Emergency Vehicle Equipment - Audible Warning Devices (sirens)

Audible Warning Devices (sirens)

When an emergency vehicle is responding, it often uses audio warning devices in addition to the visual warnings provided by its warning lights. Audio warning devices are turned off once the vehicle is on-scene. Such devices include:

• Sirens - These can be fully electronic, electric, or manual, but are all designed to create changing sound patterns. These patterns vary by model of siren. Emergency drivers are often trained to use different siren tones in different conditions, to achieve maximum effectiveness through traffic. A long standing problem for emergency services has been traffic being unable to determine the direction a siren is approaching from, and different tones have been developed on some electronic sirens to help combat this, such as the use of white or pink noise in between more conventional siren noises, which helps people to pinpoint their origin.
• Public address system - Sometimes linked to the electronic or electric siren, or possibly stand alone, this system allows the voice of the operator to be amplified to give direction. This is found especially useful in heavy pedestrian traffic.
• Air horn - These devices simply force air through a shaped pipe to create noise. They can be used to create the classic two tone 'nee naw' sound which led to the colloquial expression for lights and sirens on emergency vehicles of blues and twos. They can be both mechanical and electronic.
• Bells - Usually found on older or classic emergency vehicles, these were the original methods of notifying people of the approach of emergency vehicles. They were first hand bells, and later replaced by electric versions.
• Exhaust whistles - These are no longer used, but still referred to in some legislation, and may be found on classic emergency vehicles, an exhaust whistle is fitted to a vehicle's exhaust pipe, and functions similar to a steam whistle, with the exhaust gasses replacing the steam.

Some emergency vehicles operators may turn off their sirens when on side streets or when there are no cars on the road so as not to disturb residents. The driver will then turn on the sirens before proceeding through intersections or when traveling on potentially dangerous stretches of road.