The "hit you in the chest" tactile bass is up in the 40-50Hz region. The 18-20Hz stuff is more like a "tingling" sensation. And personally, I find the claims of hearing or feeling anything below 18Hz to be rather dubious. Those frequencies are called "subsonic" for a reason. Humans literally cannot hear them! And as for feeling them, sure, there's a pressure wave and the air is moving, but again, it's nothing more than a "tingle". There's a reason we never knew elephants were "talking" to each other with subsonic frequencies until someone recorded what seemed like silence and sped it up!
To me, all the claims of hearing of "sensing" subsonic below-18Hz frequencies amounts to hearing the harmonics that are within our normal range of hearing. For a 16Hz fundamental, we're hearing the 32Hz harmonic, not the 16Hz note.
ANYWHO, the goal for most any home theater is to have flat, linear bass down to 20Hz, with output that
remains linear no matter how loud you crank the volume. Keeping the output linear regardless of volume is the real challange. Plenty of subs look like they have flat, linear output when they're outputting 85 or 90dB, but crank things up to 105dB or louder and you'll see the shape of the frequency response change dramatically.
But there's more than just the subwoofer alone at play. Bass sound waves reflect off of the walls, ceiling and floor and "bounce" around the room, which amplifies what you end up hearing at your seat. We call this "room gain". So a subwoofer might measure nice and flat and linear all on its own, but put it in a room and you'll suddenly have a big upswing on the low end where the room gain is coming into effect.
With no filters, boosting or EQ, a sealed sub will naturally start to roll off up around 40Hz. And again, with no adjustments to the output, a sealed sub will have a natural roll off that is fairly shallow - around a 2nd order 12dB/octave slope. This will often match up nicely with the room gain in a lot of rooms, which means that the natural roll off of the sealed sub will be counteracted by the room gain and you'll end up with flat, linear response within the room again.
When you have a port or a passive radiator on your sub, that port will have a resonant frequency, which is usually called the "tuning frequency" of the ported sub. So a ported sub will often be "tuned" to 20Hz, which basically means that all on its own, its frequency response will measure flat and linear right down to 20Hz, but below that frequency, it will drop off very quickly. The typical slope is a 4th order or 24dB/octave slope, but ported subs will often have an additional filter below the port's tuning frequency, so the slope might be even steeper.
So the sealed sub starts to roll off an octave or so higher, but it does so with a shallower slope. And that shallow slope downward continues below 20Hz as well. So when you measure a sealed sub in room, and not all by itself out in an open field or something, what you'll often find is flat, linear response way on down to 10Hz or so. The sub is getting quieter and quieter, starting up around 40Hz, but the room gain is also amplifying the bass more and more as the frequency gets lower. Meanwhile, the ported sub, which measures flat all by itself in an open field, will have a rising hump on the low end when measured in a room that you need to tame with some EQ. Then, below its tuning frequency, it drops off very steeply, so the room gain is not enough to offset that steep drop off and make the output all the way down to 10Hz appear linear again.
Finally, there's matter of "group delay" or "stored energy", which is basically when the signal in the recording tells the subwoofer to stop, but the sub keeps producing sound for a little while due to resonance. With a ported sub, the sub is literally "ringing" at its tuning frequency. That resonance is how it manages louder output at that low frequency. With a sealed sub, it's entirely up to the driver to produce the sound. And mechanics come into play as well as the trapped volume of air in a sealed sub acts as a damper.
So which is "better"? Well that depends entirely on your room. And it's not purely "ported vs. sealed". There are sealed subs that have filters, boosts or EQs so that they don't have the natural 2nd order slope starting up around 40Hz, and there are ported subs that have filters or DSP or EQ so that you can alter their low end response as well.
The bottom line is that you basically have to measure and find out what's happening in your particular room. If you want, and you have the funds available, you can opt for a sub like the SVSound PB or PC13-Ultra that has a lot of DSP and EQ controls and can also be run in two different ported modes AND in a sealed mode.
The
likelihood is that in a 19 x 14 x 8 sized room, a sealed sub with a natural 2nd order roll off that starts up around 40Hz will work very well with your room's gain to produce flat, linear response to well below 20Hz (provided, of course, that the sub's output retains that gradual slope and doesn't have a filter placed on it at some point). I'm a bit surprised though that you feel you need an upgrade from your Velodyne DD15. If you're not feeling enough tactile impact from that sub in that size of a room, my guess would be that you are sitting in a spot where the 40-50Hz range is having some cancellations due to room modes and those bass waves "bouncing" around your room. Output and extension should not be a problem for the DD15 in that room size. Not at all. But with just one sub, you are certainly going to have room modes and nulls and peaks. The better solution than upgrading to a different sub would be to simply add a second DD15. With careful placement, you can even out a lot of those dips and peaks in your frequency response within your room.
The other possibility is that your 19 x 14 x 8 "room" is actually just an "area" that you've designated as the "theater", but it's actually got openings to other areas of your house. A subwoofer cannot "know" that only a portion of an open space is considered the "theater area". A subwoofer will simply try to pressurize any and all spaces where the air can move! So if you have an
enclosed 19 x 14 x 8 room, I'm shocked if the DD15 doesn't play loud enough for you. But if you have an open door, or a hallway, or another ajoining room, then it makes sense. Where ever the air CAN move, it WILL move. But yeah, if your room is closed, my guess is you're sitting in a null, and a second DD15 is a much better solution that replacing your DD15 with another lone sub
