Shout into a long cylindrical duct, and the sound reflects off the closed far end and returns a moment later as an echo. In building air-conditioning ducts and factory exhaust pipes, sound traveling through the interior can likewise reflect at the end of the pipe and become a source of noise.
A research team including researcher Jie Deng and Professor Oriol Guasch at the La Salle campus of Ramon Llull University in Spain has developed a compact device that greatly reduces this kind of reflection at the end of a duct. The mechanism is known as a "sonic black hole" (SBH). The name draws an analogy with the way an astronomical black hole traps light: the device gradually slows the propagation speed of sound waves and absorbs their energy.
What stands out in this study is that a complex sound-absorbing structure that previously had to be built into the inner wall of the duct has been redesigned as a standalone device that can be inserted into an existing pipe. The results were published in the peer-reviewed journal Applied Acoustics, Volume 243 (article number 111119). The paper was published online in 2025 and formally issued on February 5, 2026 (DOI: 10.1016/j.apacoust.2025.111119).
However, some reports describe the work as "building a black hole in the lab and making it swallow sound." That is purely a metaphor. The researchers did not manipulate gravity or spacetime as a real black hole does. What they demonstrated is a technique that uses a resin device made with a 3D printer to suppress sound reflections inside a laboratory pipe.
How a sonic black hole gradually slows sound
In astrophysics, a black hole is an object whose extremely strong gravity warps spacetime so severely that even light cannot escape beyond its "event horizon." For many years, physicists have pursued "analog gravity" research, using systems such as superfluid helium and Bose-Einstein condensates to reproduce its properties in the laboratory.
In such experiments, researchers create a boundary where the flow speed of a fluid exceeds the local speed of sound, reproducing a region from which sound waves cannot escape. Observing quantum phenomena equivalent to Hawking radiation is also a research goal.
The sonic black hole studied by Deng and colleagues, however, is not meant to reproduce such gravitational phenomena. The aim is to suppress the reflection of sound waves traveling through a duct and absorb their energy efficiently.
The basic principle is to design the duct's shape so that the effective propagation speed of a sound wave gradually decreases as it advances.
In an ideal sonic black hole, the phase velocity of the wave becomes smaller toward the end and ultimately approaches zero. As this happens, the wavelength shortens and the wave amplitude grows. The wave slows progressively as it moves toward the terminus and, in theory, would take an infinite amount of time to reach the end point.
This is reminiscent of the dichotomy paradox proposed by the ancient Greek philosopher Zeno. To reach a destination, you must first cover half the remaining distance, then half of what remains after that. Repeat the process endlessly, and it seems you never arrive.
In a sonic black hole, this is not merely a thought experiment: similar behavior arises because the propagation speed of the sound wave actually decreases.
In an interview with ScienceAlert, Professor Guasch explained that the behavior of waves approaching the end of a sonic black hole is, to some extent, similar to what an outside observer sees near an astronomical black hole. It appears as though the wave would take an infinite time to reach the end.
Using this mechanism, it is theoretically possible to prevent sound waves from reflecting at the end of a duct. Acoustically, the duct behaves as though it continues forever, with no reflecting end.
In a real device, however, the propagation speed cannot be reduced fully to zero. There are dimensional limits to the structures that can be fabricated, and the device itself has finite length.
This is where energy loss due to the viscosity and thermal conduction of air becomes important.
As a sound wave slows, its wavelength shortens and its energy becomes concentrated locally, increasing losses from the friction of viscous air and from heat exchange between the air and solid surfaces. These effects are known as viscothermal losses.
As a result, part of the sound energy dissipates as heat, and the reflected sound returning toward the duct entrance weakens.
In other words, a practical sonic black hole does not literally trap sound. It is a structure that gradually slows sound waves and concentrates their energy, promoting absorption through viscosity and thermal conduction.
A new design: rings arranged along the central axis, without modifying the duct wall
Research on sonic black holes dates back to theory developed in the 1980s by Russian physicist M. A. Mironov and others to damp vibrations in thin plates. It later came to be applied not only to bending vibrations in thin plates but also to controlling sound waves traveling through ducts.
Conventional sonic black holes for ducts typically place many ring-shaped structures inside the duct, with cavities between the rings. By varying the inner diameter of the rings toward the end according to a specific rule, the propagation speed of the sound wave is gradually reduced.
This structure, however, posed significant challenges for manufacturing and installation.
In the conventional design, the rings and cavities must be integrated with the inner wall of the duct. That meant the end section of the pipe had to be 3D-printed as a single piece or, for metal piping, cast or precision-machined.
While such a structure can be built into newly manufactured equipment, retrofitting it to air-conditioning ducts or exhaust pipes that are already installed is difficult. Introducing it into existing facilities could require work such as replacing sections of piping.
Deng and colleagues devised a design that turns this structure inside out.
In the new device, a single support rod runs along the central axis of the duct, and rings of varying diameters are attached around it in succession. Because a gap remains between the outer edge of the rings and the inner wall of the duct, the pipe itself does not need to be machined into a special shape.
Whereas the conventional design protrudes from the duct wall toward the center, the new design has rings spreading outward from the center. The research team describes it as a Christmas-tree-like structure.
