How to squeeze a new woofer in an old speaker...

First of all, we need to verify if the chosen woofer fits the volume of the speaker.

So, here is the box itself, in a first slobby stage (two way speaker with a 15" woofer and a 1" driver)

From the bottom, this is the volume where the Dayton woofer was mounted in a first stage. As you can see the 1" driver is mounted inside the volume itself:

 (Pict. 1)

This is the same volume, reduced to 85 lt approx:

                                                            (Pict. 2)













And here is the volume closed also from the bottom, for a reduced volume of 55lt:
(Pict. 3)











This is the inner volume with the speaker mounted.

Now, why all these volumes?
When we apply a given woofer to a given volume in a horn speaker like this, the first thing to look at is the impedance match.

Here you can see the graph of the Dayton woofer mounted in the volume, open (in white, look at the Fs in this position and compare to the free air resonance of the woofer.) and with different blocks of polistyrene to empyrically (or should I say slobby) reduce it and see what happens to the impedance  (red and green traces).
The two added volumes of styrene were not very different one from the other, I don't recall now how much, but the general trend was clear to me. In blue it's the impedance of the driver as in Pict.2, with the volume reduced of 30 litres almost.
I built a board to divide the first 30 litres of the volume from the rest, like in Pict.2, and measured the impedance, then I closed another 30 litres from the bottom end of the volume, like in Pict.3:

In red and green are the impedance plots of the left and right speakers, same as blue above, with the Dayton woofer and upper volume sealed (85lt, Pict.2).
In cyan the volume with two thirds closed (55lt).
For my needs, mission accomplished. The two peaks are now similar and for me this is the best I can do.





No, not really. 
I could linearize the impedance considering a possible use of tube amps, for example...
So here is the impedance plot of the woofer with and without RCL equalization for this use:

But what happens to the frequency response?
This is a too early stage for that, but a quick comparison of the green curve and cyan curve gives this:

Response measured in asnearfieldaspossible, mic on the floor, level arbitrary, in red 85lt (Pict.2, green impedance curve) and in white 55lt (Pict.3, cyan impedance curve). We loose something where really there is nothing going on, at 40Hz, but look how much more regular the graph is above 120 Hz and up, where it really counts. Did I mentioned I want to cut at 300 Hz?...