I think you basically have the right idea. An aircraft in flight is continually emitting sound (and remember that sound is air compression) - the sound of the e

I think you basically have the right idea. An aircraft in flight is continually emitting sound (and remember that sound is air compression) - the sound of the engines, but also the sound generated just by moving through air; open your car window while on a highway for an demonstration of that, and yes as you say, the volume increases with speed relative to the air. At low speeds, engine noise will dominate, while presumably there will be a speed above which 'wind noise' is louder; this is certainly true on a motorbike for example. These sound waves interfere with each other and if they're travelling at the same speed as (or slower than) the emitter, a lot of constructive interference occurs - i.e. the sound gets louder. Note this effect still occurs beyond Mach 1.0 - there's a nice animation here[0] that shows why. As you say - a sonic boom-less aircraft has to somehow move the air out of the way quietly. [0] https://physics.stackexchange.com/a/176446 https://physics.stackexchange.com/a/176446