D27TG-45 with the rear chamber removed. This leaves the vented pole piece exposed.

Two things are apparent. First, the primary resonance (Fs) is much lower, around 500hz. This is raised to a measured 700hz Fs when the rear chamber is added. It's also a fairly high Q.

Second, there is a secondary resonance at 2khz, of extremely high Q. This is, I believe, a pipe resonance due to the vent and its length. It is normally totally supressed by the rear chamber.

It is probably a function of two things. The felt inside the chamber is pressed against the vent opening, which I think changes the resonance frequency of the pipe. This places it at twice the open end frequency. A secondary benefit of this is that the felt is much more effective at absorbing sound the higher the frequency. I believe that the two combined effectively totally damp this secondary resonance.