Creacoustic: Transmission Loss Simulation of Reactive and Dissipative Creacoustic Mufflers

Silencers and industrial mufflers are used to limit noise emissions in applications like fan inlets and discharges, stacks, vents, equipment/process enclosure ventilation, power generators, etc.

They consist of ducts through which the airflow is forced to pass, providing attenuation of the propagated noise generated either by the machine/equipment upstream of the duct or by the flow itself (flow-noise).

Optimal muffler design requires maximization of noise absorption and minimization of flow pressure losses.

Noise attenuation is typically obtained through the following mechanisms:

• Reactive mufflers:
  • Destructive interference of sound waves (e.g., resonant cavities)
  • Waves back-reflection towards the source (e.g., expansion chambers providing a sudden change in the cross-sectional area of the duct)
• Dissipative mufflers:
  • Dissipation of acoustic energy into heat (e.g., damping material: foam, fibrous material, glass fiber; perforated sheets: metallic sheet punched (hole size of mm) supporting the damping material)

Creacoustic tested both types of muffler mechanisms in an acoustic laboratory for the measurement of the Transmission Loss (TL). Transmission Loss is an indicator of the noise attenuation performance (the difference between the incident power at the inlet and the transmitted power at the outlet).

Reactive Mufflers

Industrial "reactive" muffler acoustics test

Reactive muffler acoustic pressure simulation with FINE/Acoustics

Dissipative Mufflers

Industrial "dissipative" muffler with mineral wool 

Dissipative muffler acoustic pressure simulation with FINE/Acoustics

Both simulations were done with the Finite Element Method (FEM) of FINE/Acoustics, solving the Pierce-Howe Convective Wave Equation in the frequency domain.

The solver computes the internal acoustic field, taking into account:

• Sound reflection on solid surfaces
• Absorption from damping material
• Scattering (due to cross-section variations, soft-hard wall transitions)
• Convection by non-uniform mean flow
• Refraction (due to temperature variations)

The Transmission Loss spectra obtained clearly highlight the attenuation performance. We can see damping being extended over large frequency ranges at low frequencies and sharp peaks, indicating damping is restricted to narrow resonance frequencies at high frequencies. 

For the above-mentioned transmission loss simulations, NUMECA implemented a guided simulation workflow in FINE/Acoustics, exploiting its automation capability based on Python scripts and wizards driven by best practices.