How deep would a hole need to be before you could no longer hear the sound of a small object hitting the bottom? Assuming there are no obstructions, background noises, and the bottom of the pit is just flat concrete, if the average person were to drop something like a padlock (very specific, I know) into the hole, around what depth would they no longer be able to hear the sound of it hitting the bottom?
This is for a story I'm working on.
 A: Summary: Very deep - on the order of 1-10 kilometers.
Analysis:
Lets make some assumptions.  First, we are dealing with air at standard temperature, pressure, etc.  Second, lets assume that the walls of the pit are smooth, parallel, and acoustically hard so we don't have to worry about funky reflections.  Third, assume that the strike leads to an impulse with roughly equal energy from 1 kHz to 10 kHz with an amplitude of 80 dB.  I am really just guessing about the source, but this estimate should put you in the ballpark.  The high frequencies are attenuated faster than the lower frequencies, so we will focus on the 1 kHz component, which is also where the ear reaches its peak sensitivity.  At this frequency atmosphere attenuation leads to a loss of about 6 dB per kilometer.  Finally, without background noise and the listener actively searching for a signal we will assume the received signal must be below 10 dB to be inaudible.  Thus, for these assumptions the pit would need to be $(80-10)/6\approx12$ km deep.  Over this distance, the assumption of the uniform atmosphere breaks down a bit, but not significantly.  If we assume the peak frequency was actually a little higher, say 4 kHz, the absorption  is roughly 20 dB per kilometer, and the depth is about 3.5 km.
I don't know what order of magnitude you were hoping for, but these depths seem extremely large to me.  There are a variety of reasons for the large values.  For example, the still air with no background noise permits one to hear very quiet sounds.  Also, all of the sound is being directed in one direction (up the pit) rather than in all directions as in the case of open air propagation.  Another is the fact that we are neglecting any sort of phenomena that would distort the signal into something unrecognizable (like atmospheric turbulence).  If you want small depths, just allow for some moderate background noise, such as wind or motor noise, up to a level of roughly 50 dB (still pretty quiet for our modern world), and then the 4 kHz depth becomes 1.5 km.  Then you could assume the sound from the strike was actually lower for some reason (e.g., it slowed down or it was actually not very dense), and the source was actually only 70 dB, and now the pit is only 1 km.
