What's the Noise We Hear When an Airplane Passes by: Understanding Its Source

The noise you hear when an airplane passes by is primarily caused by a combination of factors related to both the airplane and its interaction with the air. Here’s a detailed breakdown of what that noise comprises and why it occurs.

Engine Noise

The engines produce a significant amount of sound, particularly in jet aircraft. This noise is generated by the combustion of fuel and the expulsion of high-speed exhaust gases. Even modern turbofan engines, which are commonly used in commercial aviation, can still produce a substantial amount of noise despite being designed to be quieter than older turbojet engines.

Aerodynamic Noise

This noise is produced by the interaction of the airplane with the air as it moves through it. As the aircraft flies, air flows over and around its surfaces, including the wings, fuselage, and landing gear. Turbulence created by this airflow leads to sound production. The faster the airplane flies, the more significant this aerodynamic noise becomes.

Mechanical Noise

Variants within the aircraft such as landing gear and flaps can also contribute to overall noise. For example, deploying landing gear creates additional aerodynamic drag and turbulence, leading to more noise. Mechanical systems, such as the flaps, can vibrate and produce sounds as well.

Shock Waves

When an aircraft exceeds the speed of sound (approximately 343 meters per second or 1125 feet per second at sea level), it creates shock waves, resulting in a sonic boom. This phenomenon is a different type of noise, distinct from the sounds produced at subsonic speeds.

In summary, the noise from an airplane is primarily due to engine sounds and the interaction between the aircraft and the air, along with contributions from mechanical systems.

For propeller-equipped aircraft, the sound is a combination of propeller noise and engine noise. The noise generated by the air displaced by the wings is typically negligible compared to the noise produced by the engine and propeller.

It's essential to understand these sources of noise to appreciate the complex dynamics at play when an airplane passes overhead. Whether it's the powerful roar of the engines or the subtle hum of mechanical systems, each component contributes to the unique audio experience of an airplane flying overhead.