However, one of the most significant sources of cabin noise from commercial jet aircraft, other than the engines, is the Auxiliary Power Unit (APU), an on-board generator used in aircraft to start the main engines, usually with compressed air, and to provide electrical power while the aircraft is on the ground.

Airplane cabins are inherently loud due to a combination of mechanical and aerodynamic factors. The primary source of noise is the jet engines, which produce a high-decibel roar from the combustion process and the rapid rotation of fan blades. Even with modern "high-bypass" turbofan technology, the shear layer instability of the exhaust creates significant turbulence. Another major contributor is aerodynamic noise; as the aircraft travels at 550 mph, the air rushing over the fuselage, wings, and landing gear creates a constant wind noise. Inside the cabin, the Environmental Control System (ECS), responsible for pressurization and air conditioning, adds a persistent mechanical hum. To keep planes light and fuel-efficient, manufacturers use minimal heavy soundproofing insulation, meaning much of this external energy enters the cabin. While newer planes like the Boeing 787 use serrated engine nacelles to reduce noise, the environment remains loud because complete silence would require a weight penalty that airlines aren't willing to pay.