A little off topic...

[Train Fan]

At 12:12 here in this video, he gives a demo of his shop vacuum and it (And the planer its hooked up to) makes this wonderful low tone. What port ration would a siren have to have to make it?

http://videos.americanwoodworker.com/vi ... Collection

[thewoog34]

A 7 port siren on 50 hz would sound roughly around that pitch.