The main differences between the two approaches are as follows.
| Comparison | Conventional sonic black hole | New plug-in type |
|---|---|---|
| Basic structure | Rings and cavities integrated with the duct wall | Rings of varying diameters mounted on a central support rod |
| Manufacturing | Requires integrated printing of the pipe, metalworking, etc. | The device can be 3D-printed independently |
| Installation in existing pipes | Often requires replacing or machining the pipe | Can be inserted into the end of a pipe if the dimensions fit |
| Materials | Requires materials and manufacturing methods compatible with the pipe | Can be made from a material different from the pipe |
| Main acoustic characteristics | Depending on the design, local cavity resonances can dominate at high frequencies | The sonic black hole effect is maintained over a wider band |
| Performance vs. conventional type | Baseline for comparison | Lower reflection coefficient in numerical analysis under the same free cross-sectional area |
| Experimental validation | Extensively demonstrated in existing research | Validated in a circular laboratory tube using a 3D-printed prototype |
The idea of inserting a standalone sound-absorbing structure into a duct is not entirely new. According to the research team, Mironov and Pislyakov previously built a similar prototype and conducted experiments.
The novelty of this study lies in building a systematic theoretical model and numerical analysis for this approach and comparing its acoustic performance with that of the conventional type in detail.
In the plug-in type, the diameter of the rings changes along the axial direction, so the acoustic properties created by the cavities between the rings and the duct wall change as well. This gradually reduces the wave's propagation speed and promotes energy dissipation.
When the research team compared the two types, numerical analysis showed that, with the same free cross-sectional area available for sound to pass through, the plug-in type achieved a lower reflection coefficient.
Because the device can be manufactured independently, there is also no need to rebuild the sound-absorbing structure itself to match the pipe's material. If appropriate dimensions and a fixing method can be secured, it may be applicable to existing pipes of different materials, such as metal or resin.
Measuring reflections with a 15 cm device and 29 rings
To confirm whether the designed device actually works, the research team built a prototype using a 3D printer.
They used an FDM (fused deposition modeling) 3D printer, which heats and stacks resin filament. The material chosen was PLA (polylactic acid), a common 3D-printing resin.
The finished device is 15 cm long, with 29 rings arranged along a central support rod. The ring diameter changes gradually along the axis, forming a structure that controls the propagation of sound waves.
The PLA material properties given in the paper are a density of about 1250 kg/m³ and a Young's modulus of about 3.5 GPa.
The paper's numerical analysis also includes a model with 40 rings of 1 mm thickness. This is an analytical model used to investigate design parameters, and is distinct from the 29-ring prototype used in the actual acoustic measurements.
The experiment used an "acoustic impedance tube," a setup used to evaluate the performance of sound-absorbing materials and acoustic structures.
In this apparatus, a loudspeaker at one end of a straight tube sends sound waves, and the sound reflected from the opposite end is measured. In this experiment, the plug-in sonic black hole was inserted into the closed end of the tube.
Sound waves of different frequencies are sent from the speaker, and sound pressure is measured by microphones placed in the tube. By analyzing the sound pressure distribution formed by the overlap of incident and reflected waves, researchers can determine how much sound was reflected.
Using the transfer function method, the team calculated the reflection coefficient at each frequency. The reflection coefficient indicates how large the amplitude of the reflected wave is relative to the incident wave. The smaller the value, the more the reflected sound is suppressed.
In the experiment, the prototype suppressed sound reflection, and the measured frequency characteristics of the reflection coefficient broadly matched finite element method (FEM) simulations carried out in advance.
The simulations also accounted for losses from air viscosity and thermal conduction. The agreement with the experimental results supports the validity of the numerical model the team built.
That said, the experiment did not eliminate reflections entirely.
In an ideal sonic black hole, the reflection coefficient is expected to approach zero, but a real device has rings and cavities of finite thickness, and neither the design nor the manufacturing can perfectly reproduce an ideal continuous structure.
Some reflection remained in this experiment as well. What was confirmed is that the prototype has the effect of reducing sound wave reflection.
The experiment was also carried out in still air with no airflow, under conditions in which mainly plane waves propagate through the tube.
In air-conditioning ducts and industrial piping, airflow, temperature changes, and complex sound wave propagation arise. These results do not guarantee performance in such real-world environments; they are better seen as a first step confirming that the basic sound-absorption mechanism works in an actual device.
Why it works over a wider frequency range than the conventional type
The study not only achieved a structure that can be retrofitted but also showed the possibility of suppressing sound reflections over a wider frequency range than the conventional type.
To understand the difference, it is necessary to distinguish two phenomena that occur inside a sonic black hole.
One is the "sonic black hole effect," in which sound waves gradually decelerate along the axial direction of the duct and dissipate energy. This is related to Fabry-Pérot-type resonance modes arising along the duct axis.
The other is a phenomenon called "rainbow trapping," in which the cavities between rings resonate at specific frequencies and locally trap the sound energy.
Rainbow trapping is a mechanism in which wave energy concentrates at different positions depending on frequency. Strong absorption can be obtained at particular frequencies, but at frequencies outside the resonance conditions, sufficient effect may not be achieved.
