Understanding Jet Exhaust Noise

The sound of jet exhaust is a combination of several distinct noises, each with its own unique characteristics. In this article, we will explore the causes and sources of this noise, ranging from modern commercial airliners to supersonic fighter jets. By understanding the mechanics behind these sounds, we can appreciate the advanced engineering that enables modern aircraft to operate more quietly yet still achieve exceptional performance.

Modern Airliners: Buzzsaw and High Velocity Exhaust

On takeoff, modern airliners generate noise from two primary sources. The first is the characteristic ‘buzzsaw’ sound, caused by the large multi-bladed fan of the engine. Its tips exceed the speed of sound, producing this distinctive whine. The second noise source is the high-velocity exhaust gas from the engine's core, the small jet engine that drives the large fan. Together, these create a unique auditory signature that is significantly quieter compared to early jet airliners.

Fighter Jets: Supersonic Ordnance and Afterburners

Fighter jets are equipped with low-bypass jet engines capable of flying at supersonic speeds—approximately 1225 km/h (761 mph/661 kn). These engines are often supplemented with afterburners, which provide a significant boost in speed and thrust. The combination of supersonic engines and afterburners results in an ear-splitting sound that can be heard from miles away. The noise is a mix of the whine from the compressor and bypass fan blades, the rushing-roar from the supersonic exhaust, and the shock waves generated by the extreme speed.

The Sounds of Combustion and Airflow

The noises you hear from a jet engine are complex and multifaceted. They originate from the burning of jet fuel in the burner cans, the exhaust passing through the turbine discs, and the airflow being sucked into the intakes under extremely low pressure. This low-pressure airflow encounters stators and Inlet Guide Vanes (IGVs), generating additional noise. Surprisingly, some high-bypass turbofan-powered jets can be quieter at full throttle on takeoff than they are at idle power. This is due to the efficient design of the engine, which directs the airflow in a way that minimizes noise as much as possible.

The noise from jet exhaust is not just a simple roar; it is a symphony of sound produced by the interactions of aircraft components and the air around them. By understanding these sources of noise, we can appreciate the remarkable engineering that allows modern aircraft to achieve their missions while operating with a level of quietness that would have been unimaginable in the past.

Conclusion

From the buzzsaw sound of a modern airliner's fan to the ear-splitting noise of a supersonic fighter jet, jet exhaust noise is a fascinating byproduct of advanced aerodynamics and engineering. By deconstructing the individual sounds and their sources, we can better understand the complexity of these noisy yet essential machines.