Jerry Rouse – jwrouse@sandia.gov
Sandia National Laboratories
Albuquerque, NM 87185
United States
Additional authors:
Cameron McCormick and Benjamin Treweek
Popular version of 1aSA6 – Acoustic Black Hole Effect Due to Variation in Duct Wall Impedance
Presented at the 187th ASA Meeting
Read the abstract at https://eppro01.ativ.me//web/index.php?page=IntHtml&project=ASAFALL24&id=3767696
–The research described in this Acoustics Lay Language Paper may not have yet been peer reviewed–
Consider a black hole in outer space, where gravity is so strong that not even light waves can escape. Now, imagine a device here on Earth that can slow sound waves so much that they cannot escape. Scientists call this intriguing phenomenon an “acoustic black hole” (ABH). An ABH structure can trap sound waves and produce a unique environment for acoustic measurement and manipulation.
How can a structure be designed to trap sound in this way? The acoustic black hole effect is achieved by altering the way sound travels down a duct. Traditional ABHs are based upon the pioneering research of Mironov and Pislyakov (2002) that used specific shapes to guide sound waves, such as rings with inner radii that vary down the length of the duct. However, in this work, the approach is different: varying the mechanical impedance of the duct walls themselves (see Figure 1). Mechanical impedance refers to how much a structure resists motion when sound waves press against it. By engineering an impedance profile—essentially, the way the walls respond to sound throughout the duct—researchers can create a situation where sound waves decrease in speed as they travel through the duct. A gradual reduction in speed effectively simulates the event horizon of a black hole, causing the sound waves to be trapped and significantly attenuated (see Figure 2).
Figure 1. A sound wave enters a duct where the walls are stiffer at the entrance and softer at the base. As the wave moves through the duct, it slows down due to the changing properties of the walls.
To better understand this phenomenon, the researchers derived and solved governing equations using two methods. First, they used a mathematical technique called the WKB approximation, which helps find approximate solutions to wave equations. Second, they used numerical simulation, which involves using computers to model complex systems. The solutions they obtained from these approaches revealed that specific impedance profiles could effectively decelerate and absorb acoustic waves, resulting in very little reflection or transmission of sound.
To verify their findings, the researchers employed a sophisticated program called Sierra/SD. This program uses a fully coupled structural-acoustic finite element algorithm. In brief, this algorithm allows researchers to create a computer model of any design they want and test how it responds to any sound source. This tool allows for detailed simulations of how sound interacts with various structures and provides a robust framework for testing theoretical predictions.
Overall, this research not only enhances understanding of the acoustic black hole effect, but also paves the way for the development of innovative acoustic materials and devices. By using the principles of ABH, these advancements could lead to improved noise control and enhanced manipulation of sound waves, with potential applications in various fields such as engineering, architecture, and environmental science.
Figure 2. An illustration of a sound wave vanishing in an acoustic black hole structure. The ABH effect is seen from the wavefronts becoming closer together (slower sound speed) and lower in amplitude (lower peaks, higher troughs) at the right end.
Figure 2. An illustration of a sound wave vanishing in an acoustic black hole structure. The ABH effect is seen from the wavefronts becoming closer together (slower sound speed) and lower in amplitude (lower peaks, higher troughs) at the right end.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.