Environmental noise is a growing public health issue, particularly in dense urban environments and areas adjacent to transport infrastructure. Passive acoustic barriers have been a widely adopted solution, although their functional and aesthetic limitations have driven the development of alternatives, such as Sonic Crystal Acoustic Barriers. These structures can incorporate resonant elements, such as Helmholtz cavities, with the aim of enhancing attenuation in particular frequency bands. However, determining the precise dimensions of these resonators is a persistent challenge, given that classical models are based on ideal geometries and do not incorporate interaction with the periodic structural environment. This study sets a new frequency-dependent correction factor, obtained both numerically and experimentally, which allows the classical Helmholtz resonance expression to be adjusted to the case of resonators embedded in a Sonic Crystal (SC). The proposed model is validated through simulations and experimental measurements, showing a significant improvement in the prediction of the resonance frequency. The results obtained in this study allow us to move towards a more efficient and realistic design of passive acoustic barriers that are lightweight and adaptable to the urban environment.

Acoustic lenses based on Fresnel Zone Plates provide an efficient and low-cost approach for ultrasound focusing, with potential applications in Focused Ultrasound, High-Intensity Focused Ultrasound, and blood–brain barrier opening. This work presents the design, numerical analysis, and experimental validation of a bifocal Fresnel acoustic lens capable of simultaneously generating two independent acoustic focal spots. The lens was designed by combining two optimized Fresnel zone distributions under spherical-wave incidence conditions, introducing geometric tuning parameters ($c_1=0.97$ and $c_2=0.40$) to maximize the effective aperture and balance the acoustic energy delivered to both foci. The acoustic response was investigated using finite element simulations with an axisymmetric model. Numerical results predicted well-defined focal regions with FLHM values of $2.25\lambda$ and $3.15\lambda$, and FWHM values of $0.73\lambda$ and $0.75\lambda$. The lens was subsequently manufactured from brass and experimentally characterized in an automated immersion tank using a piston transducer and a needle hydrophone. Experimental measurements confirmed the simultaneous generation of both focal spots. The measured lateral beam widths agreed well with the numerical predictions, with deviations below 11% and an average discrepancy of approximately 7%. Furthermore, the spatial acoustic gain maps showed that the acoustic energy remained concentrated within the intended focal regions despite the complex interference pattern inherent to the multifocal field. These results demonstrate the feasibility of the proposed optimized bifocal Fresnel acoustic lens as a passive multifocal ultrasound focusing device for future biomedical applications.