Bah, 18"? How about a 30inch woofer? Remember these big guys?
http://www.pa-anlagen.ch/Manuals/Electro_Voice/Speakers/EDS/30W%20EDS.pdf.

Trying to find the specs from later versions in the 70's. As I recall, they kept improving the things and finished up with a bigger, more efficient voice coil.

Biggest speakes on my own system are 15s. Still, if ya want to reproduce those reallllly low notes, ya gotta have big woofers