LOUDEST SOUND IN RECORDED HISTORY

Saw a show on some HighDef channel (I'm a sucker for anything on Hidef TV) about Krakatoa.

The loudest sound in recorded history occurred on August 27, 1883, just off the coast of Jakarta, Indonesia.
A volcano on the small island of Krakatoa was undergoing the cataclysmic stage of its months-long eruption. At 10:20 AM the volcano exploded with a sound heard, literally, around the world.
Scientists estimated the explosion's sound to be around 180 decibels, which is a great enough force to instantly kill all hearing tissue within a human ear.
For comparison, 180dB is about 13x as loud as a jet engine from 100 ft, or as loud as a rifle shot at point blank range. People 2,200 miles away in Perth, Australia could clearly hear the eruption immediately after the explosion.
The explosion spewed a cloud of lava and ash that killed all 3,000 people living on an island 8 miles away. Tsunami waves caused by the volcano crested at over 100 ft.
The eruption sent a shockwave of energy that traveled around the world approximately 7 times and registered on measurement instruments for 5 days after the eruption.
Tsunami waves reached the coast of South Africa over 8,000 miles away, and smaller waves registered on tidal meters as far away as the English Channel.
However, the English waves occurred too soon after the explosion to be remnants of the tsunami. Scientists believe these disturbances were instead a result of air displacement caused by the sound of the eruption

More nerdy sound factoid stuff.. dont know if this is true or not but sounds feasible i suppose

But if you're talking about sound in decibels at the recording source, it's a different story. The loudest sound possible in air is 194 decibels. What does this mean? It means that anything with a higher energy density cannot travel as a sinusoidal wave through air--the wave breaks down, causing turbulence and friction at the wavefront (like comparing smooth ocean waves to the breakers on a beach that happen when a wave gets too big). Such energy in air would turn into heat and even possibly light or plasma rather than sound; it only turns into sound under 194 decibels. You could imagine that in the first split second of a nuclear blast, the fireball has an energy density greater than the equivalent of 194 decibels, and instead of "sound" you get instant supersonic wavefront of destruction. Once the energy density breaks down, the shockwave travels out at the speed of sound, starting at 194 decibels and getting quieter as it radiates outwards. Probably the people who live to tell about nuclear blasts have heard sounds in the lower 100s of decibels or less. Recording equipement and ears actually get destroyed at sounds lower than 194dB, for example supposedly your hearing tissue gets destroyed pretty instantly at 180 decibels, and even 150 decibels will make you go deaf in a short while. So: loudest absolute sound is some sort of cataclysmic explosion volcano, meteor impact, big bang, etc. Loudest sound in energy density is 194dB and occurs at the edge of destructive events, such as major explosions, suicidal blue whales, supersonic aircraft, etc, but this sound is not heard or recorded by the observer; it diffuses by the time it is heard or else it kills the listener.