The Siren Board

http://tinyurl.com/2aecuyn
For all who may be interested, here is a patent for a modulated air loudspeaker driver. Think of it as a voice coil driven diaphone. These unit are capable of much greater acoustical output than conventional dynamic compression drivers.

In case anyone is wondering how it works, an air stream is modulated by a series of driven slots aligned with stationary slots to a horn. Think of it as a voice coil driven diaphone. These units are capable of much greater acoustical output than the conventional dynamic compression drivers that are currently used on electronic voice capable sirens.

With one of these you would not need the highly directional horn arrays currently being used, which can cause skip and dead areas of coverage in order to achieve high SPLs, but you would need a high (CFM) volume air compressor in addition to an amplifier.