Q2B

History
Produced by the Federal Signal Corporation, 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. 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 amp 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.

Present day
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 at an operating current of 100 amps. 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 one 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.

Controversy
Some fire departments and cities have banned the siren due to its loudness. The Fire Department of the city of New York (FDNY) has banned the siren because it is too loud. For years FDNY has used the Federal Signal PA300 electronic siren. Federal Signal is now currently producing the E-Q2B, the electronic version of the electromechanical siren. Its tone sounds similar to the Q2B but is not as effective due to its lack of square sound waves produced by a mechanical siren. Recently the FDNY purchased a 2006 Pierce Arrow XT assigned to Rescue 1 which is equipped with the EQ2B. 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".

A Q2B example wail can be found here