Low frequency impedance bump

The peak you are seeing in red is the impedance curve.

its not really a design feature but more a behavior of any voice coil
but then again for a LF driver, then yes it is the designers goal to hopefully have a driver with a relatively low Fs or resonant frequency.
in this case it is the free air resonant frequency of the speaker or Fs at 35hz

since its a 21 inch driver and more designed for low end reproduction the resonant frequency is rather low at 35hz

not really related to size, but for the most part a LF or low frequency driver would ideally have a low resonant frequency.



likewise this is how you measure or determine the resonant frequency or Fs of a driver by measuring the free air impedance curve and where the peak you observe occurs is the resonant frequency.


when the driver goes into a sealed box, resonance will rise to a higher value,
this is all related to mechanical and electrical values of the driver itself and the overall volume its placed in.
in general the smaller the box the more the impedance peak will rise.
likewise in ported or reflex cabs, Fs is a needed value to design a good port tuning frequency, or if a actual filter alignment is wanted.
for the most part again ported enclosures are usually tuned about 7 to 12hz above or below Fs or resonant frequency.
and just like a sealed box resonance will rise when a speaker goes into basically a box

to understand speaker parameters and what they mean i suggest you read the Eminence site
and it will give you a explanation of the mechanical and electrical parameters of a driver.

Understanding Loudspeaker Data | Eminence Speaker

 

^This.