![]() ![]() 1(b)) exhibits a unidirectionally collimated acoustic beam and its radiation pattern calculated in the far field (Fig. When sources at P1 and P2 inside the enclosure radiate airborne sounds of wavelength λ = 1.14 D, the acoustic pressure field (Fig. The dissipation effect will be considered in the latter case). The simulation is done using the acoustic module preset in COMSOL 30, without considering the dissipation effect (To reveal the essential physics, the simulation excludes dissipation. To demonstrate enhanced directivity by the enclosure, we choose P1 and P2 as two in-phase monopole sources, which are selected for optimizing constructive interference outside the enclosure. In practice, an enclosure with D = 30 cm can accommodate cell phone transducers ( ∼3 mm) well inside its interior. a constant volume flow rate from sources, as a universal model 28, 29. The sources are simulated with a constant source strength, i.e. In the interior of the enclosure, point sources can be deployed at four locations denoted by the blue dots (P1, P2, P3, and P4) as shown in the right panel of Fig. Additionally, a circular structure with coiled channels should have the radially dependent density as later evidenced by our derivation, which was missed in prior studies. Note that the previous effective medium model simplifies a similar space-coiling structure as a homogeneous and isotropic medium 20, which is different from our proposed effective medium model that accurately characterizes degenerate Mie resonances and the feature of inhomogeneity and anisotropy, as will be shown later. Here, the enclosure is adapted for generating resonances that are responsible for directivity enhancement. The configurability of our setup for achieving different types of directivity patterns can be achieved by customized source layouts. Such directivity enhancements by our configurable sound system are attributed to the interplay between wave interferences and degenerate Mie resonances in the subwavelength space. By deploying the sources inside the enclosure, we numerically demonstrate the formation of directivity enhancements such as unidirectional and bidirectional beam forming. The enclosure contains several monopole sources at low frequencies that radiate otherwise sound of indistinguishable directivity in free space. In this paper, we propose to achieve enhanced radiation directivity of acoustic sources by encompassing configurable multiple sources inside an enclosure. Efficient emission of configurable and highly directional sound beams from a subwavelength acoustic system remains as a challenging task. In all of these studies, the source-structure system has a limitation on the flexible adjustment of the radiation directivity. A latest attempt imitated a dipolar field from a monopole source enclosed in a subwavelength structure of hybrid resonances 21. The degenerate Mie resonances were caused by the super anisotropy of the enclosure, which cannot be inferred from the previous isotropic model 20. proposed enhancing multipole source emission by using a deep-subwavelength cavity of degenerate Mie resonances to enclose the source, whose radiation pattern and directivity is preserved. There are a few recent attempts on achieving efficient emission of directional sound waves from a miniaturized subwavelength source. Such a large overall scale limits the capability of device miniaturization. ![]() Even so, the entire sound system in these studies is much larger than the working wavelength. These schemes modify either boundary conditions, resonances, or the entire propagation region to adjust sources’ directivity. Recently, with the rise of acoustic metamaterials, controlling directivity or steering acoustic beams has been discussed in various schemes, such as grating diffraction 7, 8, band structure engineering 9, 10, 11, 12, 13, 14, transformation coordinates 15, acoustic metasurfaces 16, 17, and dual metastructures 18. To achieve high radiation directivity, traditional methods have adapted to use a large horn mouth to confine the sources’ radiation space or use a long phased array to incur wave interference have been adapted. However, directivity of acoustic sources at low frequencies in free space or in a subwavelength sound system is always poor 6. Sound radiation with high directivity is important in many areas including loudspeaker design, room acoustics, medical ultrasonics, and psychoacoustics 1, 2, 3, 4, 5. Radiation directivity, as one of the important characteristics of a sound source system, describes the quality of a collimated acoustic beam. ![]()
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