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I'm looking to reduce the vibrations going downstairs as much possible. How well will concrete blocks acheive this? Any recommendations on size/mass?
Mass would be useful, but your real problem isn't mass: it is "decoupling". Right now the speakers are transmitting their vibration directly into the floor, where it becomes a "structure-borne" problem, rather than an "air-borne" problem. Solid structural things are really good at transmitting sound, and when it arrives at something large and flat, such as a ceiling, it transmits the sound into that, turning it into a giant speaker. Mass will help a bit, and is part of the solution you need, but the real "magic" that you need is to to prevent the vibrations from getting into the floor at all. You need to "decouple" the speakers from the floor.
The best way of doing that is with something called Sorbothane. It's a special type of rubber that is used as a shock-absorber and vibration damper in many, many applications. There's nothing better. It ain't cheap, but it does the job wonderfully. With must types of rubber, if you take a piece and drop it on the floor, it bounces. Sobrbothane does not: it just sort of "flubs" onto the floor, and stays there.
However, it's not as simple as just putting any old piece of sorbothane under your speakers! You need to "tune" the system in order to get good isolation (decoupling) at the frequencies where you need it. When you put something heavy on something resilient, in scientific terms you have created a "mass-spring" system. In this case, the "mass" is your speaker, and the "spring" is the sorbothane.
All Mass-Spring systems have a natural resonant frequency which is governed by the amount of mass, and the resilience of the spring (how "bouncy" it is). At the exact frequency where resonance occurs and for about one octave either side, the mass-spring system transmits the vibration energy from one side to the other very, very well, and can even amplify it. That's what is happening with your speaker right now: there are some frequencies where your floor vibrates naturally, and it passes those right through to your neighbor, perhaps even amplifying them. So, by putting sorbothane pads under your speaker, you will prevent that from happening... provided that you "tune" the resonant frequency of that Mass-Spring system such that it is at least one octave lower than the lowest frequency that you need to isolate. If you get that right, then the speaker will "float" on top of the rubber for all other frequencies, and you won't have any more structure-borne vibration.
So the trick is to tune this correctly. To do that, you need to know the weight of your speaker, and the lowest frequency that you want to isolate. Based on that, I can calculate which type of Sorbothane you will need, how many pads, how thick they need to be, and how big they need to be.
There are two ways you can do this: if you want some basic isolation down to reasonable frequencies, then just buy a large concrete paving stone from your local Home Depot, put it on the floor, then put the sorbothane pads on top of that, and the speaker on top of the sorbothane. If you want even better isolation, then get two paving stones for each speaker, and put the sorbothane in between them, with the speaker sitting on top of the upper paving stone. The calculations will be different in each case, since the second paving stone adds a lot more mass to the system, which makes it easier to tune down to a lower frequency.
So that's the rough outline of what you'd need to do.
Of course, I'm assuming that your problem really is structure-borne sound, not air-borne sound. You might want to do some tests to confirm that this is the case before buying concrete blocks and rubber!
- Stuart -