Realizing broadband sub-wavelength focusing and a high intensity enhancement with a space-time synergetic modulated acoustic prison†
We experimentally demonstrate broadband sub-wavelength focusing and a high sound intensity enhancement using an acoustic prison. The acoustic prison refers to a sub-wavelength metamaterial reflection cavity surrounded by rigid walls with some openings, which could provide a space regulation to the sound field. By employing a time-reversal method to focus the sound, and arranging an acoustic prison at the focal position during the convergence stage, broadband sub-wavelength focusing with a focal size equal to the inner size of the prison and much smaller than that of the operating wavelength was demonstrated. Theoretically, the time-reversal focusing process is equivalent to providing a time manipulation to the sound field, which constitutes a space-time synergetic manipulation system together with an acoustic prison for space manipulation. In such a system, most of the waves entering the prison were multi-reflected by the cavity walls and trapped in the cavity much like a prisoner, to form a high acoustic intensity region. The formation of such high sound intensity regions depended on the synergetic superposition of cavity resonance and multiple reflections, and the resonance sound field participated in the focus construction to enhance the sound intensity inside the prison and created subwavelengths outside at frequencies around the resonant frequency of the cavity. The multiple reflections of the prison walls determined both the characteristics of the broadband focusing effect in the prison and the stable focal size under frequency change. In addition, by using multi-level prison structures to increase the effective acoustic reflection area of the walls, the focused sound signals could be further improved.