Modern civil aircraft all carry an APU, a compact gas-turbine engine to supply compressed air for engine-start and air-conditioning on the ground and electrical power on or off the aircraft. Because the unit is completely enclosed by a sound-reduction, fire-proof titanium shroud, its external signature is already muffled, yet it remains one of the major noise sources at many airports.
A cause of APU noise is combustion noise whose peak frequency is concentrated in a narrow band between 250Hz and 350Hz. This sound pressure level rises with the square of the fuel consumption rate, so even small increases in load make the tail seem louder. Internally, flight-deck crews of 777s and 787s rely on microphones that capture an inside APU sound for monitoring, while externally, brake fans and packs sometimes mask the unit, making it seem a little quiet.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is the sound of a plane APU?
From inside the cabin you first hear a soft blowing sound; this is not the APU itself but the air-conditioner that starts as soon as the APU is turned on. The APU is located inside a compartment in the tail section of Boeing B737 and B747 aircraft, and the compartment is completely enclosed by a sound-reduction fire-proof titanium shroud. Because of this heavy shroud the APU's own mechanical noise stays outside the cabin. Therefore, the most noticeable interior noise is the rushing airflow of the air-conditioning packs, especially on the 787. Passengers perceive the gentle blowing noise that follows the moment the APU switch is moved to ON.
How loud is an aircraft APU?
An aircraft APU is loud when bleed air is used to heat or cool the cabin on the ground. Its exhaust reaches roughly 80 dBA (80 dBA) within 20 m (65.6 ft) of the tail cone before any silencer is fitted. ICAO recommends a service-area limit of 85 dBA at 20 m (65.6 ft) and a ramp perimeter limit of 90 dBA. These levels are below the 118 dB (118 dB) produced by turbo-fan engines during take-off at 200 ft (60.96 m), yet they still sit within the same range as a diesel truck at 50 ft (15.24 m, 84 dB), a power mower (96 dB), or the 97 dB recorded one nautical mile (1.85 km) from a Boeing 737 on final approach. While not loud enough to rupture an eardrum like a jet at 25 m (82 ft), the APU signature is distinctive and is one reason many airports prohibit its use between 10:00 p.m. and 6:00 a.m.
What causes noise in an auxiliary power unit?
A contributor to APU noise is combustion noise produced when fuel burns inside the small turbine combustor; large-scale turbulence structures within the flame itself radiate broadband sound. Mechanical vibration is another common source of noise within auxiliary power supplies. Inside the APU, bearings in the centrifugal compressor and turbine degrade, producing a high-pitched whine as metal rubs against metal. Any misalignment of these components will multiply mechanical vibration and add harmonics to the overall sound spectrum.
Additional acoustic signatures originate from the centrifugal compressor and exhaust sections. A set of silencers exists in the APU, yet the bulk of the engine noise is exhaust turbulence arising from the differential between high-velocity exhaust gases and the ambient airflow. The spectral shape at the exhaust remains the same regardless of directivity or power setting, such that even at idle the APU still projects a broadband hiss. A poor ground connection between the unit and the aircraft metal structure turns stray electromagnetic waves into audible electrical whine.
