What is the chime that can be heard roughly 30 seconds after takeoff? This may be confirmation bias on my part, this might be impatient passengers pressing the call button, and it might be me imagining things, but it's one of those I've wondered about. I'll try to describe as best I can.



Of all the times I've flown recently on an airliner, of which most of the time it's a short haul carrier often flying smaller Boeing/Airbus aircraft*, I have been aware that there is an audible chime which can be heard throughout the cabin (ie, not only when sat in the vicinity of crew seats) very soon (30-45 seconds) after take-off. 

It is heard well before the passenger seat-belt signs are switched off.

Can anyone describe what I am hearing and what its purpose is? Who is this a signal for, and who is sending the signal?



* the two times I've flown on an Embraer I don't recall hearing it
It's signalled from the cockpit (the cabin chime) to let the cabin crew know it's OK to move around.  If you watch, this happens before the seat belt sign goes off which at the earliest, is at 10,000 feet.