Re: What is the loudest sound in the world?

Phew! That's a pretty wide ranging question you put there. I think we'll 
have to limit the subject to "sound" in the strictest sense, ie. that it 
is in the "audible range" of 20Hz to 20kHz and that it is in air. This 
means we're not talking about whale song or earthquakes (which are "sound" 
waves travelling through the earth's crust).

I also have to leave explosions out of it, because the formal definition 
of a "detonation" is that the flame spreads faster than the speed of 
sound. Apart from that, explosions can be made as loud as you want, given 
enough explosives...So, our "loudest sound" will quite probably be a roar 
or a rumble of some sorts.

The absolute loudest steady-state sound level that can exist anywhere is 
given by atmospheric pressure. A sound wave is compression (pressure 
increase) and rarefaction (pressure decrease) of air. In the latter case, 
it would be impossible to create a pressure of less than zero (vacuum), so 
the limit is a peak pressure change of 1Atm (or 100kPa). How sound level 
in dB's relates to a sound pressure in Pa can be found in the FAQ linked 
below: http://www.point-and-click.com/Campanella_Acoustics/faq/faq.htm
Doing the calculation for ourselves we'll find that 20*log(100kPA/20uPA)
=194dB SPL peak (or 191dB SPL average, which is the more normal measure).

Quite obviously this means that you'll have to be "in the heat of the 
event" to find this sound level. For practical reasons, sound levels are 
measured at a distance several times larger than the sound source itself. 
Usually, we're only interested in the loudness of an event from an 
environmental point of view. To determine if a disco is playing too loud, 
measurements are made at a few 100 metres from it. For louder sounds, the 
measuring distance is even further away.

Now, to get back to the real point: the loudest known sound, for all 
intents and purposes, is a space shuttle launch. Some sound levels are 
quoted in the link below: http://www.jhu.edu/~cpia/database/820143bb.html
They are speaking of up to 106dB(A) SPL at 6 kilometres. What this means 
is that any normal person will automatically cover their ears - at such a 
distance!