OK, getting down near the end of my historical progress photos archive....
One of the weakest points to soundproof was the basement steps. From the 1st floor you come down two steps to a 3x3 landing, both of which protruded into the theater.
Here's how it originally looked after I ripped out my original soundproofing work:
Here you can see how the landing framing was completely open to the area underneath the steps.
I made a double layer OSB sandwich out of 3/4" OSB and Green Glue cut to fit this first bay in the landing framing precisely:
Fit as-seen from underneath the steps. Since the stair landing was made from 2x6 framing, I added R19 insulation before permanently installing this piece:
And fit as-seen from the theater:
Next up was two layers of 5/8" drywall with Green Glue (after removing a ton of the excess construction adhesive)
Next, I made a double layer sandwich to fit precisely underneath the first stair tread:
And installed with acoustic caulking filling every gap:
Because of the bevel cuts, I had to split the angled piece into two for installation. Here's the first piece installed with some insulation to fill the void. More insulation was added as the second half was installed:
I added some screws through the front of the stair riser for extra security while the adhesive and caulking set:
...and used acoustic caulking to fill every other available gap from the upstairs:
A poor camera angle, but I added 5/8" drywall with Green Glue to the sides of the step stringer to fill the gaps which led straight upstairs. This is the same gap caulked from above with acoustic caulking, FYI. Not shown, but I also filled the area I couldn't reach with low pressure spray foam.
Added another double layer sandwich of 5/8" with OSB / Green Glue / 5/8" drywall underneath:
Added on piece of 5/8" with Green Glue to the front. I beveled the top of the piece to match the angle at the top precisely then used acoustic caulk to seal the entire thing:
Now you see the methods I used to take soundproofing the stairs to the extreme. When you see the framing pictures, you'll see how I positioned the framing to stand proud of the face of these stairs by 1/4" - 3/8" so the double layer sheet material of the walls will pass in front of *most* of the stair structure for even more protection. And once the ceiling goes in I'll make an angled 'cap' to cover the small part of the stairs which is still protruding into the theater. This will be inside the perimeter soffit, so it will be hidden.