How does engine noise change during descent / approach? When on the ground beneath a STAR transition I hear commercial jets making a dramatic change in sound. I assumed it was the sound of high-bypass engines being pulled back at stepdown fixes, but when a friend recently asked me what the sound was, I wanted to make sure I was providing the correct answer. I am familiar with the sound made by the Fuel Over Pressure Protector (FOPP) cavity on A380s but this is a different, much lower pitched and shorter duration sound. Any help?
It is important to notion that an aircraft does not provide a uniform sound but features several sources of noise. The noise generated by the individual sources has separate characteristics and is transmitted into different directions.
For example, the sound of the engine front fan is sent into the direction of flight, creating a cone. The sound of exhaust jet and low-pressure turbine is transmitted backwards. Passive sound sources sent according to their positioning on the aircraft. Underside structures transmit downwards.
When you experience an aircraft transition overhead, the sound created by the aircraft may be steady but you experience the sound of changing sources.