Dissimilar layers and reduced coupling raise real-world sound isolation.

Evidence

In practice, the partitions are not independent, although the isolation is improved dramatically, but not as much as would be predicted by simply summing the dB attenuations. … Coincidence resonances also reduce the effectiveness of a partition, and it is important to ensure that the two partitions have different resonances. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2119

Categorization

• SEED: A short piece on why ‘more of the same’ fails in soundproofing—and where the same logic of dissimilarity-as-strength applies elsewhere (double walls, layered defenses, diversified teams).
• XREF: connects to: Sound isolation depends on flanking paths more than the barrier itself — David Howard, Jamie Angus, coustics and Psychoacoustics, 2169; In homes, air leaks dominate sound isolation more than the glazing itself — David Howard, Jamie Angus, coustics and Psychoacoustics, 2173; Flanking transmission paths, not the partition itself, limit sound isolation — David Howard, Jamie Angus, coustics and Psychoacoustics, 2170