A doorway’s flex reduces wall stiffness only below 50 Hz without harming sound

Evidence

The simple explanation for all of this is that in spite of the solid construction of the room, the stiffness of one end wall of this room was reduced by the presence of a door in the center of it. The wall became more flexible, with possibly more mass, at very low frequencies. The performance at higher frequencies—above about 50 Hz—was unchanged. Does this matter? Not really. Low-frequency absorption is a good thing, damping the mode. The small frequency shift is normally of no consequence. It is of interest to note that, in the spatial domain, as plotted in dB, the peaks are broad and the dips are narrow. Attenuating a peak is beneficial to more listeners than elevating a dip, although damping achieves both. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1565