I have two questions about the physics of sound. As a background, I know the process of sound production can be understood as 3 stages that happen continuously:
An object oscillates back and forth and displaces the molecules of the medium. The result is a change of density (When the object oscillates to the front, it also pushes the nearest molecules to the front, increasing the density in a certain region. When it oscillates to the back leaving a "vacuum", there is a decrease of density)
The change of density is related to a change of pressure (regions with higher density of molecules will have higher pressure)
The change of pressure leads to the displacement of molecules (movements from higher pressure to lower pressure regions)
My first doubt is: Is there some kind of threshold imposed to the beginning of the object vibration that results in the production of a wave travelling at the speed of sound? Since every movement leads to a displacement of the air, there must be a condition that separates the phenomena of sound and simply the dragging of the air.
My second doubt is: When an object starts its vibration, it moves forward, thus "pushing" the molecules forward, and then backwards when it leaves a region with a lack of matter. The molecules that were ahead come back to fill the "vacuum" due to pressure difference. In their way ahead, they push yet another column of molecules, and so a wave of molecules being pushed and going back to fill a gap is formed. Why is the case that the molecules that were ahead go back to fill the void and not the molecules that were behind? If that were the case, there would also be a dragging of the air.
Doubt 1: Feynman gives some information in its lectures:
" (...) if an object is moved at one place in the air, we observe that there is a disturbance which travels through the air. (...) Of course, if the object is moved gently, the air merely flows around it, but what we are concerned with is a rapid motion, so that there is not sufficient time for such a flow."
The threshold is about a "rapid motion". Since the velocity of the object will be completely determined by the amplitude and frequency of the movement, what is exactly this speed he talks about? Is he really talking about this amplitude/frequency combination (when the object is vibrating and there's not only one single push)? For example, if the amplitude is high, the frequency couldn't be too low or the average speed is too low.