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 |