The relative dislocation density of aluminum and copper samples is quantitatively measured using linear Resonant Ultrasound Spectroscopy (RUS). For each metallic group, four samples were prepared with different thermomechanical treatments in order to induce changes in their dislocation densities. The RUS results are compared with Nonlinear Resonant Ultrasound Spectroscopy (NRUS) as well as Second Harmonic Generation (SHG) measurements. NRUS has a higher sensitivity by a factor of two to six and SHG by 14–62%. The latter technique is, however, faster and simpler. As a main result, we obtain a quantitative relation between the changes in the nonlinear parameters and the dislocation density variations, which in a first approximation is a linear relation between these differences. We also present a simple theoretical expression that explains the better sensitivity to dislocation content of the nonlinear parameters with respect to the linear ones. X-Ray diffraction measurements, although intrusive and less accurate, support the acoustics results.

It has recently been shown that the mechanical properties of thin, rectangular wooden plates can be tuned by carving them with specific patterns of perforations, effectively realising a 2D wooden mechanical metamaterial. Such a material is of great interest for the construction of musical instruments, as it could allow a new degree of creative control for makers. Furthermore, issues with the shrinking supplies of tone-woods could be alleviated as wood samples that don not meet the desired requirements could simply be altered, instead of being discarded. In this work, we study the effect of the use of these metamaterials in the soundboards of classical guitars. By way of simulations, we evaluate their impact on the modal behaviour and on the sound pressure level of the instrument, as well as on its ability to sustain the load exerted by the strings. Our results show that the metamaterials can tune the instrument’s response without compromising its structural integrity. We thus conclude that the use of wooden mechanical metamaterials in the soundboards of classical guitars is feasible and, in many ways, beneficial, not the least since it opens the door to using non-traditional woods with bespoke density and stiffness.

Se presentan los primeros resultados de una investigación sobre el diseño y fabricación de una guitarra impresa en 3D. En particular, se introduce y analiza un modelo modular optimizado para su producción en impresoras de escritorio de tamaño estándar. El objetivo es visibilizar el modelo, reflexionando sobre el proceso de diseño digital y las técnicas de fabricación aditiva, enfatizando sus intersecciones y diferencias clave con la luthería tradicional. Para ello, esta investigación se sitúa en el contexto actual, exponiendo el estado del arte sobre la aplicación de la fabricación digital en la construcción de guitarras. Luego, se detallan las decisiones de diseño, materiales y fabricación que guiaron el desarrollo del modelo, entre ellas la modularidad y dimensiones. Se exponen las principales virtudes y falencias del modelo, y se proponen cambios a implementar en una segunda versión. Finalmente, se presentan reflexiones sobre futuras exploraciones y los impactos esperados en la luthería local y en investigaciones relacionadas con la respuesta vibroacústica de instrumentos de cuerda. Es importante enfatizar que este trabajo se centra en los aspectos estructurales del modelo, los cuales deben optimizarse antes de abordar el estudio del timbre del instrumento. No se incluye un análisis vibroacústico de la guitarra, el que es parte de trabajos en desarrollo.

Knowledge of the state of plastic deformation in metallic structures is vital to prevent failure. This is why non-destructive acoustic tests based on the measurement of first order elastic constants have been developed and intensively used. However, plastic deformations, which are usually heterogeneous in space, may be invisible to these methods if the variation of the elastic constants is too small. In recent years, digital image correlation techniques, based on measurements carried out at the surface of a sample, have been successfully used in conjunction with finite element modeling to gain information about plastic deformation in the sample interior. Acoustic waves can penetrate deep into a sample and offer the possibility of probing into the bulk of a plastically deformed material. Previously, we have demonstrated that nonlinear acoustic methods are far more sensitive to changes in dislocation density than linear ones. Here, we show that the nonlinear Second Harmonic Generation method (SHG) is sensitive enough to detect different zones of von Mises stress as well as effective plastic strain in centimeter-size aluminium pieces. This is achieved by way of ultrasonic measurements on a sample that has undergone a three-point bending test. Because of the relatively low stress and small deformations, the sample undergoes plastic deformation by dislocation proliferation. Thus, we conclude that the nonlinear parameter measured by SHG is also sensitive to dislocation density. Our experimental results agree with numerical results obtained by Finite Element Method (FEM) modeling. We also support the acoustic results by X-Ray Diffraction measurements (XRD). Although intrusive and less accurate, they also agree with the acoustic measurements and plastic deformations in finite element simulations.

Wood remains the material of choice for musical instrument soundboards, but its inherent physical and mechanical variability limits the reproducibility of vibroacoustic behaviour across nominally identical instruments. This study quantifies the vibroacoustic reproducibility achieved by additive manufacturing in soprano ukulele construction, compared against two traditional tonewoods (Engelmann spruce and Chilean laurel), across three successive stages of assembly: free soundboards, soundboards coupled to identical 3D-printed bodies (full soundboxes), and complete assembled instruments. In the first two stages the soundboard is the only variable; at the complete-instrument stage the comparison extends to three independently fabricated ukuleles, so that the additional variability introduced by assembly, stringing and setup is also included. Inter-specimen similarity was quantified using metrics matched to the nature of each measured signal: the Pearson correlation coefficient for free-plate spectra, FRAC-based metrics for full soundbox admittance, and the Modal Assurance Criterion for mode shapes of complete instruments. Free PLA+ plates showed substantially higher intra-group correlation ($\bar{r}=0.87\pm0.02$) than Engelmann spruce ($0.58\pm0.11$) and Chilean laurel ($0.39\pm0.07$), a pattern that persisted against laurel but not against spruce at the full soundbox stage, where PLA+ and spruce became statistically indistinguishable, behaviour mainly attributed to the manufacturing process, specifically to the coupling between the body and the soundboard. Inter-specimen level differences in radiation among the three PLA+ ukuleles remained at or below 1.7\,dB at all first four identified frequencies. A Hybrid instrument sharing the PLA+ body and cavity but fitted with an Engelmann spruce soundboard was used as a separate material-sensitivity control, departing from the PLA+ group by up to 6.0\,dB at two of the four frequencies, while damping ratios showed no systematic difference between the PLA+ specimens and the Hybrid. These results indicate that additive manufacturing offers markedly high and uniform vibroacoustic reproducibility, supporting its use as a controlled platform for experimental lutherie and acoustic research, within the scope of the two tonewoods and single geometry examined here.