How to distinguish the alarm sounds of police cars, fire engines and ambulances?

Distinguish according to the length of the sound.

The siren of the fire engine lasts for three seconds, and the interval is one second, and it goes back and forth: low-high-low-high-low is a long soothing sound of "fire ~ ~ ~ ~", and the warning light is red.

Ambulances pitch for one second, pitch for one second, interval for one second, and so on. High-low-high-low-high-low-high-low, which is the painful sound of "hey ~ yo ~ hey ~ yo ~", the warning light is blue and the flashing frequency is very high.

The siren of the police car is very urgent and there is no interval: low-high-low-high-low is a very short sound of "over", and the warning light is red and blue.