Impedance curve from tuned Altec VOTT A7 (828) cabinet.
Bass driver is Altec 416-8B, the old poor speaker surround
compleately cleaned, and a new thin coating applied.
Free air resonance of the speaker is 22,0 Hz,  13,21 Hz in
the finished tuned cabinet.
Measuring frequency:   1Hz – 1100 Hz,  swept sine.
Copyright April 24, 2010.
Hasling Audio Systems, Denmark.
Please see LETTERS below gallery
18 Ohm
8 Ohm
0 Ohm

Altec VOTT A7 (828) cabinet impedance with final tuning (port area and internal damping material).   
The speaker is Altec 416-8B, free air resonance 22,0 Hz,
the surround has been cleaned completely from the old poor coating material, and a new thin coating applied.

The speaker resonance peak at 53,44 Hz is fixed in frequency (cabinet determined), it doesn´t move during the tuning.

The very deep cone resonance on 13,21 Hz is created by the (my)  port/horn loading, typical for horn cabinets,
the only type of cabinets where the free air resonance goes DOWN, when installed in the horn.  This tuning  gives a lower impedance in the very deep end, resulting in a better loading and higher sound pressure here, and later roll off.

This tuning is not a Thiele/Small tuning.  The blue line at 8,0 Ohm is a reference calibration with 8,00 Ohm 1% resistor.  The small rise over the 8 Ohm line comes from the DC resistance in the connected measuring cable (twisted pair) between the measuring soft/hardware and the speaker in a distant room. 
The vertical ohmic impedance on curves are linear.  The top is 78 Ohm.

Per Hasling, Hasling Audio Systems Denmark.   April 24, 2010.