Have you ever wondered why jet engines are so noisy?
According to ScienceABC, people who live near airports are prone to stress, memory loss and even heart disease. These diseases are caused by the noisy sound pollution which constantly hits their ears. We are all familiar with the roar of a jet plane that makes windows and furniture shudder.
The roar of the jet made the windows and furniture tremble.
In fact, at 140 decibels (a unit of measurement for sound intensity), jet noise only needs to add 40 decibels to damage our hearing tissues, and it’s 54 decibels less than noise. The biggest we’ve ever known. So what causes the terrible noise of jet engines?
A jet engine can be divided into several separate parts. The first part is the fan that is responsible for drawing air. A large amount of air is drawn inside the rotary compressor through the air intake. The air compressor will compress the air up to 12 times the original pressure.
Structure of a jet engine.
Combustible air is passed into the combustion chamber, where the fuel is injected. An electric spark ignites the mixture. This combustion causes the gas temperature to rise greatly. High-temperature combustion products exit the combustion chamber and run through the turbine to rotate the compressor. Although this process reduces the temperature and pressure of the gases exiting the turbine, their parameters are still higher than those of the outside conditions. The airflow inside the turbine escapes through the exhaust pipe, creating a counter-jet thrust. If the jet speed exceeds the airspeed, the aircraft will gain forward thrust.
A sudden change in velocity is observed at two times: when air enters the engine through the fan and when hot air exits the exhaust duct. The flow of air at such a high speed creates turbulence in the air and it transmits a large amount of pressure waves to the surroundings. So is sound simply pressure waves traveling through some medium, such as air?
Hot air escaping at the other end of the engine also creates pressure waves everywhere.
When air is drawn into a jet engine, it is compressed with high pressure. Before completing one cycle of engine rotation, the pressure of the air is restored. But when the propeller completes one revolution, the air is compressed again. The cycle of compression and recovery continues as the fan continues to spin. The fan of a jet engine rotates at 20,000 rpm, so the frequency of these changes (air pressure) is very high.
Likewise, the hot air escaping at the other end of the engine also creates pressure waves everywhere. If this disturbance is in the audible spectrum, it will be perceived by the human ear. We can consider this to be a very large speaker.
To reduce the intensity of this noise, engineers often attach large mufflers to the end of the engine. This unit will mix hot and cold air flowing around the combustion chamber to reduce the temperature of the combustion gas and thus its velocity. Of course this will affect the performance of the jet but this is a compromise that engineers have to accept.
Longer exhaust pipes will help reduce noise.
Another way to reduce noise is to use longer exhaust pipes, so air turbulence is more controlled. Again, this also reduces the jet’s speed but also reduces the sound intensity by about 15 decibels. The number of changes seems to be very small, but it contributes to making the aircraft operate “quiet” a lot.