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 noise or pink noise in between more conventional siren noises, which helps people to pinpoint their origin. Some sirens have dual tones. These tones are able to warn passing motorists or pedestrians to stay to the side as they pass by. 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 force compressed air from the vehicle's air brake system against a diaphragm, creating a loud noise. Air horns used on emergency vehicles usually have a distinctive tone so they can easily be distinguished from other large vehicles, commanding urgency. In Europe, they are sometimes 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. Due to the need for compressed air in order to operate, they are primarily found in fire engines and large ambulances but not in police cars and other small vehicles since they do not contain air brakes. Most ambulances have electronic and electric horns that work in conjunction with the sirens to give a signal to clear the way. 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 conventional hand rung bells, and later replaced by electric driven "Gong"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 vehicle operators occasionally turn off their sirens when on side streets or when there are no cars on the road so as not to disturb residents; however, there is seldom a mandate