Woofer Excursion Below Ported Cabinet Resonance

Can anyone explain the reason that the woofer excursion increases like crazy (exponentially??) below the cabinet resonance of a ported cabinet?

Might be helpful for us to know under what conditions you're
observing the problem.

Is it occurring all the time without exception??...or with
certain volume levels or with certain discs, material or
genres of music?? When did you first notice it and had you
undergone any other system changes before you saw the
problem?? What kind of impact is there on the sound or
system performance?? Is this problem on the sub listed in
your system??

Any info you could supply might help. Have you checked your
sub crossover settings, or changed your sub's location, etc.
Unless you were talking about this in general maybe?? Not sure I understood your question...
Can anyone explain the reason that the woofer excursion increases like crazy (exponentially??) below the cabinet resonance of a ported cabinet?

Hi Bob,
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.
Bombaywalla, My previous Paradigm Studio/60 - 2 1/2 way speakers had two 6" drivers, one working for low-mid range while other was only a bass speaker to obtain lower -3dB frequency rating. My current (Hyperion HPS-938) 3-way speakers have two 8" bass speakers, but rated frequency range is worse. The difference is that bass sounds better. It is not even issue of being less muddy, but attack and decay of the bass notes sounds very pleasant and natural. Bass line is more melodic while older speakers had pronounced mid-bass resonances. My suspicion is that port can be tuned to increase frequency response or to lower distortions. It might be also better quality of transducers, since speakers were more expensive. What do you think?
it's hard to say if there is one particular reason that your Paradigms with a better freq range performed worse than your present Hyperion which have a worse freq range spec. it's likely to be a multitude of reasons.
I've read & heard that it is easy to screw up a ported design - either the cabinet does not have the correct air volume or the drivers don't have sufficient Qts (which is a combine electrical & mechanical Q factor of the driver & is the inverse of driver damping factor) or they don't have the correct BL (the force factor) which plays into the Qts eventually. I know a particular speaker manuf who does really deep dives into finding speaker drivers that measure appropriately for his speaker designs. I'm told it's not an easy task & that most drivers available to OEMs do not qualify. It's also a time consuming task. So, it's entirely possible that the Paradigms had drivers that reflected their retail sale value. We've seen many posts on this - how a speaker is 4X marked up for the eventual buyer compared to the actual manuf cost. This gives a perspective on how much must have been spent on the drivers themselves.
When the ported speaker is not designed correctly it has a bad frequency response which means the port exaggerates the bass & there is bass overhang very often. The frequency range might have been extended but it was not worth it. OTOH, when a ported design is well executed it not only extends the bass freq response but also does not introduce any discernible distortion. So, I certainly do think that ported designs can do both items - extend bass freq resp & create very little distortion.
Hi Bombaywalla, thanks for the explanation. I'll need to give it some thought and time to digest. I have seen a few speakers with a high pass filter to protect the woofer. Maybe most just don't advertise its existence.
Bombaywalla thank you. Most likely all factors that you mentioned play role.

As for woofer huge excursions below port frequency - it would mean incredible frequency response of the speaker, while in reality response drops rather fast below port frequency. I would look for different reasons. It might be vibration from TT (or feedback), could be in the recorded material (one studio in New York was adding vibration of the subway nearby) or could be the amplifier. Some amps use DC servo to null DC voltage on the output. Servo might go crazy if amplifier lacks high-pass input filter. I suspect that my small Rowland has 5Hz filter just for that.

All woofers exhibit huge excursions below the port frequency (acoustic suspension enclosures don't have that problem); nature of the beast and I was curious for an explanation.
Bob, I found this article explaining operation of the port:
Thanks for the link Kijanki.
Did you read it? If you look at point #2 it is exactly what I wrote in my post above.
in your 09-30-15 response you wrote " I would look for different reasons. It might be vibration from TT (or feedback),....". I don't think there's another reason for large woofer excursion below the port tuned freq. What I wrote in my post & what is written in point #2 on the Barefaced Bass website is the reason....
Yes, and I realize my mistake. I assumed that excursions have to correlate to frequency extension while in reality port defeats it outputting out of phase signal.
I REALLY appreciate your reply (members usually don't post at this point).
It's cool, no problem. Thanks.