In conventional sonic black holes, local resonances in the cavities between rings can become dominant as frequency increases.
In that case, the influence of resonance in individual cavities becomes stronger than the mechanism by which the whole device gradually slows the wave. Absorption performance becomes skewed toward particular frequencies, and it can be difficult to suppress reflections across a wide frequency band.
In Deng and colleagues' numerical analysis, switching to the plug-in type shifted the frequency band in which such local resonances dominate toward higher frequencies than in the conventional type.
As a result, in the plug-in type the axial sonic black hole effect is maintained over a wider range from mid to high frequencies.
In the finite element comparison as well, the plug-in type showed a lower reflection coefficient over a broad frequency band than a conventional type with the same free cross-sectional area.
In other words, moving the rings from the duct wall toward the central axis appears not only to make manufacturing and installation easier but also to affect the resonance characteristics of the sound waves, leading to improved absorption performance.
However, the advantage over the conventional type shown here is based mainly on numerical analysis. The physical experiment measured the plug-in prototype, and no direct experimental comparison with a conventional type built under the same conditions was carried out.
The research team is continuing to study the difference between these two absorption mechanisms.
On October 10, 2026, Deng, Junjun Li, and Guasch published a related paper, "Sonic black holes without rainbow trapping: Separating two distinct mechanisms" (article number 119889), in the Journal of Sound and Vibration, Volume 640 (DOI: 10.1016/j.jsv.2026.119889).
That study analyzes sonic black holes with rectangular cross-sections, among other designs, and examines in detail the frequency bands in which the sonic black hole effect and rainbow trapping each dominate. It also explores methods of suppressing rainbow trapping and achieving high absorption over a wide band by dividing the structure into multiple smaller units.
Meanwhile, in the Applied Acoustics paper discussed here, the separate evaluation of the contributions of the two mechanisms rests mainly on theoretical models and numerical analysis.
The impedance-tube experiment measured the reflection characteristics of the device as a whole and did not measure the two mechanisms independently. The agreement between experiment and numerical analysis indirectly supports the validity of the proposed absorption mechanism.
Real HVAC systems will require measures against airflow and dust
The result is an important step toward making sonic black holes easier to introduce into real facilities.
However, the fact that a 3D-printed prototype suppressed sound reflections does not mean it can be attached directly to an air-conditioning duct or factory exhaust pipe.
Several challenges remain before practical use.
The first is the effect of air flowing through real piping.
In building ventilation ducts and engine exhaust pipes, airflow with a certain velocity is present. Flowing air adds convective effects to the way sound waves propagate, and may also generate vortices and turbulence around the rings.
Depending on the device's shape, such airflow could create new noise. And because a structure is being inserted into the duct, the possibility that it increases flow resistance, affecting the power required by fans and the pressure loss in the piping, must also be considered.
Because the experiments and numerical models in this study assumed still air, it has not been confirmed whether the same absorption performance is obtained in environments with airflow. Practical use will require evaluation that covers not only acoustic performance but also fluid resistance and flow-induced noise.
The next problem is the buildup of dust and oil.
The performance of a sonic black hole depends heavily on the dimensions of the cavities between the rings and of the gap between the rings and the duct wall.
In factory exhaust pipes and ventilation ducts, fine particles in the air could accumulate in such narrow spaces. If dust or oil blocks the cavities or changes the shape of the gaps, the device may no longer maintain its designed acoustic characteristics.
In the ScienceAlert interview, Professor Guasch also cited particles clogging the cavities, along with the effects of temperature, pressure, and airflow, as challenges for practical use.
Dealing with temperature changes is also essential.
Because the speed of sound in air varies with temperature, in facilities where exhaust gas temperature changes significantly, the characteristics of sound waves traveling through the device change as well.
In high-temperature environments, the heat resistance and dimensional stability of the device itself also become issues. The prototype used PLA resin, but for applications such as high-temperature exhaust pipes, materials suited to the operating environment would have to be selected.
Whether sufficient absorption performance can be maintained as these conditions change is not yet known. Long-term performance and durability in real environments have not been verified.
The research team also mentions the possibility of extending the concept beyond one-dimensional ducts to two-dimensional sound-absorbing panels and three-dimensional acoustic spaces in the future.
If realized, it might be applicable not only to reflected sound in piping but also to room acoustics in buildings and to sound control in more complex spaces. These uses, however, are subjects for future research, and their practicality has not been established.
What this study showed is that the theoretical mechanism of the sonic black hole can be realized as a standalone device that is relatively easy to manufacture, and that it can actually suppress sound reflection.
The significance of replacing a complex structure that previously had to be integrated with the duct with a 15 cm resin insert is not small.
Numerical analysis further suggests that it may suppress reflections over a wider frequency band than the conventional type.
The focus going forward will be whether this performance can be reproduced in real facilities with airflow, dust, and temperature changes.
A sonic black hole does not trap sound through gravity. It is a technology that uses structure to control how sound waves travel and dissipates their energy as heat. Whether the small device developed here grows into a noise-reduction technology usable in air-conditioning systems and industrial piping will depend on future verification in real-world environments.