One of the symbols of the Leningrad blockade is metronome. During the first months of the blockade, more than 1,500 loudspeakers were installed on the streets of Leningrad, and about 450 radio points were installed in institutions and houses. People in Leningrad were cut off from the country, and working radio showed them that the city is still alive and continues to fight. During breaks in broadcasting, the radio was not silent – the sound of a working metronome was broadcast on the air. The metronome signal confirmed that the radio network was still working, and also warned people about artillery attacks and air raids. The rapid pace of metronome meant an air alert or bombardment, a slow one meant a stand-off or no alarm. Residents of Leningrad who survived the siege said that they perceived the sound of the metronome as the heartbeat of the city.