What area of the freq response does this affect?

[Booger weldz]

The ~35 cm (250 hz or so? ) offset stub at the exit end (and closed end) ?

 

[Booger weldz]

I tried to sim it but didn’t have that particular drivers TS parameters

 

[Booger weldz]

i think i got it close?

 

[GM]

Right, it adds a peaking notch with the offset setting the shelf's BW, ergo ~ an acoustic BSC filter.