To sound the air-raid siren

鸣响防空警报

An order was issued to black out at the sound of the air - raid siren.

已下令听到警报声就实行灯火管制.

At first the all -day-and-night tone sounded something like an air - raid siren.

一开始，整天不间断的单调声音就像是空袭警报。