Turbojets and turbofans are both types of jet engines, but they differ in their design and how they produce thrust. The difference in noise levels between turbojets and turbofans can be attributed to their respective configurations and operating principles.
Turbojets are known for their characteristic high-pitched noise. This noise primarily arises from the engine's exhaust jet, which is produced by the rapid expulsion of hot gases from the engine core. In a turbojet engine, all of the incoming air passes through the combustion chamber, where fuel is burned, and then it is accelerated through the nozzle, resulting in a high-velocity exhaust jet. The high-speed exhaust flow creates intense turbulence and pressure fluctuations, generating the loud noise associated with turbojets. Additionally, the absence of a bypass air stream in turbojets means that the noise produced by the engine core is relatively more pronounced.
On the other hand, turbofan engines consist of a core engine surrounded by a fan, which is responsible for producing a significant portion of the engine's thrust. The core engine operates similarly to a turbojet, but a substantial portion of the incoming air bypasses the combustion chamber and flows around the engine core. This bypass air, also known as the bypass stream, helps generate additional thrust and contributes to the engine's efficiency. The presence of the bypass stream helps reduce the noise produced by turbofans.
The noise reduction in turbofan engines can be attributed to two main factors. First, the bypass air acts as a noise shield, surrounding and muffling the noise produced by the core engine. The additional airflow helps reduce the turbulence and pressure fluctuations associated with the exhaust flow, resulting in a quieter engine. Second, the slower velocity of the bypass stream compared to the exhaust jet in turbojets leads to lower-frequency noise generation. Lower-frequency noise is generally perceived as less loud and more tolerable to the human ear compared to the high-frequency noise produced by turbojets.
In summary, turbojets tend to be louder than turbofans due to the absence of a bypass stream and the high-velocity exhaust jet generated by the engine core. Turbofans, with their bypass airflows, offer noise reduction benefits by shielding the core engine noise and producing lower-frequency noise, making them comparatively quieter.