How Quick is iPhone's Movement/Immobility Detection?

Hello,

I would like to know how quickly can the iPhone detect Movement and Immobility

More specificaly..

1.

Imagine I'm standing still, I put the iPhone in my pocket and then I start walking. How quickly can the iPhone pass from immobile state into detecting actual movement?

More importantly, let's say that I'm walking and suddendly I stop. How fast can the iPhone detect the new immobile state?

Does passing from movement to immobility take longer to be detected, because the accelerometer sensors need some time to "calm down" first in a way in order to then detect the new immobile state?

Or is this not the case and the response from the accelerometer sensors is immediate when it comes to passing from movement to immobility?

To put everything into perspective imagine I want the iphone to play a continuous sound when I start walking & while I'm walking and this sound should stop when I stop walking.

2.

Now I understand ther might be some accidental movement inside my pocket even though I'm standing still for whatever reasons, and the accelerometer might detect that as actual movement.

However, is it possible to come up with some code where only data of value greater than X coming from the accelerometer, would be accepted as movement and therefore only data of greater than X value could trigger the event - sound starts/sound stops?