Hey Ghost. The best thing you can do is KNOW what at what level your neighbors begin to HEAR! Maybe you can introduce yourself to your neighbors and ask them to help identify the point at which Low Frequency transmission occurs. Maybe conduct a test with some bass heavy music. I wouldn't even worry about mid or high hz. But once you know at what level/hz, then you have something to work with. And actually, thats the whole point. Why try to prevent transmission if the existing transmission loss is already good enough for the level you monitor at.
BTW, here is the deal. Even though the actual apartment shell is concrete, is there a layer of drywall over the concrete? Most likely there is. If so, most likely it is either furred out or even maybe a double wall. I've seen it. Ask the building manager if he knows anything. If it is furred out or a double wall, then the walls will be whats known as a THREE LEAF system. That means, one leaf(drywall) on your side of the partition wall, an airgap, the second leaf(concrete), another airgap, and the third leaf(neighbors drywall). IF this is indeed the typical scenario to ALL of the apartment PERIMETER walls, this implys something. Normally, TWO leaf systems will perform MUCHO better than a THREE LEAF system, as systems with TWO airgaps create a double airspring/resonant system, which allows transmission to occur at a HIGHER frequency and lower STC. Believe it or not. Tests have confirmed this. So even though the inner leaf is concrete, transmission may occur at a lower volume and higher frequency than expected. This is why its BEST to know exactly at what level(dba) transmission occurs, which only your neighbors can tell you.
Personally, I wouldn't depend on those spiked steel stands, even though they are filled with shot. This is the reason, EVEN if, they are loaded with mass, there is AIR between the shot balls, Resonance can still be transferred to points, which means structural transmission can still occur at that resonant frequency, even though you may not hear it. What I would suggest is MASSIVE stands which will dampen the vibration. Maybe not as elegant, although you could build a screen around them, I would use concrete blocks or cubeical concrete piers, or even those 2" thick x 12" square patio steps, glued together with concrete adhesive. Then decouple the speakers from the blocks with a layer of 1/2" or 1" rigid fiberglass. The 1/2" stuff can be found at any Home improvement place. They sell DROP IN fiberglass acoustical ceiling tiles, with a vinyl face on one side. Just peel that off. All you need is one 2'x4' panel. Even complete rooms are now being floated on fiberglass isolators. You could even drape fabric over the whole thing to cover them. Let me tell you how I know this is so.
A few years ago, a Belgium acoustician told me a story of a test they did at a university. They built a ROOM WITHIN A ROOM, whereby BOTH the outer and inner shell was poured with 12" thick concrete, with a 3" airgap between them. The inner shell was floated, and both shells had an access where they could enter the inner shell. These access's were sealed AIR TIGHT when closed. With witness's placed around the shell at 2 meters, a 9mm pistol blank was fired from within. NO ONE heard anything. Then, through this access, a small dowel was wedged into the airgap, as such it TOUCHED both the inner shell and outer shell at a tangent to the diameter. Then a small portable radio was placed inside and the access's closed. With volume at normal listening level, it could CLEARLY be heard by all witness's. Decoupling is not a myth. Even points can transmit ALL frequencies via structural transmission. Not only that, but concrete will transmit FASTER, and FURTHER through a building than air. Ths is why your first line of defense against transmission in a concrete building is DECOUPLING your speakers.
The next is positioning your speakers so that your REAR WALL is an interior partition wall. It will recieve the first reflection WAVEFRONT, which has the most energy, AND is the farthest from the speakers direct sound. DIFFRACTED around the speakers behind them is far weaker, although at high db levels may not make a difference. In this case, and in your situation,(depending on where your speakers are in in relationship to a neighbors wall, AND how long your lease is
, AND if transmission occurs at rediculously low levels, this is the one time I would suggest the use of a LIMP MASS product that you can purchase on a roll. I would fasten a decoupled cleat to the ceiling and hang it about 3" from the wall(from ceiling to floor, and the length of the wall(behind your speakers most likely). Then I would EITHER hang 3 or 4" thick rigid fiberglass broadband absorbers from the cieling also, about 2"(whats a few holes to fill later-big deal
) with 1/4" ply backs and a one inch airgap between the backs and the fiberglass. Cover with fabric. Concrete enclosures such as an apartment, even though furred out walls will offer some bass absorption via membrane principles, need extreme bass trapping due to low frequency not escaping through the walls etc. Corner SUPERCHUNKS of rockwool or rigid fiberlass offer the best solution to this problem.
Ghost, if you will draw up a plan of your room, I'll draw up the stuff I mentioned here IF YOU NEED IT.
But confirmation from your neighbors of transmission levels will tell you the best information about monitoring levels, which translates into what you REALLY need if anything.
fitZ
