A whisper is 30 dB. A jet engine is 140 dB. Krakatoa in 1883 was 310 dB at the source, heard 3,000 miles away. But there's a hard ceiling at 194 dB where the wave stops moving and starts breaking. Move the dial. Watch air give up.
A rock concert from the floor. Your ears will recover from one night, mostly. Three nights in a row and they won't. The wave is still a wave, the air is still air, and the math is still benign. We are nowhere near the ceiling.
A logarithm of pressure ratios. The reference P0 = 20 µPa is the quietest pressure a healthy young human ear can detect at 1 kHz, defined in the 1930s. Every 20 dB is a 10× increase in pressure; every 10 dB, a 10× increase in intensity. The scale is logarithmic because human hearing is.
A sound wave oscillates above and below atmospheric pressure (101,325 Pa at sea level). The trough cannot dip below zero, because there is no such thing as negative absolute pressure. So the maximum amplitude a real wave can have is exactly one atmosphere. Plug that in, and the answer comes out a hair above 194.
Sound intensity is power per unit area, proportional to the square of pressure. So a 10 dB jump is exactly 10× the intensity; a 60 dB jump (whisper to chainsaw) is one million times. The reference here is normal speech at 60 dB. The scale is unforgiving once you start counting orders of magnitude.
A point source spreads energy over a sphere, so intensity falls as 1/r². That means the level drops 6 dB for every doubling of distance. Solving for when it falls to 85 dB (the OSHA 8-hour exposure limit) gives the radius of permanent hearing damage. Atmospheric absorption isn't included, so this is a lower bound.
Sound is a longitudinal compression wave in some material: gas, liquid, solid. Air at sea level carries it at 343 m/s. The molecules don't travel; they jiggle, bumping their neighbors, and the disturbance propagates. Loudness is the size of those jiggles, frequency is how fast. Without a medium, no jiggling, no sound. Space is silent because space is empty. This is why the ceiling exists at all: the medium has limits.
Engineers at Bell Labs needed a unit for telephone signal loss in the 1920s. They settled on the bel, base-10 log of a power ratio, then divided it by ten because whole bels were too coarse. The reference 20 µPa was chosen because that's what the average healthy ear can just detect at 1 kHz. So 0 dB isn't silence; it's the threshold of human perception. Negative dB exists, you just can't hear it.
The page started with a thread that wandered: what's the loudest possible sound, then do sonic booms count, then what about a nuclear bomb. Two answers emerged. There is in fact a hard ceiling, set by the pressure of the atmosphere itself. And there are several ways past it, but none of them are sound anymore. The dial above is the conversation, made visible. Move it. Watch air give up.