Diego Martinez Plasencia

Acoustic levitation enables mid-air displays using physical particles to create 3D visuals, but stability limits the achievable animation complexity. Stability depends on factors including the acoustic solver, particle count, motion speed, and path geometry. This paper analyzes these factors, characterizing their effects, identifying constraints, and allowing particles to successfully follow the paths. We then propose Acoustic Actor-Critic (AAC), a closed-loop motion planning system that maximizes stability for multi-particle trajectories with minimal changes to the intended visual content. This follows a plan-detect-repair strategy: i) the Actor plans trajectories under the established constraints; ii) the Critic evaluates their stability and detects instabilities; iii) the Repair modules trigger localized repairs upon unstable path segments. Results showed that AAC can automatically refine and repair multi-particle trajectories, reducing failures from 21% to 6% across 100 paths. Our findings enable creators to produce more stable levitation paths, while AAC automatically refines trajectories with minimal deviation from the original animations.

Acoustic techniques have emerged as a transformative approach in additive manufacturing, enabling contactless, high-precision manipulation of particles, droplets, and cells through acoustic wavefields. While acoustic patterning has demonstrated remarkable control over microscale spatial configurations, its scalability into three-dimensional (3D) fabrication has remained constrained by challenges in bonding limitations and vertical stacking. To solve this challenge, hybrid acoustic-assisted 3D printing integrates acoustophoresis with established methods, such as direct ink writing and stereolithography, enhancing material deposition accuracy, microstructure alignment, and porous structuring of 3D printed products. However, these hybrid systems remain tethered to layer-by-layer architectures, limiting the full potential of acoustic techniques. Recent breakthroughs in standalone acoustic 3D fabrication, leveraging levitation, focusing, and ejection, have unlocked potential in contactless, layerless, and flexible multi-material assembly. Acoustic levitation systems enable mid-air construction on non-planar surfaces, and focused ultrasound techniques facilitate deep-penetration polymerization and acoustic droplet ejection advances nozzle-free droplet-based bioprinting. This review systematically evaluates the evolution of acoustics in additive manufacturing, addressing critical challenges in material compatibility, resolution, and scalability, while outlining the future of acoustics in additive manufacturing technology.

Cooperative transport, the simultaneous movement of an object by multiple agents, has been widely observed in biological systems such as ant colonies, which improve efficiency and adaptability in dynamic environments. Inspired by these natural phenomena, we present a novel acoustic robotic system for the transport of contactless objects in mid-air. Our system leverages phased ultrasonic transducers and a robotic control system onboard to generate localized acoustic pressure fields, enabling precise manipulation of airborne particles and robots. We categorize contactless object-transport strategies into independent transport (uncoordinated) and forward-facing cooperative transport (coordinated), drawing parallels with biological systems to optimize efficiency and robustness. The proposed system is experimentally validated by evaluating levitation stability using a microphone in the measurement lab, transport efficiency through a phase-space motion capture system, and clock synchronization accuracy via an oscilloscope. The results demonstrate the feasibility of both independent and cooperative airborne object transport. This research contributes to the field of acoustophoretic robotics, with potential applications in contactless material handling, micro-assembly, and biomedical applications.

Additive manufacturing is critical in modern production with advanced applications across various domains. However, achieving omnidirectional printing that can adapt to varying substrate geometries is still a challenge. Moreover, multi‐material in situ printing without cross‐contamination presents another hurdle. In this study, an acoustophoretic 3D fabrication system capable of omnidirectional and multi‐material in situ 3D printing is reported using acoustic levitation. This system harnesses a phased array of transducers (PAT) to finely tune the ultrasonic field, generating acoustophoretic forces that levitate and transport objects in mid‐air. It allows omnidirectional in situ printing of materials onto complex substrates with diverse orientations in a contactless and voxel‐by‐voxel manner. It is capable of manipulating a broad spectrum of materials, including liquids with zero‐shear viscosities from 1 to 5,000,000 mPa·s, as well as solids. AcoustoFab has successfully printed structural, conductive, and biological materials in varying directions on complex substrates. Additionally, the contactless approach enables in situ printing safely on a delicate surface, as demonstrated by printing on a human hand. The flexibility and versatility of this approach demonstrate promising applications in areas ranging from biomedical engineering to industrial manufacturing.

Holographic acoustic levitation using phased arrays of transducers has facilitated innovative mid-air particle displays. However, current time-invariant methods for computing acoustic holograms often induce high phase changes and thus transducer amplitude fluctuations, severely limiting possible dynamic displays. In this work, we develop a wide range of temporal phase retrieval algorithms that suppress phase change and trade off computational time and solution optimality. We base our hardware implementation on Gerchberg-Saxton (GS), which we identify as gradient descent for maximizing focal amplitudes and thus acoustic trap quality. Following this, we adapt GS to additionally constrain transducers’ (and points’) phase changes from the previous time frame, and enable multiparticle animations without prior knowledge of particles’ paths and interactions. We experimentally showcase a series of levitated character animations depicting human motion and believe our work paves the way for natural dynamic multi-particle displays and unencumbered 3D delivery of other modalities such as haptics and audio.

Multi-point STM offers a great range of parameters (i.e., drawing frequency, number of points) to produce different tactile sensations. However, existing studies offer limited insight on the effects of these parameters, and ignore their effect on the physical stimuli delivered, limiting effective haptic design. We propose a two-stage model to predict response to multi-point STM. The first stage predicts physical stimulus properties with 7.8% error, while the second stage predicts mean and spread of perceived intensity with 8.0 % and 8.8% error. We report 3 studies conducted to derive this model: one to characterize physical stimuli, another one measuring user perceptual thresholds, and a third one measuring user’s perceptual response to multi-point STM. Besides, we characterize 4 effects that influence device performance, confirm if previous effects reported are due to physical or perceptual effects (or both) and derive recommendations for manufacturers, haptic designers and HCI researchers.

For more than a century, segmented displays, such as the seven‐segment display, have been a popular and cost‐effective option. They can display various commonly used characters by controlling a few custom‐shaped binary light emitters. Herein, the acoustic equivalent of segmented displays, which uses heterogeneous sound modulators to generate a limited set of acoustic holographic images, is introduced. Designing segmented acoustic displays is more challenging than optical ones due to the complex relationship between emitted sound and generated holographic images. To address this challenge, a design methodology based on unsupervised learning techniques is proposed. The approach balances the cost of acoustic displays and the quality of the images they generate, resulting in segmented displays that outperform existing general‐purpose ones when generating a finite set of acoustic images. Using simulations and physical fabrication of metamaterial‐based acoustic displays, it is proven that the approach can create segmented acoustic displays that produce high‐quality images at a lower cost. Additionally, the methodology is applied to multifrequency acoustic displays and its scalability is assessed as the number of images increases. The cost reduction through segmentation presented herein is expected to democratize sound manipulators for acoustic displays and other applications like acoustic levitation and noise cancellation.