Why Do Some Airplanes Sound So Much Louder Than Others When They Pass Overhead?

A frequent question heard when airports are nearby is, why do some airplanes make so much more noise than others as they fly over? The answer lies in several factors including the type of engine, altitude, humidity, and the aircraft's purpose. This article will delve into the details of these factors and explain why some planes are indeed louder than others.

Engine Type: Turbofans vs Turbojets

The type of engine an aircraft uses has a significant impact on its noise level. Modern airliners typically use turbofans, which are quieter compared to older turbojet engines. Turbofan engines work by using a large bypass fan to direct a significant amount of air around the engine core, which generates thrust more efficiently and quietly than a pure jet thrust from a turbojet engine. This makes turbofans much quieter than traditional turbojets, where all the engine thrust comes from the turbine exhaust, creating a lot of noise.

In contrast, turbojets are used primarily in military aircraft and some smaller commercial planes. These engines have no bypass fan and rely on compressing and expelling a smaller volume of air at high speeds, resulting in a louder noise. For instance, a fighter jet like the F-16 with a high-bypass turbofan engine will generate significantly less noise than a small private jet like the Cessna 172 with a turbojet engine.

Altitude and Air Humidity

When an airplane is flying over your area, the altitude at which it is traveling also affects the loudness of the sound you hear. Sounds are often more intense at lower altitudes because the air is denser, allowing the sound waves to travel more efficiently. Additionally, if the air is more humid, the sound will be more compressed, making it louder as well.

Imagine you are standing on the ground and suddenly hear a loud boom overhead. The sound is more noticeably louder at this point because the airplane is flying just above your head, and the dense air is allowing the sound waves to reach you with minimal attenuation. As the plane continues to fly at a higher altitude, the sound intensity decreases due to the decrease in air density and the sound waves spreading out over a larger area.

Military Aircraft and Noise Standards

Military aircraft often operate at higher noise levels because they are not required to follow the same noise standards as civilian aircraft. In contrast, civilian aircraft must adhere to strict noise regulations set by authorities such as the Federal Aviation Administration (FAA) and the International Civil Aviation Organization (ICAO).

Military planes are designed for missions that require maximum performance, and noise is often a secondary consideration. This means that military jets can produce much more noise, especially during takeoff and landing, where the engines are often at full thrust. For example, the F/A-18 Hornet, a widely used military jet, can generate much more noise than a typical commercial airliner when taking off and landing.

Cargo Aircraft and Fuel Loads

Cargo aircraft also tend to be noisier than commercial airliners, particularly during takeoff and landing when they are fully loaded. This is due to the amount of fuel and cargo they carry, which can disrupt the airflow around the aircraft, leading to more noise.

When a cargo aircraft takes off with a full load, the denser air around the plane can cause increased turbulence and noise. Additionally, the aircraft's larger surface area and the weight of the load can cause more disruptions to the airflow, resulting in a louder sound as the plane takes off and lands.

Shock Waves and Fan Blade Tips

Shock waves and the oscillations caused by engine fans and gases in jet exhausts also contribute to the noise generated by aircraft. When an aircraft moves through the air, it disturbs the air molecules, creating pressure waves that can combine and amplify. High-frequency air oscillations caused by the fan blade tips can also reflect and combine, similar to waves on the ocean combining and creating larger waves.

Inlet designs, including inlet liners, can mitigate some of this noise by reducing air disturbance. However, some inlet designs may be more effective in reducing noise than others, depending on the aircraft model. Therefore, even two aircraft of the same model can produce different levels of noise if they have been designed or modified differently.

Conclusion

In summary, the sound of airplanes can vary significantly depending on the type of engine, altitude, humidity, and other factors. While modern turbofans are generally quieter than older turbojet engines, military aircraft, cargo planes, and highly fueled planes can produce much more noise. Understanding these factors can help you appreciate why some airplanes sound so much louder than others as they fly over your area.