Charles

Thanks @Kyle, the TV1812 is actually a new model for PSA that has a similar volume to the FV18, tuned to 12.5hz with a single down-firing 4” port.  I ended up buying a pair of the Rythmik FV18s and they are very potent.  

On 11/2/2016 at 2:16 PM, Ricci said:

That's why you see 15 and 18" drivers with 3 and 4" ports or thin slot ports. PR's have advantages but also some shortcomings themselves. Cost being one and a lot of real estate on the baffle/s for them being another. Also the advantages of running them in opposed pairs make that arrangement almost a necessity. Mechanical displacement limits is another one.

Josh, after reading through this thread, I was hoping you could provide your opinion on a couple of commercial offerings.

You tested the FV18 and found it was limited by vent noise at 10 and 12.5 hz.  Rythmik then updated their design to use 4" ports instead of 3.5" ports which should squeeze a little more out of the design.  I am getting ready to purchase new subs and I am down to the FV18 Rev 2 and the new PSA TV1812.  Both are tuned to 12.5 hz and have similar power and cabinet size.  The biggest difference is that in 12.5 hz mode, the FV18 has 2 4" ports and the PSA sub has a single 4" port which is straight with flares to 6".  I suspect that this port is going to limit output around tune with compression and chuffing setting in much earlier.  I'm guessing the output loss could be as much as 4 db.  Does that sound accurate?

7 minutes ago, Ricci said:

Do you hear any scratching or feel it rubbing if you gently push in on the cones? Is the sound more similar to a tapping sound or a ticking? Myself and some others did receive some 21Ipal's which were making a tick or tap type of sound. B&C replaced mine. I suspect that it comes from a loose shorting ring, but I'm not sure. It sounds like you may have the same issue. 

I don't hear any noise when gently moving the cone.  In fact, when the noise occurs, the cone really isn't moving because the SPL level is very low.  The only time the noise occurs is when the driver is playing something, but at a very low level.  I wouldn't describe it as tapping or ticking, more like a slight popping like a radio station that is not quite tuned in.  These are 18 inch iPal drivers.  Do you have information on the issue you experienced that I can reference?  The sub has 2 drivers and the noise comes from both of them which is why I didn't suspect a driver issue.

I'm hoping that Ricci or others with more experience with high end, high efficiency drivers can help solve an issue I am having:

My subwoofer uses B&C iPal drivers.  When there is bass content at a very low SPL level (barely audible), the bass sounds scratchy.  It is almost like there is static in the signal.  Turning the volume up to a more moderate SPL level eliminates the noise and at very spirited levels, it sounds fantastic.  Luckily, this exact scenario is very rare during normal listening but it does occur in scenes where the SPL of the bass content is very low like distant background music.

Is it possible that what I am hearing is the result of the high efficiency of the drivers themselves?  My thought is that at very low signal levels, the signal to noise ratio reaches a point where noise that usually wouldn't be heard becomes audible.  Because the drivers are very efficient, they are able to pickup this very weak signal and produce noise where lower efficiency drivers would remain silent.  If that isn't the cause, what might I look at?

I have already tried different receivers, cables, power source, amplifier, etc.