What makes jet engines so loud

Connect and share knowledge within a single location that is structured and easy to search. Everyone is familiar with the whirring sound of jet engines when seeing an aircraft taking off from a nearby airport.

It is distinctly very loud on the ground and one can hear it even when the airplane is miles away. Although one can hear a 'white noise' like sound when inside an airplane, the engines don't sound very loud in spite of being just meters away from them.

I understand that the cabin is well insulated from the outside, but I would expect to hear a similar whirring sound of the engines. So what is the phenomenon that makes jet engines sound louder on earth compared to inside the aircraft cabin? Air molecules wiggle back and forth and bump into other air molecules so they wiggle too so you have a whole chain of wiggling air molecules. The jet engine moves air molecules A LOT, hence it's extremely loud. As the sound moves away from the jet engine the energy disperses over a larger and larger area and so the sound pressure level drops.

The pressure drops by half every time you double the distance. That's 6 dB per doubling of distance or 20 dB per decade. If it's dB at 10 meters, it's still dB at m, 80 dB at 1km and 60 dB at 10km. That's why you can easily hear it on the ground. There is no easy way for sound to get into the cabin, because the cabin is air tight and fully sealed. The air molecules outside can wiggle like crazy but the air molecules inside don't care. It's still fairly loud in the cabin but that's due to mechanical sound transmission through the wings and the fuselage.

Quite many small airplanes don't have mufflers in order to save on costs, space, engine performance, weight and additional maintenance. But equally, quite many small airplanes have mufflers for additional reasons related to the exhaust temperatures and yet emit excessive amounts of noise.

However from a manufacturer's point of view one should ask if it is necessary to increase design and operational costs to this end. Once the plane takes off there's no real need for it anyway. And if we take it one step further and consider the regulations we see that since the noises are within the acceptable margins there is no real need to enforce suppression and thus manufacturers don't need to add this additional cost and design changes to their planes.

Furthermore airplanes have additional noise sources resulting from their interaction with the air as you can see in the ICAO's diagram below:. Sign up to join this community.

The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why are light aircraft engines so noisy? Ask Question. Asked 5 years, 1 month ago. Active 5 months ago. Viewed 7k times. What's the quietest light pilot trainer out there and don't say glider, thank you! Improve this question. Pondlife IanT8 IanT8 2 2 silver badges 5 5 bronze badges.

To quote wikipedia "Much of the noise of a propeller aircraft is of aerodynamic origin due to the flow of air around the blades. They do try to build propellors that are quiet, but that's not really very easily accomplished I mean, you can't just put a muffler on a propellor, ya know? Show 3 more comments.

Active Oldest Votes. Is this the apocalypse? No, it is only a fighter jet preforming maneuvers. Any Madison College student taking classes on the Truax campus has likely heard the booming symphony of the jets as they fly overhead.

With the airport in close proximity to the school, it is almost impossible not to hear them. But what are these aircraft and why are they so loud?

The noise from a jet comes from two sources — the engine itself and the reaction the exhaust has with the surrounding air. A jet engine works by taking in air through the intake and compressing it as much as 12 times its original pressure. From there, fuel is injected into the compressed air and sent to a combustion chamber where the air-gas is ignited.

Once ignited, the highly pressurized air-gas mixture is pushed through a turbine and out a nozzle at the rear of the engine, giving the aircraft its thrust.

The more energy needed to ignite the air-gas mixture and expel it from the engine, the louder the engine will be. Also, the combustion chamber is another source of noise within the engine. However, because it is 'buried' within the engine's core, it does not have a predominant contribution. Mentioned earlier, the exhaust jet is the major source of jet engine noise. This can be suppressed by inducing a rapid or shorter mixing region.

Where this reduces the low frequency level, it may increase the high frequency noises, which are quickly absorbed by the atmosphere. Thus the noise that does reach the listener is outside the audible range. This is achieved by increasing the contact area between the exhaust gas stream and the atmosphere by using a nozzle incorporating a corrugated or lobe-type noise suppressor.