Today, on Pimp My Airline…
Not quite related, but the loudest plane I’ve ever heard was the Concorde. Was at an airshow, and just hearing the thing take off was bone-rattling. I can’t imagine how the sonic boom would feel.
Osbourne Reynolds, the inventor of bouillon cubes, described a dimensionless number representing the ratio of inertial to viscous forces in fluid flow. It’s proportional to velocity and size and to a property of the fluid called the kinematic viscosity. A jet engine exhaust must have a very high Reynolds number, because it creates flight power by throwing the exhaust gasses rearward at very high speed, and its diameter can only be made smaller by greatly raising the already high speed.
There is a great deal of energy in this exhaust because of its motion. As all the motion subsides, it must do so through what computational fluid dynamicists call a turbulence cascade, because the inertial part of fluid motion does not consume any kinetic energy - only the viscous part does. So larger vortices split somewhat randomly and chaotically into smaller ones, in a cascade downward in vortex size until the Reynolds number gets close enough to 1 that the viscous effects consume much of the kinetic energy (turning it to heat energy). This is quite a small size scale.
Meanwhile, because there are turbulent vortices of all these sizes, some of the kinetic energy propagates outward in sound waves. All the size scales contribute similarly, and the sound waves therefore have all frequencies contributed similarly, making a roaring noise.
Such roaring noises have maximum information content in the sense that there is minimal predictability (because when a sound is predictable it takes little information to duplicate it). If you’ve heard the sounds modems make when they are transferring compressed data, you recognize the same roar. This sound has maximum information content in the same sense. If there were something predictable about the sound, like a steady tone or a rhythmic beat, it could have been compressed out by data compression methods.