Why is the a320 so loud?

Under each A320 wing sits the fuel overpressure protector, a small vent with circular openings to the airflow. At approach speeds the air crossing that cavity sets up a resonance, the same mechanism as blowing across the neck of a bottle, and the result is a pure, penetrating tone rather than broadband rush. Because a pure tone carries much further than noise spread across frequencies, it reaches the ground clearly, and it appears precisely when the engines are throttled back and quiet, which is why people under approach paths came to associate the sound with the type rather than with its powerplants.

From 2014 Airbus offered vortex generators, angled plates mounted just ahead of each opening. They disturb the shear layer crossing the cavity so it cannot lock into resonance. The parts weigh almost nothing and fit in a few hours; Lufthansa financed a fleetwide retrofit, other operators followed, and aircraft delivered since carry them from new. As pre-2014 airframes retire the tone is disappearing from approach paths altogether.

Everything else is ordinary narrowbody sound: the CFM56 or V2500 fan, the airframe itself with gear and flaps out, which on a modern type rivals the engines during descent, and the auxiliary power unit running at the gate. The neo generation shifts that balance, with larger, slower-turning fans on the LEAP-1A and the PW1100G, and Airbus states the aircraft's noise footprint is around half that of the previous generation.