Originally Posted by a77cj7
I will add that the similar response shape doesn’t equate to “acting like sealed”. The ported design still has the excursion minima and impedance spike at tune, so instead of being amp limited on the low end, is actually much more efficient.
I disagree strongly on using an undersize port to shape response. Port compression and noise aren’t what I would consider to bd desirable outcomes under any situation. Now, if you use EQ to tame the low end and size the port appropriately for this, It could work.
Btw, my undersized HST’s do have a positive hump in the response at port tune even with the undersized box/port. Velocity is off the rails, and chuffing is present, of course. The effect of a strong motor.
One of the major attributes to a sealed box is the low group delay in the audible pass band. A very low tuned sub tends to have very low group delay in the audible ranges -hence "similar to". Obviously there are significant differences, the gain in efficiency (and reduced distortion due to less excursion) among them.
Port compression is a function of how much you're pushing through the port. For example if you lived in a fictional world where 10 watts was the maximum output of any amplifier, even your strong motor design would be fine with a simple 4" port no matter what you tuned it to.
It is an oversimplification to size a port diameter based on Vd capability of the driver(s), although that'll definitely get you in the safe zone.
I, too, am very much against port compression. I also am against using an undersize port to shape response -that would be a terrible idea. The box does most of the shaping of the curve, the port either flows enough to be compression/chuffing free enough or it doesn't at that particular tune/power level. It stands to reason, however, that if the response is rolled off, the port isn't going to see the same requirements for air flow capability before compression/chuffing. It's all about what you want to do. If you look closely there are many commercial designs that use tenets of this fact; PSA and JTR among them.
Below is four graphs showing vent velocity freq response and group delay for a dual 21" subwoofer. The blue line is a 15" diameter port tuned to 30hz. The pink line is a 6" port tuned to 8hz. The grey line is sealed. The red line is the same 6" port tuned to 8hz with a 10hz high pass. All are being fed 2000 watts. The smaller diameter port with high pass actually has less velocity, and less compression in the audible pass band at maximum power (2000 watts) although both are certainly good. It'll exhibit LESS compression, and LESS chuffing. This is expected; the 8hz tune is going to give you a frequency response shape that looks very very similar to a sealed box; i.e. the port ain't doing much in this alignment
. As it isn't doing much, you don't need it to be as large. This is what happens with a medium box and a very low tune. Note: you can manipulate this, and make it all untrue (or control), with DSP.
This is just basic acoustics/physics. The real question is if there are any good real world applications for this, and depending on the application, yes, the caveat being that it is, absolutely a design compromise. They all are though, which is why we have so many different alignments to choose from.