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.
I think you will often hear the engines spooling up and down on final because they are adjusting altitude, and they might be coming in too low or high so they have to adjust accordingly.
Also flaps and gear are extended in the final because the glidepath is quite shallow which cause more drag, so to keep the airspeed where it should be and not higher (for safety) the engines need to put out more thrust, thereby causing the extra noise heard on final. In the initial descent, you probably are gliding with the engines at idle causing more drag than thrust. It is the most efficient way to descend in a jet.