The prediction of the pressure drop of packed beds requires an accurate estimation of the packed-bed porosity as its error propagation multiplies the occurring errors by a factor of about four. For a better understanding of the packed bed's local porosity characteristics, a comprehensive x-ray tomography study was performed investigating and correlating the void structure of packed beds made of smooth, mono-sized spheres in cylindrical confining walls having tube-to-particle diameter ratios lambda = 3.0 to 9.0 reinforced by numerically generated packed beds of lambda = 1.1 to 9.0. An in-depth analysis of the obtained oscillating void profiles is performed, discussing the locations and heights of porosity extrema. Most importantly, a significant and in parts surprising extrema formation in the tube's center is described, especially present for lambda < 6, which is assumed to have a predominant effect on the packed bed's pressure drop. (C) 2020 Elsevier B.V. All rights reserved.