You have a room, 75'x75'x15' (23m x 23m x 4.5m)

rectangular with hard reflective surfaces on six sides.

Ambient noise in the room is about 98dB(C). Noise is caused by World Soccer Cup on big screen tv, Muzak (pop music, elevator music), and the conversations of 200 people seated at open tables on movable wooden chairs.

How much quieter would the room get if seventy-five 2'x4'x4" acoustical absorbers were placed around the walls of the room (i.e. 1 every 4 feet, each with a 2' horizontal spacing between, mounted long vertically).

Volume of the room: 84,375 cubic feet. (2400 cubic meters)

Surface area of the room: 15,750 square feet (1450 square meters)

Surface area of proposed absorption: 600 square feet. (55 square meters)

I vaguely remember there exists a formula for this,

that presumes that people won't turn up their big screen tv, muzak, and voices.

(A local restaurant 'upgraded' -- removed the carpet, all the padded booths (tables with head-height padded leather couches), a dividing wall between the table restaurant area and the bar area (where the big screen tv is) -- and my goodness is it loud in there now)