"Ideal"/Better Subwoofer Isolation Strategies?

[Timothy A. Kogstrom]

To Whom it May Concern:

I have gifted my two J.L. Dominion 108 sub-woofers to a friend who lives on an upper floor in a condominium, (placement very limited by her desire), such that one is in a corner, the other on a front wall, both on the same plane, very near/below the active monitors, (Adam Audio A5X's), on A.V. cabinet on front wall,

THAT said, due to her concern with vibration transmission through the floor to the condominiums below and around(?) her, I have isolated the sub-woofers by creating a "mass-loaded" foundation below the D-108's...

Each foundation is comprised of TWO, 16 lb., 12" X 8" X 2" concrete blocks. Each sub-woofer, (manufacturer's feet attached), placed on top of the upper concrete block, (26.4 lbs. + 16 lbs. = 42.4 lbs.)...below which are 4, (corner placed), 30 durometer, 1.75" dia., "Sorbothane" hemispheres, (appropriate size to adequately suspend and damp the 42.4 lb. mass)...
Below this is the identical, lower concrete block, which is suspended above the tiled floor by 4, (corner placed), 30 durometer, 2" dia., "Sorbothane" hemispheres, (appropriate size to adequately suspend and damp the 42.4 lbs. + 16 lbs. = 58.4 lbs. of the total mass of the subwoofer + foundation system).

HOWEVER, this now raises the sub-woofer(s) approx. 5 inches off the intended/(engineered for), ground plane...

Questions:
1. By raising the sub-woofers by 5 inches, in what frequency range/(frequencies?), will the subwoofer frequency curve be adversely affected, and how much of a deviation must be corrected?
(J.L. Dominion D-108 Sub: 31 - 112 Hz (±1.5dB), -3 dB at 29 Hz / 119 Hz)

2. Is 32 lbs. of mass-loaded, damped suspension sufficient to isolate these, 24.4 lb. sub-woofers from the floor...(i.e., how much isolation/vibration transmission can I expect?).

3. Is there a better mass or configuration to use...?...(space/placement requirements are VERY restrictive...)

Thank You for you patience in this if you have read this far!

Thank you in advance for your responses/assistance, and I avidly look forward to your response!

Sincerely,
Synergist

 

[lovinthehd]

I would first measure the current placement before worrying about any correction....5 inches might make a small difference but I wouldn't worry about it.

Best way to see if what you have done works, is to go visit the neighbors and see where the sound intrudes (have someone in your gf's place turn up the volume while you chat on the phone or something?)....but sounds like if anything is going to stop transfer from these little subs, that would do it. Won't stop what the majority of the output is (i.e. radiated from the cone into the room) particularly, tho. I'd probably have not gone as far as you but don't have the need either....