When asking the question "How Loud is a Jet Engine?" you've got to be a bit specific with what you mean exactly. Are you referring to just the engine isolated from the rest of the jet? Or the entirety of the aircraft as a whole. Believe it or not there is more to aircraft noise than simply the engine, though that certainly is the noise source that sticks out in my mind when i think of a jet.
The aerodynamic noise (sometimes called airframe noise) is the noise produced by airflow over the fuselage, fuselage cavities, control surfaces, and landing gear. In modern high-speed jet aircraft this aerodynamic noise is the principal source of noise above 600 Hz under cruise conditions. However, under landing and takeoff conditions, when the velocity of the aircraft is much lower than for cruise conditions, the overall sound pressure level of the aerodynamic noise is about 10 dB below the propulsion system noise. The importance of aerodynamic noise at cruise conditions is that it is the dominant contributor to aircraft interior noise. For operations near an airport, aerodynamic noise represents a lower bound for noise reduction and becomes increasingly important as power plant noise is reduced.
Turbojet Engine Noise
Jet engines affect propulsion through the acceleration of air masses. In the turbojet engine, air is compressed in an axial or centrifugal mechanical compressor, heated in a combustion chamber, and then accelerated by expansion through a jet nozzle. A gas turbine in the path of the expanding gas serves solely to drive the compressor. These processes produce three types of noise:
1: Inlet noise radiated from the air intake, primarily as a result of the compressor noise plus aerodynamic noise.
2: Noise radiated from vibrations of the shell of the engine.
3: Exhaust Noise
The exhaust noise includes contributions from noise sources inside the engine, such as combustion and turbine noise, but exhaust noise is generated primarily outside the jet nozzle in the mixing region of the high-velocity jet and the surrounding air. This latter so-called aerodynamic jet noise is the turbojet engine's principal noise source under normal, full-power operation, and far outweighs the other noises on a sound power basis.
6. Hot section
7. Turbines Low and High pressure
8. Combustion chambers
9. Cold section
10. Air inlet
An aircraft flying at supersonic speed creates, in addition to the noise radiated by its propulsion system (which is also present at subsonic speed), a sonic boom which may be experienced by ground observers. This phenomenon is not connected with the stationary emission of acoustic energy by the aircraft. It consists of two, three, or even more pressure pulses separated by a time delay of 0.1 to 0.2 seconds. It is generated by the shock-wave pattern formed around any body moving with supersonic speed. Such a pattern for a sphere in a wind tunnel at mach number 3.0 is shown below. Measurements of the pressure across this pattern indicate that in the front (bow) wave, the pressure increases suddenly from the normal atmospheric pressure in front of this wave to a certain value; then it decreases continuously to a negative value of pressure and jumps back to normal pressure at the back wave.
The U.S. Navy has taken the study of its jet propulsion engine noise quite seriously and understands the environmental impact such noise can have on a community.
This image shows a comparison bar graph of different types of jet engines.
This image shows the evolution over time of noise mitigation for jet engines.