{early PA?}
# I recall reading of a phonograph which used mechanical amplification.
# Compressed air was squirted out of a valve which was controlled by the
# pickup.  The result was noisy and distinctly lo-fi, but much louder
# than a conventional phonograph.  It tended to wear the disks out
# pretty quickly though.

An now-deceased prof told us willing students about a project he had
worked on during WWII.

They needed a mega-power PA with very clear audio quality. The purpose
was to bellow at refugees from aircraft.

Their solution was a giant compressed-air source, and a horn with
parallel shutters worked by a small audio system. I think he said it
worked very well, thus the War Dept. cancelled the project ;_}.

