The aerodynamics phenomena of a particular bass-reflex port

AES 140th ConventionParis, June 6, 2016

Convention eBrief 257

MotivationBass reflex design with unexpected broadband noise when excited at high levelsHelmholtz frequency = 50 Hz

Problem statement

AERODYNAMICSACOUSTIC

VIBRATION

Root cause for noise

• Aerodynamic origin: vortex shedding at port’s edge.• Predicted by low value of Strouhal’s number (St) at

the Helmhlotz frequency.

MEASUREMENTFlow visualization

using Particle Image Velocimetry (PIV)

SIMULATIONNumerical approach using computational fluid dynamics (CFD)

Hypothesis

Simulation• Open-FOAM® Computational

Fluid Dynamics toolbox• Finite Voume Method with

dynamic mesh• PIMPLE Algorithm (SIMPLE +

PISO)

MeasurementFlow visualization using PIV

Fog seeded with 1.5μm particle oil

Photron FASTCAM APX-RS Function

generator

IR LASER

Numerical results• Representation of parameter Q

o Is the invariant of the velocity gradiento Takes positive values inside a vortex, negative values

outside

Numerical resultsFrequency response at probe located 20cm outside the port

Experimental results

Experimental resultsFrequency response with open/closed port, when exciting at the Helmholtz frequency

ConclussionNumerical and experimental results show vortex shedding at the port creating broadband noise around 1200Hz

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.