I'll address one issue that is a common mistake that people make far too often.
Don't make the screen so big that it leaves no room for speakers. Or... makes the screen impractical to view. There are on-line screen size calculators that can help you determine the best size screen for your room size and viewing distance.
Now you can use an Acoustically Transparent Screen, which is a nice idea, but have you checked the cost of them? I'm suspecting they are not cheap.
Also, can you explain the uneven floors or uneven ceiling.
I'm just trying to understand how it can be 74" (6' 2") on one end and 81" (6' 9") on the other? I'm not saying it is not, I'm just trying to understand how and why. For example is the ceiling actually that low or are those the low spots where heating or pumping run under the ceiling? The more we know the better we can help.
You say a 156" (13ft) Screen, but is that diagonal? If so, with a 2.35:1 aspect ratio, the screen will be 5ft x 12ft (roughly).
You say the space will be 17ft wide and 20ft long, one assumes that does not count the open space to the side?
If you have two rows of seating, you will have to raise the second row probably at least 6". And it turns out, if I understand correctly the back of the room is 7" higher than the front of the room. So, a platform made of 2x6 plus a layer of 3/4" Plywood, is going to take you very close to 6". Whether a 6" platform is enough for you, in your circumstance, only you can determine. With the screen only 1ft from the floor, the rear seat are not only going to have to see over the heads of the front row, but they are going to have to be able to see down to see the bottom of the screen.
If you go for a 4ft x 9.4ft screen (1:2.35)
, that is still pretty big, and would allow the screen to be 2ft off the floor (roughly). Easier to view from the back row.
You have to treat every aspect of the room with respect to every other aspect of the room. Sound, lighting, seating, sound system, speaker placement, screen size, and room acoustics - each will effect the other.
If you can afford an Acoustically Transparent Screen
, so much the better. In that case, it is possible for you to have 3 identical speakers across the Front. Say 3 floorstanding speakers ...on the assumption...
that you can find someone to sell you ONE (or 3)
Floorstanding speakers. In the USA, floorstanding speakers are frequently priced as EACH, but are only sold in PAIRs.
There are many places you can buy Acoustic Panels, and Acoustic Foam. There are many videos on YouTube about both. There are many YouTube videos on making your own Acoustic Panels, which really isn't that hard and can be done with only the most basic tools.
I would say, generally you want some treatment behind the speakers. Then at the First Reflection Points
for both rows of seating. These can be determined by sitting in the first row Prime Seat Center, then having someone slide a mirror along the wall until you can see the speakers in the mirror. Keeping in mind you will have TWO speaker (left/right) and you will not see them at the same point along the wall. Then repeat the process for the second row of seating.
With low ceilings, though we don't know the nature of those ceiling, you might want to put Foam Acoustical Tile on the ceiling. It doesn't need to be 100% coverage. Again you can use a mirror to find the First Reflection Point on the ceiling and place Acoustic Tile there. More at the point of first reflection, and less else where. You want some reflection in the room, you just don't want the reflection to dominate the sound.
As to the far back wall, perhaps a blend of Acoustic Diffusers
and Acoustic Panels
. There are also videos about how to make your own Acoustic Diffusers
. Again not a complex task, and do-able with even basic tools.
and Acoustic Foam
break up a solid wave front, and scatter it as many smaller wave-front moving in all directions and with different timing.
That's all for now.