5 - 25 Hz

[mashaffer]

Do you like the pipe organ?

Yes, Very much.

 

[Wavebourn]

It is not indicated whether any of the sketches are to any kind of scale; by their roughness I am assuming they are not. Like this one that looks fairly simple to build:
Note that this is not the Graham Holliman velocity coupled......etc All sizes | Graham Holliman | Flickr - Photo Sharing!

It looks like a ported box with Helmholtz resonator attached to it's back.