Let me give more detail

Let me try this again. My sincere apologies for not providing enough detail the first time. In finalizing my attic studio plans, I'm concerned about sound leakage through the floor. I am unable to treat the floor from below. It's hard to tell how much sound transmits through the floor and how much is now going out the sides and roof, since I haven't yet built the interior acoustically-treated walls. Now the space is still wide open with no insulation. I own the apartment below the attic, so I'm not concerned about disturbing myself. However, I am concerned about how much sound reaches the apartment BELOW mine from the attic above. I played music through an amp at live-band volume up there and while it wasn't oppressive, you could hear the lower frequencies in the apartment below mine. It'd be nice to have a bit more isolation, though, again, I'm not sure how much was coming out through the roof and down the exterior walls of the building. The house is 112 years old and built solidly -- framed in oak. (I own the building BTW.) My apartment provides additional insulation from the sound traveling from attic to the ground-floor apartment below. The drums are Roland V-Drums. Drummer will generally use head phones. The only live part of the drums will be hi-hat and ride (maybe the snare, though very rarely) . I was planning to put the V-Drums on thick rubber matting such as the type used under weight-lifting apparatus in gyms. Though electronic, the physical kick pedal does transmit without some decoupling. A drummer friend used rubber of this kind in his apartment with his V-Drums and complaints from below his place have stopped. We don't play very loudly, thanks to the use of the V-Drums. The typical TV set can go louder than we play. We typically play at a volume where we can talk to each other without mics and without screaming. Vocalist does not use or need a mic to be heard for example. We also never play late at night. I currently am unable to measure volume since we're not rehearsing right now, but it's definitely a lower than average volume. The floor is currently very old tongue-in-groove hard wood. It's far from air tight. I have considered self-leveling gypsum concrete to help cut sound transmission, but after talking with a sales rep at Trademark Soundproofing, I'm leaning toward two layers of plywood with two layers of green glue. I thought of adding a layer of soundboard, but sales rep didn't think I would get much bang for the buck with that given my needs -- especially since foot noise is not a concern. What do you think? Two layers of subfloor with the green glue sound like it'd help? Sales rep certainly thought it would. Thanks guys. Hope this is enough detail.

I own the apartment below the attic, so I'm not concerned about disturbing myself. However, I am concerned about how much sound reaches the apartment BELOW mine from the attic above.

The problem might not actually be the isolation of the floor itself. It is very likely that you have "flanking" paths through the structure of the building itself. It works like this: the sound waves inside the room eventually hit the wall surfaces, causing those to vibrate slightly. Since the wall surfaces are nailed to the studs / joists behind, they transmit the vibration directly to those. And since the very same studs/joists are also solidly connected to the wall surfaces in the apartment two floors down, those wall surface will vibrate too, thus transmitting your music sound directly into the apartment of your downstairs neighbor. Even if you isolate the floor to extreme levels, it still might not reduce the sound levels in your neighbor's apartment by much, since the sound is "flanking" around your floor, through the building structure. It might also be leaking into the wall cavities through air gaps in your walls and out again through air gaps in his walls, or through any HVAC system, or even out your window and back in through his window. They only real way to isolate a room successfully is to isolate all parts of the room equally: walls, floor, ceiling, doors, windows, HVAC and electrical. They all need to be isolated to the same level. So, while adding mass and damping to your floor might help, it make not make enough difference to be worthwhile, unless you do the rest as well.

I was planning to put the V-Drums on thick rubber matting such as the type used under weight-lifting apparatus in gyms.

That might or might not work. Depends on the rubber. That would help eliminate impact noise. Another way of accomplishing this is by building a drum riser. Basically just a couple of thick sheets of plywood screwed together, resting on a 4" thick panel of OC-703 semi-rigid insulation. That is known to work well.

I currently am unable to measure volume since we're not rehearsing right now, but it's definitely a lower than average volume.

Next time you do play, take a sound level meter with you and check to see what your acutely level is. Set it to "C" weighting and slow response.

The floor is currently very old tongue-in-groove hard wood. It's far from air tight.

Bingo! There's a big part of your problem. That needs to be sealed air tight, as do all other possible cracks and gaps around the room.

I have considered self-leveling gypsum concrete to help cut sound transmission,

Did you check with a structural engineer? You can't go adding loads of mass to your floor until you do that. Get him to come in and take a look at what you have, and tell you how much extra mass you can add safely.

What do you think? Two layers of subfloor with the green glue sound like it'd help? Sales

Perhaps, but perhaps not. It all depends how much of the sound is actually getting through the floor, and how much is flanking around it. - Stuart -

This could be a chicken and egg situation. I suspect a lot of the leakage is now going down through the walls. When I construct the inner walls as shown in the diagram, that should attenuate a lot of that. I intend to use two layers of 5/8" dry wall with green glue attached to some type of whisper clips and channels. Shouldn't the floor construction come first though? Or doesn't it matter? I can't measure the sound of the group because we aren't going to do anything until after I construct the space. Besides it varies. The last place we rehearsed had virtually no sound insulation and no one ever complained. I once asked my neighbor if the noise bothered him and he said he had never even heard us. None the less, I'm in a new place and want to do things right the first and only time. I know I don't need much attenuation, but I want to play it safe. I'm shooting for 50 to 55 STC in the studio. That should eliminate all street and neighbor noise, which is my primary goal.