Keywords
1. Acoustic Cloaking
2. Backscattering Suppression
3. Liner Surface Modes
4. Audible Range
5. Acoustic Metamaterials
Introduction
In a rather compelling advancement in the field of acoustics, scientists have devised a way to cloak objects within a duct, making them “invisible” to sound waves. This groundbreaking research, using liner surface modes to suppress backscattering in acoustic ducts, has significant implications for various industries, from architectural acoustics to naval stealth technology. The study, detailed in a 2019 issue of Scientific Reports, presents a numerical model demonstrating how a specially engineered liner can guide sound waves around an object, effectively creating a zone of silence.
The Science of Acoustic Cloaking
A team of researchers at the Laboratoire d’Acoustique de l’Université du Mans in France, associated with the Centre National de la Recherche Scientifique (CNRS) and led by Maaz M. Farooqui, Yves Aurégan, and Vincent Pagneux, proposed an innovative concept of acoustic cloaking inside ducts. What sets this method apart is its ability to bend plane waves around objects using the surface modes of a liner, conceived from a resonant liner based on an array of tubes.
The Phenomenon of Backscattering
Traditionally, when sound waves encounter an obstacle, they tend to scatter in multiple directions, a phenomenon known as backscattering. This scattering can cause echoes and noise pollution, issues that are often counteracted using sound damping materials that absorb the waves. However, these materials do not make the obstacle “invisible” to sound.
Liner Surface Modes
Utilizing liner surface modes presents a radical departure from this traditional approach. By employing a resonant liner, an acoustically tuned material with specific properties able to guide sound waves, the waves are coaxed to curve around the object smoothly, with minimal reflection. This resonant liner, with its peculiar structuring, captures and channels the sound waves, effectively turning the obstacle “reflectionless.”
Zone of Silence
The numerical model showed that the resonance of the liner creates a zone of silence where the obstacle within is cloaked for an extensive range of frequencies. This “silence” does not translate to an absence of sound but rather an area where the sound waves do not interact with the obstacle. The liner’s ability to deflect the sound wavefronts mirrors the characteristics of an ideal invisibility cloak.
Frequency Band and Impedance
The cloaking capability depends on the frequency band, which is a function of the liner’s impedance and the height of the obstacle relative to the surrounding duct. Significantly, for smooth-shaped obstacles, the researchers identified an inherent self-cloaking ability that further extends the cloaking frequency band.
Dispersion and Wave Phase
One of the challenges in utilizing this technology is mitigating dispersion effects, which can lead to slow sounds and wave phase distortions. Consequently, tuning the liner’s properties is critical for minimizing these undesirable effects and achieving effective cloaking.
Practical Applications and Future Perspectives
This novel cloaking technique has vast potential applications. In the realm of architectural acoustics, it could help in designing quieter ventilation systems that do not interfere with the surrounding environment. In industrial settings, machines with noisy components could be made acoustically transparent, reducing the overall noise pollution.
One of the most intriguing applications lies in the military field. Submarines and naval vessels could be tailored with such liners to become less detectable to sonar, enhancing their stealth capabilities. Additionally, the same principles could eventually apply to medical imaging techniques like ultrasound, preventing artifacts from obstructing the clarity of images.
References
To further understand the context and significance of this study, here are five references summarizing the evolution of cloaking technologies and related acoustic principles:
1. Schurig, D., et al. (2006). Metamaterial electromagnetic cloak at microwave frequencies. Science. doi: 10.1126/science.1133628.
2. Farhat, M., Guenneau, S., & Enoch, S. (2009). Ultrabroadband elastic cloaking in thin plates. Physical Review Letters. doi: 10.1103/PhysRevLett.103.024301.
3. Stenger, N., Wilhelm, M., & Wegener, M. (2012). Experiments on elastic cloaking in thin plates. Physical Review Letters. doi: 10.1103/PhysRevLett.108.014301.
4. Parnell, W. J., Norris, A. N., & Shearer, T. (2012). Employing pre-stress to generate finite cloaks for antiplane elastic waves. Applied Physics Letters. doi: 10.1063/1.4704566.
5. Zhang, S., Xia, C., & Fang, N. (2011). Broadband acoustic cloak for ultrasound waves. Physical Review Letters. doi: 10.1103/PhysRevLett.106.024301.
Conclusion
The discovery and application of liner surface modes in acoustic ducts have opened new avenues in sound management and stealth technology. By creating zones of silence where obstacles become invisible to sound waves, this technique is poised to revolutionize the way we approach noise control and acoustic cloaking. While further research is required to optimize this technology for real-world applications, its potential is unquestionably promising. The ability to manipulate sound with such precision can lead to quieter, more harmonious environments and a leap forward in covert operations.
DOI: 10.1038/s41598-019-43538-3