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International audience

International audience; The work presented here use guided wave characteristic to localize impact source by a correlation technique that mimick time reversal. Time reversal in acoustic is a very efficient solution to focus sound back to its source in a wide range of material including reverberating media : a wave still has the memory of its source location. Previously applied in ultrasonic in the 2000's at Institut Langevin [Ing, 2005], we implement localization technique for building use. Guided waves in concrete slabs have specific characteristic : we show through experimentation that it is possible to adapt the localization technique. State of the art In plate localization technologies :-Surface acoustic wave absorption-Triangulation process-Amplitude disturbed ultrasonic wave dif-fraction pattern-Pressure sensors in tile Applications Applications to localization of foot load in concrete slabs are multiple :-Flow measurement-Man-machine interface-Home automation Guided wave in plates The system function on the detection of the anti-symmetric Lamb waves, which is the main acoustic energy propagating in thin plates (where λ 2h with λ the wave length and 2h the plate thickness).-Lamb waves are guided disper-sive waves formed by interference of longitudinal and shear waves [Viktorov, 1967]-At low frequencies, only the S 0 and A 0 mode exist. The latter dispersion curve approach the bending wave velocity and varies as √ f Lamb waves in plate : (a) S 0 and (b) A 0 mode Feat of the dispersion curve of the concrete slab of the laboratory (only A 0 mode is visible) Results In a second step, the new impulse response is time reversed and virtually re-emitted by the sensor. We compare the impulse response from one point of the surface with the recorded ones.-Band width 500-1500Hz-Grid : 20cm-Area> 10 × 10m 2 Contrast C t The maximum to ground level ratio of the correlation coefficient is theoretically linked to the band width, which is in good agreement with our results.-Maximum peak to ground level ratio C t ≥ 2-N sensors : C t → √ N C t Resolution The focusing patterns show a −3 dB width in good agreement with the diffraction theory and corresponding to the dispersion law in a cavity.-Experimental resolution at −3dB width : 36cm-Theoretical resolution : J 0 (kr) = 34cm (with k the wave number)-Diffraction theory limit : λ 1,5kHz 2 = 50cm Typical normalized correlation patern Theoretical spot width Correlation coefficient of a footfall on a concrete slab (2 × 1.6m 2) maximum of correlation : 87% ; contrast : 2, 3 Localization technique The localization technique is based on two fundamental principel : the time reversal focus sound back to its source [Ing, 1998] and reciprocity enable to reverse emitters and sensors position [Cassereau, 1992]. It involves the comparison by cross-correlation between impulse responses recorded on a discretized surface and impact on the surface. This technique of acoustic imaging is comparable to a simulation of time reversal : the cross-correlation gives a focalisation pattern all the stronger as the signals are related. Impulse response block diagram : position of impact P , medium H and sensor S Cross-correlation coefficient C ij,1 between sensor S 1 and positions P i and P j is : We use a 2 nd sensor to avoid P phase [Ing, 2007] : C i,12 = |P i

International audience

National audience; First proposal of negative index materials by V.G. Veselago [Sov. Phys. Usp. 10 (4), 509514 (1968)] has triggered interest for flat lens design. In 2000, J.B. Pendry proposed [Phys. Rev. Lett. 85, 3966-3969 (2000)] to take advantage of negative refraction and flat lens geometry to achieve subwavelength focusing. An alternative is based on photonic crystals. Such materials reveal strong anisotropic dispersion relation that may lead to band folding and negative refraction. Flat lens behavior is observable in a narrow spectral range called the All Angle Negative Refraction (AANR), where all incident wave vectors are negatively refracted. Negative refraction in phononic crystal has been recently demonstrated for elastic waves in a solid, and for Lamb waves at high frequencies in a plate with a square lattice of square holes [Appl. Phys. Lett. 96, 101905 (2010)]. This paper presents the experimental study of bending wave focusing by a solid flat lens in a thin plate with a 45°-tilted square lattice of circular holes. The dynamics of the focusing effect is observed below the first stop band in the acoustic dispersion surface at the carrier frequency predicted by theory.

During the last ten years, time-reversal of acoustic fields have been shown to be a very useful technique in ultrasonic imaging and testing. With the use of transducer arrays, it is possible to steer the sound beam to an arbitrary position within the medium, even if the medium is inhomogeneous or contains scatterers. The major drawback with traditional beamforming and time-reversal techniques is that they require the use of transducer arrays. Recently a new technique was presented that, with the use of a waveguide, makes it possible to focus sound arbitrarily, with only one transducer elements. A problem with the setup is that the signal-to-noise ratio (SNR) is degraded because of interfering echoes from the waveguide. In this paper, we present an echo-cancellation scheme that results in an SNR gain of approximately 33 dB. This enables the new technique to be used in pulse-echo mode, where this was not previously possible. Godkänd; 2002; 20061024 (ysko)