

NASA and related agencies apparently used this same principles to create
the loudest reported reproduced sound. They used an "analog" electrically
controlled valve to control the flow of air across a horn throat. If I 
remember correctly it was called a "modulated air blast transducer". There
were reports of the thing being able to produce 106 dB @ 80 Hz @ 10 mile
distance, communicate directly with fighter pilots @ 5000 ft, etc.

Regards,
Gordon.
