A model for acoustic transmission through a 2D square crystal of R-radius bubbles with a lattice constant L was previously proposed. Assuming a purely monopole response of the bubbles, this model offers a simple analytical expression of the transmission. However, it is not applicable when the bubbles are too close to each other ( L / R < 5). This article proposes an extension of the model by including the dipole response of the bubbles. Comparisons with numerical and experimental results show that the new expression gives a good estimate of the concentration at which the monopole model is no longer valid, but fails at properly predicting the transmission.

We present a theoretical and experimental study of the acoustic second-harmonic generation by a single layer of bubbles. This simple system allows us to investigate the subtle interplay between nonlinear effects and multiple scattering. A perturbative model is shown to give an excellent agreement with the experimental measurements, and we demonstrate the existence of an optimal concentration of bubbles, for which the harmonic generation is maximum. The potential of bubble screens as efficient subwavelength acoustic nonlinear sources is discussed.

We report a quasi-flat and sub-wavelength acoustic lens built using a soft matter process for broadband ultrasonic 3D focusing in water. By using a simple emulsion templating method, a soft porous material with a high acoustic index relative to water (∼5) has been molded and shaped into a convergent lens with a very low curvature and a thickness smaller than the working wavelength in water. This kind of acoustic metasurface generates a narrow focused beam, and its focal length can be adjusted. These acoustic lenses may be of great interest for underwater applications, where acoustic focusing and imaging are highly demanded.