How objects with very low frequency generate sound? From what I understand if an object vibrates with a frequency between 20-20k Hz audible sound is generated (for humans), I have also observed this with speakers. However, many times I get very confused in understanding why a certain object is generating audible sound. For example, if we move a stick very fast, it makes a "vrooom" sound, in this case, the motion of the object (stick) is not periodic, but a sound is still generated. I have observed that an audible sound is generated only when we move the stick very fast. Maybe an audible sound is generated if the stick is at least as fast so that the air replaced by it returns back to its original position in 1/20th of a second or less. What I am trying to say is that when we move a stick in the air it could generate a vacuum for an instance of time, which is then filled by the air surrounding the vacuum, so if the motion is so fast that the vacuum is filled in 1/20 of a second or less, we could hear a sound. Am I correct? Is this how sound is generated?
If this is the case, please help me in understanding why the needle representing seconds in the wall clock makes a sound, it must have a frequency of 1 Hz which should not be audible. And the sound is not because of the machinery in the clock, as if I remove the second's needle, no sound is produced. I do understand that this is a very basic question to ask at a physics forum, but a google search didn't provide me an answer.
 A: 
if we move a stick very fast, it makes a "vrooom" sound, in this case, the motion of the object (stick) is not periodic, but a sound is still generated.

This sound is made by air rushing past the surface of the stick. The rough surface of the stick, combined with the speed you're moving it at combine to make the air vibrate at some frequency (or a bunch of frequencies) in the audible band.

why the needle representing seconds in the wall clock makes a sound, it must have a frequency of 1 Hz which should not be audible. And the sound is not because of the machinery in the clock, as if I remove the second's needle, no sound is produced.

It's hard to be sure without examining the particular clock you're asking about, but this might still be because of the mechanism within the clock, which will include parts that move at more than one cycle per second. But the needle is needed to transfer the vibrations from the mechanism to the air outside the clock, so when you remove the needle the sound no longer gets emitted into the air.
A: In order to produce a 1 Hz signal that is detectable to your ears, the amplitude of movement of the vibrating object has to be enormous, on the order of tens of feet if not hundreds. Thus, the second hand of a wall clock is indeed producing 1 Hz sound waves, but they are too small to hear by many orders of magnitude.
