Sirens

[natalie_1982]

Why do the sirens of emergency vehicles have different sounds? Also I've noticed that police cars have several different noises - why is this?

[undercovers]

well each type of emergency vehicle has a different sound so you can / could tell them apart. But they have started to vary the sounds they each make, apparently this is because one continous siren is very hard to judge where it is coming from, whereas if the pitch and type of noise keeps altering people have a much better chance of guessing where it is coming from

[Peter Pedant]

a friend of mine said as a six year old he used to refer to them as nar-nars. And they came in three colours, red white and blue.

In those days red narnars had bells

Fifity years later, I note that the association of colours with sounds is called synaesthesia. And in the mean time people have done research on localisation and sound and found the results as above. The most easy to localise is white noise of varying intensity - a bit like a rush of bubbles coming through water. Two tone horns are now unlawful unless used by the police

[natalie_1982]

Thanks for your answers, I'm glad no-one really knows and a lot of people haven't even noticed they sound different. I'm sure there must be an actual reason though, something to do with the type of emergency? How fast they are driving as to how well you'll hear them how far away? What type of road they are on - is there a lot of pedestrians?

[booksworth]

Nat, Peter is right. You can hear the sirens from a long way off, so they catch your attention. The blasts of white noise help you locate where the vehicle is coming from.

[dan_999uk]

There are three main siren tones in use in the UK:

• Wail - slow, continuous rising and falling tone
• Yelp - fast, continuous rising and falling tone
• Hi-Lo - or "two-tones" - alternating high and low pitched tones

There are other tones such as Phaser and Airhorn but these are not as familiar to the public and are seldom used.

The reason for having different tones is because they are more effective in certain conditions - for instance, wail can be heard from a longer distance on high speed roads, yelp is good for getting attention at junctions, traffic lights etc, and hi-lo is good in dense crowds or very slow moving traffic.

Most sirens have an on/off switch and the tone is changed either by a dial on the siren control box or by tapping the vehicle horn. Most sirens have all three of the above tones, some have other things like a public address system.

When I'm going to an emergency, the siren will usually be on wail, and I'll switch it to yelp approaching hazards such as roundabouts, traffic lights etc to get attention and hopefully ease my passage through traffic.

If you have an emergency vehicle behind you and you hear a long horn blast on their normal car horn, it means SHIFT NOW!