Originally Posted by LINEARX
In your opinion Bosso, what is the law of acoustical/electrical physics at work here?
It's not my opinion, just a fact of low freqs. As I've posted before, an excerpt on the subject from Dan Wiggins and Dave Hyre, 2001:
Boundary gain arises from the driver operating not in free space but in a constrained space. That is, the driver is typically referred to as operating in 4π spacefree air, but ½π space in-room. Each boundary cuts the total “space” in half. Thus the floor boundary cuts the space to 2π, the side wall cuts the space to π, and the rear wall finishes reducing the space to ½π (also referred to as eighth space).
Pressure vessel gain comes from the fact that, below a certain frequency, the room no longer supports standing waves; that is, the room is too small to contain a full wavelength. Contrary to legend, this does NOT mean the room cannot “reproduce” such waves! Rather, it means that the room is completely and uniformly pressurized by the input signal (we can’t call it a wave, since it’s not a full wave). This results in a gain in acoustic pressures in the room that grows as the frequency decreases (more gain for lower frequencies). Note that this effect is the primary reason one can get tremendous bass levels within a car; the gain starts at a very high frequency, thanks to the small size of the pressure vessel (car interior).
A third point that I've made in the past is part my opinion and part fact. The lower you go in frequency, the less the SPL is affected by the distance you sit from the subwoofer, according to the inverse square law, which states that every time you double the distance from the source of the sound, you lose 6dB intensity.
At single digits, you don't lose any intensity at the average distance from a subwoofer to the LP. This makes the gains achieved seem larger than they actually are, proportional to the actual specific distance and the higher frequencies in the subwoofers BW.
The 4th point is that if, for example, you see 18dB of gain from BG/PVG, your harmonic distortion, as a percentage of the dBSPL, is decreased by that 18dB because room gain is linear.
The main point I've tried to make of the advantages of using a sealed system in conjunction with a signal chain with the widest BW is that if your system (sub alignment/signal chain) is attenuated above 10Hz, you won't have much in the way of sound wave input for the room to act on.
A 5Hz sound wave is over 200 feet long, so pressure vessel effect will be seen in most every home, regardless of the openness of the floor plan. Boundary gain will also be realized according to the quality of the rooms construction and the specific placement in the room.
I have seen 30dB and more gain from 3-12Hz in various rooms. Some were very large (6,000 cubes), but sealed off. A recent HT I just completed and measured is in a 3,000 cubes basement room, but open to the rest of the basement, which is a total of 8,600 cubes and the sub is flat to 5Hz. Since the anechoic FR of that particular sub is 16Hz with no HP filter or early signal chain roll off, it's down around 25dB at 5Hz, so it's easy enough to do the math.
Unfortunately, some genius set the standard for posting graphs of in-room FR with REQW at 10Hz to 100Hz, so there are very few posted graphs to below 10Hz. I've attached a graph of Ssab's LLTs in-room with the notations to show the huge gain his room exhibits below 10Hz.
It will be very interesting to see that same graph with the sealed system he's building as we speak. It should be attenuated 12dB less, and if he uses an amp (just for comparisons sake) that's only down 3dB at 5Hz, he should see 15-24dB more dBSPL at 10Hz and below than the posted graph.
That would put him flat to 3Hz with no audible distortions below 10Hz. We shall see.