A method of making a panel for installation in the high velocity air flow region of an engine nacelle and other applications requiring sound absorbing material and increased air frame strength. The panel utilizes a single layer of either a metallic or nonmetallic cellular core with a solid or perforated septum disposed internally therein. The septum may have a planar or nonplanar configuration within the individual cells of the core thereby providing means for reducing the noise level of various sound frequencies created by a jet engine or any other sound producing devices for that matter. The planar or nonplanar septum provides structural stabilization and/or vibration dampening to a bonded sandwich assembly. The panel may also be used in strengthening air frame construction and other applications besides sound attenuating panels.