPR) are very sensitive to the water content of the medium through which they propagate. For all this, the present experimental study analyzed the sensitivity of GPR to assess the location of the waterfront in concrete by means of the comparison of the energy level increments with the absorption coefficient. For this purpose samples of concrete were fabricated and after curing were introduced into water for different time intervals. The results showed that depending on the placement of the antenna regarding the location of the waterfront the agreement varied considerably. Best agreements were found when the antenna was placed in the same surface that was immersed into water and the reflected waves were analyzed.

Non-destructive characterization of maritime pine sawn timber dielectric anisotropy by means of GPR

The dielectric response of timber to electrical fields is influenced by many factors, one of the most important being the internal structure of the wood. Thus the anisotropy of the wood should be taken into account when the ground-penetrating radar (GPR) technique is used to explore the properties of timber. The work carried out analyzes, by means of a GPR with a 1.6 GHz antenna, the dielectric behavior of Maritime Pine timber according to the direction of the electrical field with respect to the grain. After the acquisition process, the propagation velocities, amplitudes, and spectra variations were compared for all the studied grain directions. Significant differences were found between these parameters when the field was propagated in a parallel direction compared with perpendicular to the grain. However, when the field was propagated in various perpendicular directions to the grain (radial & tangential) the studied parameters were not found to differ greatly.

Ability of the Direct Wave Amplitude of Ground-penetrating Radar for Assessing the Moisture Content Variation of Timber

The moisture content (MC) has a significant influence in wood’s physicochemical properties and therefore in most types of wood pathologies. The MC variation leads to modifications of its electromagnetic properties and to specific effects upon waves’ characteristics. Given the advantages of the direct electromagnetic wave (DW) observation, the aim of this work is to asses the ability of this wave to provide access to timber MC characterization. For this purpose, GPR measurements were carried out on timber joists (1.6 GHz central frequency). The dielectric behaviour of wood varies according to the direction of the fiber. Therefore, it is necessary to distinguish between differences attributable to the wood anisotropy and those due to variations in MC. With this aim, prior to the MC analysis, assays were carried out in different directions regarding the fiber. The results of this study show very satisfactory correlations between DW amplitudes and MC in all fiber directions. These results showed, in agreement with theory, that a decrease of MC results in an increase of the DW amplitude. These results demonstrates the GPR efficiency and the innovative application of this technique as a non-destructive evaluation tool for the rapid sounding of timber structures.

Refractive acoustic devices for airborne sound

We show that a sonic crystal made of periodic distributions of rigid cylinders in air acts as a new material which allows the construction of refractive acoustic devices for airborne sound. It is demonstrated that, in the long-wave regime, the crystal has low impedance and the sound is transmitted at subsonic velocities. Here, the fabrication and characterization of a convergent lens are presented. Also, an example of a Fabry-Perot interferometer based on this crystal is analyzed. It is concluded that refractive devices based on sonic crystals behave in a manner similar to that of optical systems.