A shock applied to a road structure generates different physical phenomena according to the characteristic of the shock (frequency, intensity). When the load is applied by a Falling Weight Deflectometer (FWD), static surface deflection of the roadway is measured. When the impact echo method is applied, compression waves propagate through the structure and the surface response is measured at a higher frequency. With an intermediate solicitation, the vibratory response on surface road can be measured and the Frequency Response Function (FRF) can be evaluated. This function is sensitive to the presence of internal damage such as voids or cracks. Sensitive frequency band depends on the investigated structure and also the type and the location of damages. To interpret the results, it is necessary to standardize them using a reference function. The paper presents the data processing principle, the statistical process to deduce the reference function which is representative of the healthy structure. It also presents the Colibri device which implements the method. Finally, the method performances are illustrated by several applications on test sites where internal damages are detected.