| NSF Org: |
CMMI Division of Civil, Mechanical, and Manufacturing Innovation |
| Recipient: |
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| Initial Amendment Date: | December 22, 2020 |
| Latest Amendment Date: | May 20, 2021 |
| Award Number: | 2039463 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Yue Wang
yuewang@nsf.gov (703)292-4588 CMMI Division of Civil, Mechanical, and Manufacturing Innovation ENG Directorate for Engineering |
| Start Date: | January 1, 2021 |
| End Date: | December 31, 2024 (Estimated) |
| Total Intended Award Amount: | $380,909.00 |
| Total Awarded Amount to Date: | $380,909.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
201 OLD MAIN UNIVERSITY PARK PA US 16802-1503 (814)865-1372 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
Graduate Program in Acoustics university park PA US 16802-1503 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | Dynamics, Control and System D |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.041 |
ABSTRACT
Sonic crystals are artificially engineered materials that can control sound in unconventional ways, such as to camouflage sound or confine sound at desired locations. While single layers of sonic crystals have been primarily studied in the past, this research seeks to study a fundamentally new type of sonic crystals, where two sonic crystals are stacked with a small angle misalignment. Such a bilayer configuration forms an artistic moiré pattern, commonly found in textiles. This project will broadly advance the field of acoustic functional materials by endowing sonic crystals with a new set of capabilities to manipulate sound. The resulting bilayer sonic crystals are expected to facilitate applications such as energy harvesting and enhanced acoustic emission and sensing. The research activities will involve undergraduate students, as well as students from under-represented groups. Innovative outreach activities will also be enabled, such as a partnership with the Palmer Museum of Art at Penn State University for a special exhibition on bilayer sonic graphene, with the theme to forge an unexpected bond between art (moiré pattern) and science.
Twistronics is the field that studies electronic behavior that can be dramatically altered by controlling the twist between layers of two-dimensional materials, such as graphene. A recent major discovery in twistronics is the so-called magic angles, which are extraordinary twist angles between two sheets of graphene that give rise to utra-flat bands, creating the Mott insulating state and unconventional superconductivity. This research draws inspiration from the recent development in twistronics and seeks to exploit twist and interlayer coupling as two new degrees of freedom to devise a new family of sonic crystals, i.e., twisted bilayer sonic crystals. Analytical and computational models will be developed to shed light on the band structure of twisted bilayer sonic crystals with a wide range of twist angles and interlayer coupling strength. A framework will be established to identify the acoustic version of magic angles in twisted bilayer sonic crystals. Important properties of acoustic magic angles, such as their corresponding eigenmodes, physical bounds, and robustness to defects, will be revealed. At last, important insights will be gained on the topological features of twisted bilayer sonic crystals, as well as on how loss interacts with their bands, either favorably or adversely.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
As a result of this award, our research has advanced the frontiers of wave physics by exploring how sound waves behave in specially engineered structures known as phononic crystals. These materials are designed to control sound in ways that go far beyond what traditional materials can achieve—similar to how semiconductors control electrons in computers.
Our work focused on bilayer phononic crystal systems, where two patterned plates are placed in parallel, and the orientation between them is slightly rotated, or “twisted.” This twist creates complex overlapping patterns called moiré patterns, which are known to produce novel and sometimes surprising behaviors in other areas of physics, such as twisted bilayer graphene in electronics.
We began with a detailed numerical study of how sound passes through two perforated plates with square arrays of holes. By varying the twist angle, plate separation distance, and frequency, we created a comprehensive map of how these factors interact. Surprisingly, we found that when the plates are very close together, even a small twist (as little as 5–10°) can dramatically reduce sound transmission. This was counterintuitive, as we initially expected better alignment to improve sound passage. Instead, the misalignment can cause destructive interference, blocking the sound in narrow angular ranges.
In the second part of our study, we examined moiré lattices of phononic plates with two types of designs: one with twisted arrays of holes (moiré holey plates) and another with lattices of tiny pillars placed on both sides of a plate (twisted pillared plates). These structures allow us to trap sound in extremely small regions, creating highly confined acoustic modes.
For example, we discovered a special resonant mode in a twisted holey plate with a quality factor (a measure of how long sound stays trapped) of 300,000, which is exceptionally high. This indicates very low energy loss and strong confinement—features that could be useful for sensing, signal filtering, or energy harvesting. In contrast, the twisted pillared plate also showed confined modes but with lower quality factors (~8,000), suggesting different design trade-offs. In addition, we found that twisting arrays of pillars can create exotic elastic wave behaviors, including analogues of topological edge states known from condensed matter physics.
Overall, this research provides a new toolkit for controlling sound in ultra-compact systems and introduces twist and layer interaction as powerful design parameters. Our results challenge prior assumptions in the field and open up promising new directions for fundamental wave physics and practical acoustic technologies.
Last Modified: 03/31/2025
Modified by: Yun Jing
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