There are more than a hundred different acoustic instruments, some of which are very loud. What is the loudest acoustic instrument?
Under a strict definition of "instrument": pipe organ -- they'll shake your bones like a rock concert.
The claim is that the Atlantic City Music Hall organ is the world's loudest instrument, with one of the stops being louder than a train whistle. Several articles, inluding the Wiki article, this one and the link above indicate that this is acknowledged by the Guinness Book of World Records, though I haven't been able to confirm it directly.
Another contender, under looser interpretation of "instrument", would be church bells, though the audience usually is not right up with them when they are being struck.
Loudness is a subjective sensation, so I will talk about an objective measure, which is the acoustic power that different instruments can generate. In contrast to the pipe organ answer, this answer is restricted to instruments that derive all of their power from a single human performer.
This page indicates that drums easily produce more acoustic watts than other instruments, which seems to match experience.
It also indicates that the trombone has the greatest power output of the instruments that can play different notes (not counting tuned drums). Experience also agrees that brass instruments are pretty darn loud.
I've read other sources that are not easy to cite here that closely match the numbers listed on the cited page.
Maybe the calliope?
From Wikipedia: A calliope (see below for pronunciation) is a musical instrument that produces sound by sending a gas, originally steam or more recently compressed air, through large whistles—originally locomotive whistles. A calliope is typically very loud. Even some small calliopes are audible for miles.
Another option could be the cannon, as used in the 1812 Overture.
As already said, there are many different ways how this could be judged. Acoustic power is certainly a good criterion, though it should be weighted with the ear's response: bass instruments require much more power to sound loud. That also allows their sound to reach further, though.
Subjectively, I'd name a couple of contenders. I'll not count church bells, these would actually win in all categories, nor instruments that can't be fully human-powered.
- Most painful at close distance: piccolo, cowbell.
- Biggest distance where it's still painfully loud: trombone or snare drum, some woodwinds (shawm, bagpipes).
- Farthest to be heard: bass drums or horns.
- Biggest efficiency (loudness achieved with little effort of human power): probably trumpet (but I've never played one), cymbals or tubular bells.
- Physical efficiency (acoustic power achieved with little effort): timpani, tuba (again no idea how exhausting it is), else possibly double bass.