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In a published in Science Advances, researchers from Technical University of Denmark and Universidad Politécnica de Madrid demonstrate a new device called an acoustic rainbow emitter (ARE) that takes in broadband white-noise signals from a point source that radiates sound equally in all directions and scatters it up so that different sound frequencies or pitches are emitted.

Similar to how a prism splits white light into a rainbow, the ARE device steers each frequency in different directions, creating an acoustic rainbow.

In nature, some animals—like humans, bats, and dolphins—have evolved intricate ears (pinnae) that can catch, shape and direct sound in amazing ways, helping them sense and navigate their surroundings.

Despite ample natural inspiration, humans have struggled to design systems that can work on a wide range of frequencies. Unlike nature, which utilizes passive structures to shape sound, most artificial sound control systems require active devices or resonance-based systems.

Existing acoustic systems have demonstrated sound splitting in closed environments, but they have yet to match the fully controlled, broadband auditory manipulation in free spaces, similar to biological systems.

The researchers of this study set out to change that with an approach powered by computational morphogenesis—a process that utilizes algorithms for structural optimization and finite element analysis to generate complex shapes.

With tools like topology optimization, accurate wave-based modeling, and modern fabrication techniques such as 3D printing, the researchers had unprecedented freedom to design complex structures that can manipulate sound in entirely new ways.

This allowed them to iteratively adjust the shape of solid material to control the sound emitted to match a specific pattern across frequencies. The researchers also used the Helmholtz equation to simulate sound propagation and scattering in the air around a rigid, sound-reflecting structure.

Based on the data collected from the computational models, the team created a new single-material solid object with intricate scattering properties that could capture a mix of sound frequencies from a single source and split them into their spectral components, creating an acoustic rainbow.

Apart from the ARE, the researchers also designed a lambda splitter that takes in a mix of sound frequencies and directs low and high-frequency sound waves in different directions.

Both devices operate on the principle of passive scattering, where the acoustic system is purely driven by interactions between the hard plastic surface and sound waves, requiring no electricity. The rainbow emitter and the lambda splitter are excellent examples of how a smart arrangement of passive structures can be used to control sound without relying on energy-intensive resonance or active components.

The researchers note that this study establishes the potential of computational morphogenesis to precisely shape how sound fields are emitted and received, providing valuable insights for disciplines that deal with wave sensing and control.

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