A particular bass-reflex port design causes unwanted noise in the audible frequency range. After discarding structural and mechanical vibration issues, the hypothesis that vortex shedding could be the source of the noise has been considered. Experimental and numerical evidences of the vortex, an analysis of its noise and the similarities between the real performance and the simulated one are presented. The simulations have been performed with axisymmetric geometries with the open source OpenFOAM toolbox. Additionally, three different experiments were carried out. First, acoustic signal experiments were done to analyze the response of the bass-reflex ports. Second, a mechanical vibration was tested in order to discard this source of noise. A good agreement has been found between numerical and experimental results, especially in the frequency band of the detected noise, around 1200 Hz. The presented CFD approach has proved a useful and cost-effective tool to face this kind of phenomena. Presented at the AES 140th convention

1. The aerodynamics phenomena of a
particular bass-reflex port
AES 140th Convention
Paris, June 6, 2016
Convention eBrief 257

3. Motivation
Bass reflex design with unexpected broadband
noise when excited at high levels
Helmholtz frequency = 50 Hz

4. Problem statement
AERODYNAMICSACOUSTIC
VIBRATION
Root cause for noise

5. • Aerodynamic origin: vortex shedding at port’s edge.
• Predicted by low value of Strouhal’s number (St) at
the Helmhlotz frequency.
MEASUREMENT
Flow visualization
using Particle Image
Velocimetry (PIV)
SIMULATION
Numerical approach
using computational
fluid dynamics (CFD)
Hypothesis

6. Simulation
• Open-FOAM® Computational
Fluid Dynamics toolbox
• Finite Voume Method with
dynamic mesh
• PIMPLE Algorithm (SIMPLE +
PISO)

7. Measurement
Flow visualization using PIV
Fog seeded with
1.5μm particle oil
Photron
FASTCAM APX-RS
Function
generator
IR LASER

8. Numerical results
• Representation of parameter Q
o Is the invariant of the velocity gradient
o Takes positive values inside a vortex, negative values
outside

9. Numerical results
Frequency response at probe located 20cm outside
the port

10. Experimental results

11. Experimental results
Frequency response with open/closed port, when
exciting at the Helmholtz frequency

12. Conclussion
Numerical and experimental results show vortex shedding at the
port creating broadband noise around 1200Hz

13. Full paper :“Numerical an experimental study of the
aerodynamics sound sources in a bass-reflex port”
by
Garcia-Alcaide, V.M, Palleja-Cabre, S., Castilla, R. Gamez-Montero, P.J.
LABSON; Dept. of Fluid Mechanics,
Universitat Politècnica de Catalunya.
Romeu, J., Pamies, T. ,
LEAM, Dept. of Mechanical Engineering,
Universitat Politècnica de Catalunya
and
Amate, J.and Milan, N.,
Amate Audio, S.L.