Can anyone explain the reason that the woofer excursion increases like crazy (exponentially??) below the cabinet resonance of a ported cabinet?
briefly, in a ported woofer design, the mass of air in the plastic tube that creates the port acts like a mass & the air enclosed in the woofer cabinet acts like a spring. I'm equating the port & the woofer cabinet to the familiar mass & spring equivalent that we learnt in high school. For a port to work correctly, the woofer cabinet has to be tuned to a frequency below the natural resonance frequency of the woofer driver. When you look at the Stereophile measurements of speaker impedance & phase for ported speakers you see 2 humongous peaks in the bass region. The peak at lower freq is the freq to which the cabinet is tuned & the higher freq peak is the woofer resonance peak. The port is tuned to operate in between these 2 peaks.
For bass frequencies above the natural resonance frequency of the woofer driver, the port does nothing i.e. has no influence.
For bass frequencies below the tuned cabinet resonance, the back wave air movement due to the woofer is in opposite phase with the front wave air movement of the woofer & the back wave is cancelled. The woofer driver now behaves as tho it suspended in free air (& not inside a cabinet). What this does is that the port unloads the woofer cone & woofer driver cone excursion can be much more than its safe allowable mechanical limit & can be easily damaged. Thus, in many speaker systems that are rated to play at very high SPLs, the manuf includes an electrical high-pass filter associated with the port which cuts off the low frequency content below the cabinet resonance such that it never reaches the woofer driver in the 1st place. This helps to protect the woofer driver. Unfortunately, this comes at a price - this high pass filter introduces frequency dependent delay & can smear the bass response of the woofer when its is operating normally. The manuf can adjust this high-pass filter response to be least invasive but some effects might still be present whereby you lose the ambience of the recording, the hall effects, etc.
Hope this helps.