Police Car - Equipment - Audible and Visual Warnings - Audible Warnings

Audible Warnings

In addition to visual warnings, most police cars are also fitted with audible warnings, sometimes known as sirens, which can alert people and vehicles to the presence of an emergency vehicle before they can be seen. The first audible warnings were mechanical bells, mounted to either the front or roof of the car. A later development was the rotating air siren, which made noise when air moved past it. Most modern vehicles are now fitted with electronic sirens, which can produce a range of different noises. Police driving training often includes the use of different noises depending on traffic conditions and manoeuvre being performed. In North America for instance, on a clear road, approaching a junction, the "wail" setting may be used, which gives a long up and down variation, with an unbroken tone, whereas, in heavy slow traffic, a "yelp" setting may be preferred, which is a sped up version of the "wail". Some vehicles may also be fitted with airhorn audible warnings. Also in some European countries, where a hi-lo two tone siren is the only permitted siren for emergency vehicles, a "stadt" siren will be used in cities where it has loud echo that can be heard from blocks away to warn the traffic an emergency vehicle is coming, or a "land" siren will be used on highways to project its noise to the front to produce more penetration into the vehicles ahead to alert the drivers.

A development is the use of the RDS system of car radios, whereby the vehicle can be fitted with a short range FM transmitter, set to RDS code 31, which interrupts the radio of all cars within range, in the manner of a traffic broadcast, but in such a way that the user of the receiving radio is unable to opt out of the message (as with traffic broadcasts). This feature is built into all RDS radios for use in national emergency broadcast systems, but short range units on emergency vehicles can prove an effective means of alerting traffic to their presence, although is not able to alert pedestrians, non-RDS radio users, or drivers with their radios turned off.

A new technology has been developed and is slowly becoming more popular with police. Called the Rumbler, it is a siren that emits a low frequency sound which can be felt. Motorists that may have loud music playing in their car, for example, may not hear the audible siren of a police car behind them, but will feel the vibrations of the Rumbler. The feeling is that of standing next to a large speaker with pumped bass.