Originally Posted by bossobass
I have to say that, after a decade of spirited debate, it's been great to read an intelligent poster who had his "eureka" moment, gets it and posts the information for others to absorb. Great stuff, FOH.
The debates over the years and that continue to this day of "Less filling!"... "Tastes great!" over this alignment vs that alignment have always been polluted with nonsense about which sounds "better, faster, tighter", etc. In fact, it has nothing to do with all of that and all of that only ever served to cloud the basic truths.
Tom Nousaine said it years ago... "Bass is bass". I've grown to love those simple, indisputable, black-and-white basic statements. Alignment is irrelevant if the reproduction is flat, clean at the desired playback level and properly calibrated with the rest of the system.
BTW, I also love that you've taken up the use of the word "system". Since the SW signal is never input directly into a box of whatever alignment with a driver in it, the low end is, by default, always handled by a system of components.
To the excellent post on PVG, I would make a simple correction; "Room Gain" is a phenomenon that comprises 2 elements: 1) Boundary Gain and 2) PVG. Boundary Gain results from reinforcement of the pressure waves from the driver (or horn mouth, or port, or passive radiators), which is the original point source, that result from additional point sources created by the reflective surfaces according to the placement of the sub in close proximity to boundaries.
So, the Room Gain phenomenon consists of an area of purely PVG, an area of transition, which is an interaction of PVG and BG and an area of purely BG.
As you mentioned, the PVG+BG are free lunch. LOTS of free lunch. Conventional wisdom says "There is no free lunch" and "There is no replacement for displacement". BG can add a theoretical +18dB and PVG can add a theoretical +18dB. That's an increase equivalent to adding 7 more of your subwoofer with no cost in space or $$. Sounds like free lunch to me. That's also an increase equivalent to adding 63 more of your drivers to your existing driver. Sounds like quite the replacement for displacement to me.
I discovered early on that the only way to properly utilize PVG was with the alignment that has the shallowest roll off with the least THD. Sealed wins. It's just a simple fact of life. All other alignments are band pass contraptions that have steeper roll off with much higher THD out of band (down low).
As you mentioned, there is a second advantage to the sealed alignment and PVG. This would be the fact that PVG happens to coincide nicely with reduction of harmonics as a percentage of the fundamental.
For example, if you see +9dB of gain per octave in your room, beginning at 30 Hz. At 8 Hz, say your sub gives 50% THD from HD2 (16 Hz) + HD3 (24 Hz) + HD4 (32 Hz) + HD5 (40 Hz) at ground plane.
PVG boosts the fundamental of 8 Hz by +18dB, but boosts the harmonics less, reducing their levels as a percentage. 2HD (16 Hz) is boosted by only +8dB, 3HD (24 Hz) is only boosted by +4dB, 3HD and 4HD & 5HD (32 Hz & 40 Hz) are not boosted at all. Essentially, this reduces the THD through 5HD from 50% to 10% and, since 2HD is still in the infrasonic range, the THD in the range of hearing is even less.
For those who might argue against this example, I took it from one of Sir Edward's reviews:
Unfortunately, it's not as simple as purchasing a bang for buck ID sealed sub. As you mention, it is a system that's required and starting with a 2nd order alignment doesn't assure you're ending with a 2nd order alignment.
Essential requirements for digging deep include having a signal chain that provides the signal as well, without rolling off too early, and an amplifier that can provide the juice below 20 HZ, as amplifiers tend to behave differently in the subsonic realm than their specs might indicate.
Warp, notnyt and others have done it right and are enjoying the results, room size notwithstanding (after all, 1/2 wave at 5 Hz is 110 feet, so I doubt room size matters for anyone at single digits). I've enjoyed it for many years now. I've asked many folks if and what difference they sense playing scenes with and without <20 Hz many times (mostly because it's great fun) and I only ever engaged in the debates with folks like the legendary Callas to keep the record straight, FWIW.
Thanks for the great post, I thoroughly enjoyed it.