Joannès Chambon - Doctorant LVA & MicroDB - Séminaire du LVA

Three-dimensional beamforming for wind tunnel applications using ESM based transfer functions

As an inverse problem, sound source localization in three dimensions relies on two distinct cornerstones.
One is the physical model chosen to describe the acoustic propagation of the sources to identify
and the other is the algorithmic process used to derive information from measured acoustic data. Mainly
focusing on the first point, an Equivalent Source Method (ESM) aiming at the simulation of realistic
Frequency Response Functions (FRF) is proposed in this paper. The underlying idea is to substitute the
acoustic behaviour of a radiating object by a set of acoustic monopoles calibrated with respect to the
boundary condition on its skin. Such a method allows to perform 3D Conventional Beamforming (CBF)
with FRF taking into account the acoustic environment and the influence of the radiating or scattering
structure. Misleading sound source localization outcomes due to ground reflections or diffraction are
therefore prevented.
As a first step, the ESM process is validated thanks to the Spherical Related Transfer Function which
provides a rigorous analytical framework for FRF comparison. ESM boils down to an inverse problem
in itself upstream to CBF, and various ways of solving it are assessed.
With a view to present an industrial application, FRF are computed on a car mesh and validated
thanks to a commercial Finite Elements software. These FRF are then used to carry out 3D CBF with
the experimental pressure scattered by an omnidirectional source placed near the rear-view mirror of the
car, measured by a 160 microphones array in the Daimler automotive wind tunnel. At the cost of a brief
study of CBF sensitivity to the FRF computation, the method yields promising results in comparison
with the commonly used free field assumption.
Finally, a strategy to include the contribution of wind tunnel convective effects at low Mach number
is initiated. To this end, a geometric routine devised by R.K. Amiet is coupled with the ESM boundary
condition step and assessed on wind tunnel measurements.