Why I Bought Two Monster Subs

HSU Research VTF-2 MK5 subwoofers

Now that I have gone from full-time audio equipment reviewer to semi-retired person, I am at liberty to make choices not available before. When my subwoofer expired of old age and reviewer-abuse syndrome — oh, the beatings it took — I saw this not as a loss but as an opportunity. I thereupon bought two subwoofers larger than any I’ve had before.

In my previous life, for obvious practical reasons, I shunned any gear that I could not easily uncrate, move around the room, and pack up again later. I was a highly specialized reviewer and my strong suits were audio/video receivers and compact speaker packages. Reviewing compact speakers came naturally — I live in a one-bedroom apartment, not a vast McMansion, so small speakers were what I could handle, and they were the right size for the room where my reference system lived.

However, taking early retirement (more or less) after a corporate bloodletting, I reassessed what my system needed. Some parts of my reference system survived. I decided that the five 2007-vintage Paradigm Reference Studio 20 v4 stand-mount speakers that served me as a reviewer would serve just as well in my retirement. The relative neutrality that made them an appropriate mate for a variety of amps also suited my listening preferences. I also decided to stick with the four Klipsch RP-140SA height speakers, whose limited function is to bounce the height channels embedded into some surround soundtracks off the ceiling. The nine-channel Denon AVR-X7200W surround receiver which powers all nine speakers was another keeper.

What my system lacked, especially after the untimely death of the compact sub I had used for more than a decade, was bottom-octave authority. Several months of going sub-less only make the craving keener. Eventually I decided to drop the bomb, ordering not just one but two brawny subs. The benefit of the second sub is to achieve more even room coverage, providing satisfying bass in more than one listening position. Within a few weeks a pair of 25-inch-tall cartons arrived on my doorstep, each containing a HSU Research VTF-2 MK5 subwoofer (available for $539 from hsuresearch.com).

The Boring Part

For decades I dutifully typed out grafs like this: The VTF-2 MK5 is actually the second-smallest model from the drawing board of subwoofer design genius Dr. Poh Ser Hsu. It has a single 12-inch driver (or two if you order the dual-drive model for $1029). Aside from one lone 10-inch model, all other HSU subs use 15-inch drivers. Driving the cone is an amp rated at 350 watts continuous power or 1400 watts peak power. The forward-facing driver is augmented by two front-facing ports, either or both of which can be plugged with the supplied foam bungs to tailor the sub’s bass response to the needs of the room and/or listener.

On the back panel are pairs of line- and speaker-level inputs. One of the line-level ins doubles as an LFE-in (for a receiver’s sub output) — and that’s the best option for most modern surround systems. Among the knobs and toggles, the usual suspects include volume, crossover, crossover defeat, power on/auto/off, and phase (controlling whether the driver moves in or out — a little listening will show what your room prefers).

More unusual are the Operating Mode toggle (EQ1/EQ2) and the Q Control. The former, combined with the foam bungs, can operate the sub at any of five equalizer curves, including ported (both ports open), sealed (both ports closed), and hybrid (one open, one closed). You can eyeball the frequency response charts on the HSU product page and choose the curve that suits. I chose EQ1 with one port open to get maximum bass extension. In the process I traded off some efficiency. However, just one of these two subs would have had more than enough headroom to serve in my livingroom. The two of them together had — well, more than more than enough.

HSU’s arrangement is somewhat unusual. Most subs lack EQ modes. A select number of desirable models have built-in parametric equalizers that can be set, via app or other means, to measure the room and correct the sub’s output precisely. One of those products would be perfect for the untutored listener who just wants to set perfect bass and forget about it. In adopting the HSUs, I decided to take a more esoteric approach.

The Weird Part

I had a unique situation and the knowledge to take advantage of it. My room has a pair of standing waves — in other words, bass-bloating peaks in frequency response — between 60 and 80 Hz. I decided to offset them using just the natural bass rolloff of the Paradigm stand-mount speakers, with none of the room-correction magic found in some subwoofers and most receivers (including the otherwise excellent Audyssey MultEQ XT32 room correction built into the Denon).

From 80 Hz and up, the Paradigms are reasonably if not altogether perfectly flat, as you can see in the measurements by my longtime colleague Mark J. Peterson published with my review of them. This flat response makes them tonally neutral. From 80 down to 60 Hz, their diminishing bass response cancels out the room’s natural bass peaks. At a crossover of 60 Hz and below, which is on the low side of normal, the subs take over. The result may not be ruler-flat but it is well balanced and listenable to my ears — and my ears have experience with countless speakers and subs in this room.

Having already selected which of the subs’ EQ curves to use, the only remaining task was to set their volume using the system’s volume controls — the one in the receiver and those in the subs themselves — with a sound pressure level meter. All volume settings ended up at much lower levels than I would normally use with a single sub. The process was easy and fast. The system now seemed balanced to my ears, and has since logged countless hours of music and movie listening — though I wouldn’t rule out one final tweak with a real time analyzer. (My last one died with the iPod touch on which it lived. I’ll start over with an Android phone app though I’m not in any hurry.)

If you read my review of the surround remix of Abbey Road you already know what happens next. Ringo’s drum solo toward the end of side two is one of the most tasteful and well recorded in rock history. The bass drum and toms had the right weight. But more than that, they had speed, with the drivers starting and stopping on a dime; timbral subtlety, because drums and bass have nuances like any other instruments; and of course loads of headroom, because in my room these subs are using a small fraction of their potential output.

The Sneaky Part

There was one little problem. My room doesn’t have enough real estate to fit in two giant subs with what acousticians would deem perfect bass-optimizing placement. The subs had to fit within the footprint of the front left and right speakers, which meant sitting beneath them — essentially becoming giant speaker stands. If you can visualize the Klipsch elevation speakers sitting atop the Paradigm stand-mount speakers sitting atop the HSU subs, that Frankenstein monster is my system. It is actually not much uglier than most large multi-enclosure floorstanding speakers.

This introduced a major acoustic problem in the form of bass vibrations transferring from the subs to the speakers. I could readily hear it in a track on the HSU test CD, a movement from the Saint-Saëns Organ Symphony. When the organ sounded its lowest notes, the tweeters in the speakers broke up audibly. To ameliorate this, I placed high-density acoustic foam pads between the subs and speakers. With the pads installed, the organ’s bass pedals could thunder away without audibly affecting its upper ranks. The subs now rarely produce enough vibration to color the speakers, but when they do, the pads reduce the problem below the threshold where I would notice it.

My approach is unorthodox. Because the results are not repeatable in every room — you might set up the same or similar gear in a room with different acoustics and get vastly different results — I am not irresponsible enough to recommend this combination of gear to the hapless reader. But the HSUs have given me a lot of brawn for the buck. This is my retirement system and I love it.




Author of The Friendly Audio Guide and the annually updated Practical Home Theater (quietriverpress.com).

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Mark Fleischmann

Mark Fleischmann

Author of The Friendly Audio Guide and the annually updated Practical Home Theater (quietriverpress.com).

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