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The Q2B  siren is an electromechanical siren that has one of the most noticeable sounds in the world. The Q2B or "Q-siren" is famous for its sound, which is trademarked, and the look of the siren. It is produced by the Federal Signal Corporation.
The siren was used on police vehicles, ambulances, fire apparatus and other emergency vehicles before there were electronic sirens which have widely replaced the Q-siren, except on fire apparatus and similar emergency vehicles large enough to handle the Q's current draw. In the early 1950s the only siren sound were the ones made by the electromechanical sirens. The siren had its beginnings in World War II when similar designs and similar sounding sirens were used as air raid sirens. After the war was over, Federal Signal Corporation began to sell the siren to different police departments and fire rescue departments. The siren quickly became popular as one of the most effective traffic clearing devices.
Today Federal Signal's Q2B siren is still in wide use. The majority of users of the Q Siren are fire departments, although some ambulances and heavy rescue squads have employed the Q-siren. The Q-siren produces 123 decibels at 10 feet (3.0 m) at an operating current of 100 amps at 12 V DC (1.2 kW). The Q-siren's manual and the Federal Signal website state that "this product produces LOUD sounds for requesting the right of way". Hearing protection is recommended but not required when operating the siren or for anyone in the truck. The Q2B is mounted outside the truck, usually in the front bumper, on top of an extended front bumper or in the grill. Roof-mounting is no longer common because of noise in the cab.
Some fire departments and cities have banned the siren due to its loudness, which led to them being substituted for electronic replica versions, such as the E-Q2B. Other mechanical sirens like the Super Chief were equipped on some trucks to replace the Q siren. However, electronic versions of the mechanical siren, despite sounding similar to the Q2B, are not as effective as the real Q2B due to their lack of square sound waves produced by a mechanical siren. The Fire Department of the city of New York has notably banned the siren because of its loudness. For many years FDNY has used the Federal Signal PA300 electronic siren, which then led to them being criticized for sounding more like a police car or an EMS unit than a fire truck. The FDNY then purchased an E-Q2B equipped 2007 Pierce Arrow XT, which was assigned to Rescue 1. Rescue 1 Members have been quoted as saying "the best thing about the new rig is that old fashioned siren that all other trucks have, it really gets your blood pumping hearing that. It's great that we sound like a fire engine again, not a cop car or an EMS unit". The E-Q2B became standard for FDNY units being delivered. Before then, many companies throughout FDNY have placed mechanical sirens on their rigs over the years; however, they were then confiscated during maintenance.
- http://www.fedsig.com/products/111/q_siren Federal Signal Q2B Page
All information credited to the FDNY and Federal Signal Corporation unless otherwise noted.