TL design vs driver parameters

[Henkjan]

So I built this nice sub with the Tangband W69-1042 see TL sub with 2 TB69 woofs just a straight TL with A=Sd.

Since a co-worker would like a new sub, and he thought the sub with the W69 was rather large, I thought of a similar TL employing the W6-1108SA. both fs and Qt are close to those of the W69 so it was my expectation that I could simply scale the design for the smaller Sd. NOT! The simulation of the TL with MLK's 'driver offset' sheet showed a different respons, much less low. where the W69 had a rather flat curve from 100 down to 40Hz (see pic below) the W6 rolled off much earlier (sorry, forgot to make a snapshot, will post this later) with -5bB @40Hz. I tried to tweak the design, longer pipe, taper but the basic form of the curve remained the same

so there's more than just fs and Qt ....