Sound isolation doubles only when partitions are acoustically independent of each other
Evidence
The solution is to have two or more partitions that are independent of each other. If the two partitions are truly independent, the total attenuation, or effective sound isolation, is the product of the attenuations of individual partitions, that is, the dB attenuation is the sum of the dB attenuations of the individual partitions. 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. This is most easily assured by having them made with either a different thickness or a different material. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2119
Categorization
- DEFINE: Explains the compounding rule for isolation in dB and the engineering trick of mismatching resonances.
Related
- Independent partitions sum dB attenuations but real ones underperform this (similarity 0.89)
- Partition mass provides isolation at higher frequencies until coincident resonances degrade it (similarity 0.84)
- Partition sound attenuation rises with frequency except at coincident resonances, which damping reduces (similarity 0.83)
- Real walls operate in the mass-controlled region where performance rises 3 dB per octave (similarity 0.82)
- Independent partitions sum their attenuation in decibels (similarity 0.79)
- Dimensional room symmetry does not guarantee acoustic symmetry (similarity 0.78)