Why is jet engine so noisy?

According to ScienceABC, people who live near the airport are more prone to stress, memory loss and even heart disease.

Have you ever wondered why jet engines are so noisy?

According to ScienceABC, people who live near the airport are more prone to stress, memory loss and even heart disease. These diseases are caused by noisy sound pollution that constantly affects their ears. We are all familiar with the roar of jet jets making windows and furniture shudder.

Picture 1 of Why is jet engine so noisy? Photo 1 of Why is jet engine so noisy?
The roar of jet jets made the windows and furnishings tremble.

In fact, at 140 decibels, the noise of jets with an additional 40 decibels can damage our hearing tissues and are 54 decibels less than noise. The biggest we ever knew. So what causes the terrible noise of jet engines?

Engine

A jet engine can be divided into several separate parts. The first part is a fan that pulls air. A large amount of air is drawn into the compressor through the air intakes. Air compressors compress air up to 12 times the initial pressure.

Picture 2 of Why is jet engine so noisy? Photo 2 of Why is jet engine so noisy?
Structure of jet engine.

Flammable air is transferred into the combustion chamber, where the fuel is pumped. An electric spark ignited the mixture. This burning process causes the gas temperature to increase greatly. High temperature combustion products exit the combustion chamber and run through the turbine to spin the compressor. Although this process reduces the temperature and gas pressure escaping from the turbine, their parameters are still higher than the external conditions. The airflow inside the turbine escapes through the exhaust pipe, creating a counter-propulsion force. If the jet speed exceeds the cruising speed, the aircraft will have thrust forward.

The difference in speed

A sudden change in velocity is observed at two points: when air enters the engine through a fan and when hot air escapes the exhaust duct. Such high-speed air currents produce turbulence in the air and it transmits a large amount of pressure waves around. So is the sound simply the pressure waves passing through an environment, such as air?

Picture 3 of Why is jet engine so noisy? Photo 3 of Why is jet engine so noisy?
Hot air escaping at the other end of the engine also creates pressure waves everywhere.

When the air is sucked into the jet engine, it is compressed with high pressure. Before completing a rotation cycle of the engine, the pressure of the air is restored. But when the rotor completes a spin, the air is compressed again. The compression cycle and recovery continue as the fan continues to spin. The jet engine fan rotates at a speed of 20,000 rpm, so the frequency of these changes (air pressure) is very high.

Similarly, hot air escaping at the other end of the engine also creates pressure waves everywhere. If this disturbance frequency is within the heard frequency spectrum, the human ear will perceive it. We can consider this a loud speaker.

Noise reduction and mystery of air turbulence

To minimize the intensity of this noise, engineers often attach large mufflers to the end of the engine. This unit will mix hot and cold air running around the combustion chamber to reduce the temperature of the combustion gas and thus reduce its velocity. Of course this will affect the performance of the jet engine but this is the compromise that the engineers must accept.

Picture 4 of Why is jet engine so noisy? Photo 4 of Why is jet engine so noisy?
A longer exhaust pipe will help reduce noise.

Another way to reduce noise is to use longer exhaust pipes, so the turbulence of the air will be more controlled. Again, this approach also reduced jet speed but the sound intensity also decreased by about 15 decibels. The number of changes is very small, but it contributes to making the aircraft "quieter" a lot more.