Acoustic: Sound Absorption in a Porous Pipe Silencer

Description

The use of porous media inside tubular structures has become a key strategy for sound absorption. This technique takes advantage of the inherent properties of porous materials to dissipate sound energy, thereby reducing noise pollution and improving the acoustic environment within the tube.

In this project, we simulate the phenomenon of sound absorption inside a pipe, where a porous medium serves as a silencer. The primary aim is to measure two key parameters across a wide range of frequencies:

Transmission Loss (TL) — the reduction in sound power as sound travels through the pipe filled with the porous medium. It is a critical quantity in many engineering applications where noise reduction is required.

Sound Pressure Level (SPL) — the pressure deviation from ambient atmospheric pressure produced by a sound wave. Here, the interest lies in understanding how the SPL varies over a broad frequency range as sound propagates through the porous medium inside the pipe.

The geometry was created in ANSYS SpaceClaim, and the computational domain was then divided into separate cell zones in ANSYS Meshing, generating 1,209,174 polyhedral cells.

Methodology

To achieve this, the Ffowcs Williams–Hawkings (FW-H) acoustic model was employed. This model is well regarded for its ability to accurately predict the acoustic behavior of a system, making it a suitable choice for the present simulation. The study aims to build a deeper understanding of how sound behaves under these conditions — insight with significant relevance to fields such as acoustical engineering and environmental noise control.

The inlet and outlet are placed 200 mm and 500 mm from the silencer, respectively. Air enters the tube at a velocity of 5 m/s. The flow equations are first solved in steady-state form; the acoustic equations are then introduced and the solution continued in an unsteady (transient) manner.

Conclusion

As the air enters the pipe, it must pass through the porous medium, which produces a marked pressure drop owing to the complex internal structure of the porous material. A stagnation point forms on the porous wall, and the velocity increases sharply in accordance with Bernoulli's equation. Both effects are visible in the figures below.

From an acoustic standpoint, a comparable behavior is observed. To fulfill the study's objectives, three receivers were positioned within the domain: one placed 100 mm before the porous medium, and the other two placed 200 mm and 400 mm downstream of the porous silencer. When interpreting the results, note that the reference acoustic pressure is set to 2 × 10⁻⁵ Pa, so all reported values are relative to this reference level.