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Expansion Chamber Muffler: 3D Acoustic Cavity Helmholtz Eigenmodes, Standing Waves & Sound Pressure Level (SPL)
Automotive expansion chamber reactive silencer cavity solved via the 3D Helmholtz acoustic wave equation with non-reflecting Robin outlet impedance and sound pressure level (SPL) field visualization.
Internal combustion exhaust mufflers, HVAC ducts, and intake plenums attenuate airborne noise through reactive acoustic impedance mismatches and chamber resonances. This model analyzes a 3D cylindrical expansion chamber (Ø90 × 120 mm) connected to Ø30 mm coaxial inlet and outlet pipes (expansion area ratio m = 9). Solved with WebCAE's native 3D Helmholtz acoustic cavity solver, standing wave anti-nodes and sound pressure levels (SPL) are computed for targeted transmission loss tuning.
1. Geometry & FEA Mesh
- •3D B-Rep CAD model (`muffler_cavity.step`, 8 CAD faces, total length 220 mm, chamber Ø90 mm).
- •1,680 TET4 acoustic cavity fluid elements, 475 nodes.
- •Acoustic fluid medium (air at 20°C): speed of sound c₀ = 343 m/s, density ρ₀ = 1.204 kg/m³, characteristic acoustic impedance Z₀ = 413 Rayl (Pa·s/m).
2. Loads & Boundary Conditions
- •Acoustic velocity excitation applied at the inlet port face (`surface number 33`, Ø30 mm).
- •Non-reflecting anechoic Robin impedance radiation boundary applied at the outlet port (`surface number 32`, Z = 413 Rayl).
- •Rigid sound-hard acoustic boundary condition on chamber outer walls (`surface number 27, 28, 29, 30, 31`, ∂p/∂n = 0).
3. Simulation Results & Findings
- ✓5 acoustic cavity resonance modes identified: fundamental longitudinal standing wave at f₁ = 1,560.9 Hz, followed by higher modes at 2,238.1 Hz, 2,518.4 Hz, and 2,963.0 Hz.
- ✓Maximum standing wave acoustic pressure reaches |p|_max = 118.5 Pa at the expansion chamber endplate.
- ✓Sound pressure level spans 74.2 to 132.4 dB, clearly revealing internal acoustic quarter-wave resonance nodes.
WebCAE's 3D acoustic Helmholtz solver gives NVH and automotive powertrain engineers immediate insight into muffler cavity resonances, insertion loss, and airborne noise suppression.
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