Morphogenesis of sound creates acoustic rainbows

Abstract

Sound is an essential sensing element for many organisms in nature, and multiple species have evolved organic structures that create complex acoustic scattering and dispersion phenomena to emit and perceive sound unambiguously. To date, it has not proven possible to design artificial scattering structures that rival the performance of those found in organic structures. Contrarily, most sound manipulation relies on active transduction in fluid media rather than relying on passive scattering principles, as are often found in nature. In this work, we utilize computational morphogenesis to synthesize complex energy-efficient wavelength-sized single-material scattering structures that passively decompose radiated sound into its spatio-spectral components. Specifically, we tailor an acoustic rainbow structure with "above unity" efficiency and an acoustic wavelength-splitter. Our work paves the way for a new frontier in sound-field engineering, with potential applications in transduction, bionics, energy harvesting, communications and sensing.

Figures (4)

• Figure 1: Visualization of the Acoustic Rainbow Emitter (ARE). The morphogenetical TopOpt method shapes the scattering inclusions that compose the, shown as gray material. When the ARE is excited by mono-polar source emitting broad-band white noise, the radiated sound creates an acoustic rainbow. The source is positioned at the center of the emitter (illustrated using white light), and driven with equal power at all frequencies from 7600 Hz to 12800 Hz. In the figure, the experimentally measured acoustic output (far field) is mapped to the visible spectrum of light by its magnitude and frequency content in the full 360$^{\text{o}}$ surrounding the ARE. (See the Supplementary Information for a detailed description).
• Figure 2: ARE geometry and emitted sound pressure: (left column) numerical results and (right column) experimental results. (A) Blueprint for the ARE designed by our morphogenetic computational approach, the mono-polar source position is indicated by a red dot. (B) Experimental construction of the ARE (3D printed design), sample size and angular convention are overlaid. (C-D) Maps of the max-normalized far field pressure. (C) Simulated emission, the spatio-spectral decomposition is observed as different frequencies are emitted in different angles. (D) Experimental measurements of the sound pressure emitted by the ARE, showing an almost perfect agreement with simulations (see Supplementary Information for the data treatment procedure). (E-F) Per frequency max-normalized far field power. (E) Simulated sound emission. (F) Experimental measurements performed on the 3D printed sample.
• Figure 3: Lambda-splitter geometry and emitted sound pressure: (left column) numerical results and (right column) experimental results. (A) Blueprint for the lambda-splitter designed with the morphogenetic approach, the red dot indicating the source position. (B) Experimentally constructed sample (3D printed) with corresponding dimensions. (C-D) Max-normalized far field pressure maps. The cyan and magenta vertical lines indicates the bounds of the frequency bands targeted in the design process. (C) Simulated sound emission, the majority of the energy of all spectral components are observed to be directed into the +35$^{\text{o}}$ direction or the -35$^{\text{o}}$ direction depending on frequency. (D) Experimental measurements performed on the 3D-printed emitting device showing remarkable agreement with simulations (see Supplementary Information for data treatment procedure). (E-F) Per frequency max-normalized far field power, showing a clear decomposition of the spectral components. (E) Simulated far field emission. (F) Experimental measurements.
• Figure 4: (a) Blueprint for the ARE with numbering of the 28 features and a colormap illustrating the deterioration of $\Phi$ as each single feature is removed showing that (almost) all features have significant importance for $\Phi$. (b-c) Far-field pressure map for the ARE with (b) feature 8 removed and (c) feature 14 removed showing the deterioration of the emission pattern as individual features are removed. (Compare these to the original ARE radiation emission pattern in Fig. \ref{['FIG:ACOUSTIC_RAINBOW_INVESTIGATION']}c.)