FLOOR as a bass trap? Indeed, it can be, yes.
However, simply filling it with 6 inches of insulation would be a total waste compared to what is possible.
In the area that I installed a similar floor over concrete I did just that, but the room in question didn't need any more bass traps. I did it so I would have a floating floor that collected and transferred energy through the listening chair to the listener. It added tremendous life to music vs. sitting on the concrete. It also had some nice trapping effects down to probably 150/200 Hz at best.
If you want a real bass trap, you'll want to create a diaphragmatic absorber under the floor in place of the insulation. You will use 3/4 plywood on 2x6 or 2x8 or 2x10 or 2 x 12 inch joists. The cavity that the joists create must be air tight. Joists must be glued to the floor and air tight. Install two layers of large size bubble wrap on the concrete to create effectively a 1 inch air space. Then lay down a layer of 2 inch thermafiber aka rockwool OR Ownens Corning's high density 2 x 4 panels of 701 (I think it's 701 or 702). Then install another layer of bubble wrap to create the second 1 inch air space and add ANOTHER layer of the 701 or 702. Repeat this layering process, (1 inch air space + 2 inch high density fiberglass) until you have reached the top of your floor joists. This would be a serious bass trap. 12 inches gives you linear absorption down to 60 Hz.
If you can do anything you want, perhaps create a 6 inch depth floor and wrap the outside edges of the room with a 12 inch floor- effectively creating a sunken living room. The outside part that is 12 inches deep would trap low frequencies, while the 6 inch depth floor would trap higher frequencies.
Just remember, a bass trap (a real one anyway) consists of a sealed box with no air leaks and no internal bracing filled with alternating layers of high density fiberglass and dead air spaces. It can be any shape you wish. The depth of the alternating layers determines how low it will absorb. To get down to 20~30Hz range you need about 18 inches of depth. 12 inches usually hits around 60 Hz. In comparison, a piece of regular house fiberglass insulation, or foam tiles would need to be some 15 foot thick to absorb bass in the 20~30 Hz range and it is proportional.
The reason the alternating layers of dead air and high density fiberglass works in less than 1/4 the depth is because of the layering aka diaphragmatic action of each layer. As each layer resonates independently of the other, it converts the sound energy into heat and the heat, unlike the sound energy, does not come back out of the box. The heat is caused from friction. The friction is caused from air molecules colliding with each other and with the high density fiberglass. Air molecules that are moving at the speed of sound are de-accelerated by the fiberglass particles. The air space between each layer creates impedances in the resonance that shift in phase backward in time with each consecutive layer. The contrasting phase angles meet in the dead air space between each layer causing some cancelation of the sound. This is why the layering effect is 4 times more efficient than the same depth of a single layer.
Before you begin you room, buy the book "Masters Handbook of Acoustics" by Everets. It's an easy read, has the minimal math required to calculate absorbtion, diffusion and gives several examples of extreme listening rooms, which is another term for Recording studios. It will give you the knowledge you need to be HIGHLY effective with your money.
Example, one man has a box of 2 inch foam tiles and he puts them on the walls of his listening room where he thinks they might be effective. Another man reads the handbook and places the same number of tiles in the exact locations where they WILL be most effective and actually hears a profound improvement while the first is still in the "I think I can here it zone"
Knowing where to place acoustic treatments is at least 400% more important than how many you purchase. The shits not cheap, you could by 4 or 5 times more treatment than you need because you didn't put it in the exact right spot. The book is around 30 dollars. You will know exactly where and what to do, how to do it, how to build it.
You'll also get enough of a basic understanding of all this to distinguish BS from the real thing. You'll be able to do the math and see that it takes 20 tube traps to equal one 150 lb diaphragmatic absorber. You discover that diaphragmatic techniques render linear results - meaning equal absorption at all frequencies within it's design band... Usually several octaves.
BEFORE I bothered to read this book, I took a perfectly good space and build a hideous sounding listening room. It had concrete floors. I put indoor outdoor carpet on the floor that was glued down to the concrete. At that stage I put my favorite speakers in the room and they had no bass, not to mention sounded like crap.
I ended up having to cover 2 walls floor to ceiling corner to corner with 2 inch foam tiles before it sounded just OK. I had bass now. Because the 2 inch tiles acted as bass traps... NO. Because the 2 inch tiles absorbed mid and high frequencies from 1K on up. See, the bass was always there... the highs were just 12 dB too loud.
Now the room looks stupid. I find out after reading the book, that had I installed a thick pad under the carpet and used a thicker carpet to begin with, I would have had the same number of Sabins of absorption as the two walls covered with foam. So I got new carpet and put a pad under it, removed 100 percent of the foam tiles on the walls and had the exact same frequency balance. The tiles cost 2.00 each. I used 306 of them, and then removed 306 of them.
I also learned about golden ratios and how they effect standing wave patterns in the room. I then raised my ceiling 3 inches, moved one wall in 10 inches and the other out 4 inches to hit that ratio. Now there are greatly reduced holes in the bass as you walk through the room.
Can you see how much money I could have saved, not to mention time, had I read the book before I started...
Many years of practicing the information in that book has confirmed it is accurate.
Steve