is the 10k feet ding automatic or manual?
 

That ding you hear throughout the cabin (at least on United) that tells the FA's it's time to make the announcement regarding portable electronic devices... is that manual (i.e. pilot turns on the ding) or is it automatic?

I guess I never notice this. I do always hear a double ding which is suppose to be at 18,000.

The ding tells the FAs a couple of things

1. The cockpit is going "sterile." That means don't bother the pilots about anything other than a critical emergency. They don't want to be distracted as they go about final landing duties.

2. It says that it's time to make the "electronics" announcement.

FWIW, most pilots refer to it as the "double ding." As in, Captain to First Officer, "Double Ding, me."

edited to add: it's manual 100% of the time and it's activated from the cockpit.

What about on the ascent?

I'm no pilot, but I'd put money on it being manual. I eagerly await it to pop my music back on and there have been numerous times where we are well into the flight and the FA's get antsy, give the pilot a ring and, ta da, "Ding".