What do the different sirens heard from Emergency Vehicles mean? Such as Wail, Yelp, Priority, etc?
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Traditionally the sirens on vehicles were mechanical items. They used systems like alternating between multiple air horns (like the pipes that form an organ) at different pitches, or used systems of spinning and stationary plates or cylinders that would interact and cause air to flow between and around them and create the different and changing pitches.
Different vehicles might use different versions depending on what was practical and possible to mount to different vehicle types, and what different countries and services felt was the most appropriate choice.
This is why sirens were traditionally very distinctive – certain siren types or models would be fitted to police, others to fire engines so you could easily tell them apart, and the same for different countries services.
Modern sirens tend to be electronic instead – essentially just a loud speaker that can be used to play back pre-prepared sounds.
This ability to change the sound means we can change the sounds to suit different purposes. Certain sounds will be better at carrying over long distances, while others will be more immediate and noticeable, but if more limited range. So one siren may be used on a long straight road to give vehicles warning far ahead, and a different one approaching a junction better for grabbing the attention of those nearby who may not have heard the earlier siren from a distance.
The ability to change the sound also helps to gather attention too – when you hear a regular sound for a period of time you start to tune it out as background noise, so stopping that sounds and changing it to a new one helps bring attention back to it.
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