Children seem to be able to sleep in noisy environments, even when carried around. Is this generally true and what are the reasons? How does the ability to sleep with sound around differ with age and other circumstances?
Ability to sleep through noise seems to be conditioned in early infancy. An infant who is always put to sleep in the quietest possible conditions will usually develop a tendency to wake at even the slightest noise, whereas infants who are exposed to normal noise levels during sleep times will usually develop a tendency to sleep through more noise. Of course there is a point for everyone where exhaustion simply takes over and they will sleep through noise or other discomfort that would normally wake them.
Along with the existing answers, it's also a matter of basic social instinct:
When you're walking alone at night through a dark and silent forest, then every single whisper of a sound triggers a danger reaction in you.
When you hear a crowd of happy people around you, you feel safe. When you are surrounded by familiar sounds and the voices of people you know, there's no stone-age instinctive fear of irrational dangers lurking.
This way, positive background sounds work to calm us, to soothe us, because it tells our stone-age brain that we are safe.
If you have ever commuted to work using public transportation you will find yourself doing exactly the same thing as the kids.
Basically when you are tired in the noisy environment you make the noises blur to the point where it becomes white noise and that point it's doesn't bother them when they sleep.
I'd like to add our experience with our little one (currently just over 6 months old):
The sleep is lighter at the beginning, i.e. during the first 5-10 minutes - a dropping pen or a phone conversation for example can cause waking. After that, the sleep gets deeper and waking up becomes less likely.
As a general information, our little one sleeps with pink noise during the night, but varying levels and types of noise during the day (depending on whether we're in the car, at home etc).