There's also a point, somewhere around 160 something, where you just can't get any louder in air as the density of air won't support the soundwaves. You can go louder in water. Which is why the redneck fisherman with the dynamite works. Tom Danley did some work for the government where they made these huge air horns to see if they could make destructo rays. Didn't work out. But they made some really loud noises.
If you were to have such a contest, it should be in the free field so you are only getting the wavefront off the drum. That would make the results comparable to results from elsewhere. A sound level meter wouldn't react fast enough. You'd have to use a time based computer measurement system like Smaart to pick off the transient.