Why are fighter jets so loud?

An airliner engine such as the CFM56 moves most of its air through a large fan at moderate speed; only a small fraction passes through the hot core. A fighter engine does the opposite: the F100, F110 or EJ200 have bypass ratios below one, so nearly all the thrust comes from a fast, hot core jet, and lighting the afterburner raises the exhaust temperature and speed further. James Lighthill showed in 1952 that the acoustic power of a turbulent jet grows with about the eighth power of its velocity, so doubling exhaust speed multiplies the sound energy by roughly 250 times.

The afterburner injects fuel into the exhaust and roughly doubles thrust at the expense of enormous noise, adding a low-frequency rumble and a crackle caused by shock waves in the supersonic exhaust. The characteristic diamond pattern visible in the jet plume is a series of shock cells, each of which radiates screech tones. Measurements on US Navy carriers have recorded levels around 150 dB on the flight deck during catapult launches, which is why deck crews wear double hearing protection.

Fighters train at low level and at high speed, so the distance to the listener is short and the sound arrives shortly after or even with the aircraft, in contrast to the gradual approach of an airliner on final. Airframe noise also matters: open weapon bays, external stores and slats generate turbulence. Airliners have quietened dramatically since the 1970s because of certification limits and high-bypass engines; military aircraft are exempt from those noise rules, since range, speed and thrust-to-weight ratio take priority over comfort on the ground.