Since we're all involved with rooms generating a great deal of sound, I though some here might find this interesting. Generally the most difficult aspect of high level isolation is controlling the low frequencies (bass). Keep in mind that STC doesn't measure bass, as it does not consider frequencies below 125Hz, and we're obviously dealing with rooms that put out a great deal of sound below that. Generally construction efforts to reduce low frequencies will naturally take care of the lower energy high frequencies.
In short, every compressible cavity (such as air cavities in walls and ceilings) will define a specific resonance point (frequency) in a decoupled system. If we have a double stud wall, or ceiling with clips and channel, then we have a decoupled system. Think of this decoupled system as a spring that oscillates. This system will have a calculable low frequency resonance point, defined by the Mass-Air (spring)-Mass parameters. Let's say this resonance point is 70Hz.
At 70Hz, we don't stop a lot of sound, since resonance allows that frequency to pass fairly easily. At 100Hz, we're doing much better, but as we start looking at frequencies lower than 100Hz, Transmission Loss gets worse and worse until we hit 70Hz rock bottom. So at resonance (70Hz), and just above resonance (70-100Hz) things are not great for our sound isolation. Generally the math is from the resonance point up to around 1.5X the resonance point we don't do as well in sound isolation.
If we could move that resonance point from 70Hz. to 40Hz. we would be much better off:
Scenario #1 has 70Hz resonance point, and weakness from 70Hz through 105Hz. (70 x 1.5= 105).
Scenario #2 has a 40Hz. resonance point, and a weakness from 40Hz through 60Hz. (40 x 1.5= 60).
This is why we spend time looking to incorporate methods to lower that LF resonance point as much as possible. How do we accomplish this? Keeping in mind that a decoupled system is a spring system:
We can add absorption in the form of simple (standard thermal) insulation. This will lower the resonance point (frequency) of the system a bit.
We can add mass to the system. This essentially weighs down our spring system, slowing the oscillation = lowering the resonance. The added mass is more effective than the insulation.
We can add cavity depth to the system. For the same reason that insulation helps, so does more air in the cavity. This also isn't as effective as adding the mass.
So again, if we can progressively march that low frequency point down, we minimize the frequencies that will display weakness.
Hope this helps.
