The t matrix of a hole in an acoustic waveguide is here obtained and applied to study the scattering of sound waves propagating inside a waveguide. It is found that a hole drilled in a waveguide behaves as a cylindrical unit whose dynamical mass density is lower than that of the surrounded background. This property has been used here to design an acoustic refractive lens. Numerical experiments based on multiple scattering simulations confirm the sound focusing by the designed device.

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