Real walls operate in the mass-controlled region where performance rises 3 dB per octave

Evidence

Most practical partitions operate in the mass-controlled region of the isolation curve, with coincident resonances limiting the performance at higher frequencies. Figure 6.55 shows the attenuation of a variety of single partitions as a function of frequency. In particular note that the plaster board wall has a significant coincidence resonance. The performance of a single partition increases by 3 dB every FIGURE 6. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2116