Measurements made on a square flat plastic plate reinforced with a periodic array of six beams were checked with the theoretical input impedances of an infinite plate, an infinite beam, and an infinite orthotropic plate. Comparison shows that the beam‐plate structure reacts like an infinite beam below 500 Hz. But approaches the infinite orthotropic plate prediction at higher frequencies. when excited on a rib. Excellent agreement was found with the theoretical input impedance of an infinite plate when the structure was excited between ribs at its midpoint. Comparison was also made between theoretical and measured impedances after unconstrained viscoelastic damping tile was added to the smooth side of the reinforced plate.