This paper is about the reflection of a plane acoustic wave incident on a material modeled as a dipolar gradient solid. The dipolar gradient model has been used in order to account for the micro-structure present in multi-scale materials (e.g. biological issues, composites, meta-materials). The influence of the internal lengths of the gradient model on the reflection coefficient is described and discussed. A dispersive behavior is observed at high frequency, when the wavelength of the disturbance approaches the characteristic size of the material. This topic is of major interest for understanding the role played by the micro-structure in the reflection phenomena occurring at fluid-solid interfaces and find its application to material properties characterization by means of ultrasound waves.