What it actually decides.
A ground stack is a column of loudspeakers built up from a stage deck, a riser or the ground, carrying its own weight instead of hanging from a rigging point. On a park lawn or a closed street there is frequently no structure overhead at all, so the stack is what is available unless a ground support tower goes up to fly from.
The cost is coverage. A stack starts near head height and the front rows sit above it, so level falls steeply from front to back. L-Acoustics measured this for subwoofers and published the figures. Ground-stacked subwoofers in a 100 m deep venue can produce a front-to-back SPL difference of over 30 dB. In a 50 m deep venue with a standing audience, the same paper puts a subwoofer as high as 10 m with no loss of rear efficiency, which cuts the front-to-back spread by more than 12 dB.
The efficiency penalty usually quoted for flying is overstated. That paper puts the loss of floor coupling at 3 dB of total radiated power rather than the 6 dB commonly assumed, and finds the difference recovered at distances of about five times the subwoofer height for a standing listener. Measured at 20 m, a single KS28 on the ground and the same box flown at 3 m differed by far less than 6 dB at listening height.
What decides it on site is rarely acoustics. Flying needs a point rated for the load and somebody willing to sign for it. A stack needs floor area in front of the deck, sightline clearance for the first rows, and ballast or a frame that holds it in wind. Where a roof or a ground support structure is going up anyway, the hanging point arrives with it.
We own this equipment.
Event audio production — owned inventory, our own crew, and a site survey before anything is specified.