Experimental determination of the diffusion constant for ultrasonic waves in 2-D multiple scattering media with focused beamforming

Abstract : Experimental measurements of the diffusion constant for ultrasonic waves (around 3 MHz) propagating in water through a scattering slab (parallel metallic rods) are presented. Sample thickness is around ten times the transport mean free path. Several hundreds of transmitting/receiving positions, 40 mm off the sample surfaces, are used. Focused beamforming is achieved in emission and reception in order to mimic a set of virtual sources and receivers located at the sample surface. The ensemble average of the transmitted intensity $\left\langle{I(x,t) }\right\rangle$ is estimated by averaging over all possible couples of sources/receivers apart by the same off-axis distance x. Under the diffusion approximation, $\left\langle{ I(x,t)}\right\rangle$ shows a gaussian dependence on x, which makes it possible to measure a diffusion constant D and thereby characterize the scattering medium. We discuss the experimental results and pinpoint the difficulties of measuring a reliable value for D on a real sample. As it was observed in previous works on the mean free path, the diffusion constant D strongly depends on frequency, due to the resonant nature of the scatterers.
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Nicolas Viard, Arnaud Derode. Experimental determination of the diffusion constant for ultrasonic waves in 2-D multiple scattering media with focused beamforming. Acoustics 2012, Apr 2012, Nantes, France. ⟨hal-00810710⟩

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