Fiber reinforcement is well known to dominate the post-cracking behavior of composites. The fibers’ contribution is also important however, at the pre-peak stage as they restrain cracking. The sensitivity of AE, enables characterization in the micro-scale even during this pre-cracking regime and before serious damage evolves. This study focuses on parameters obtained at low level of loading before cracks appear in any measurable form in the material. As model materials, additively manufactured fibrous polymer composites and cementitious composites are used, showing that the content and the effectiveness of the reinforcement phase and the properties of the fiber/matrix interphase are reflected on the early AE activity allowing projections to the final mechanical properties.