Interview with Dr. Poh Ser Hsu of Hsu Research
Srajan Ebaen: What prompted you to get into the
subwoofer market?
Dr. Hsu: I joined the Boston Audio Society while I
was still a student at MIT. I built a number of subwoofers for members of the society. In
1991, Peter Mitchell heard one of my subwoofers and wrote a great article in Stereophile.
That article generated enough reader interest for me to start my company in late 1991,
early 1992.
SE: Do you feel that subwoofer requirements for
music and movies are different?
PH: The requirements are different,
but subs can be designed to handle both types of programs well. Home-theater subs need not
have very extended bass -- high output capability down to 25Hz is sufficient for movie
soundtracks. However, home-theater subs do need to be able to play loud, or at least be
able to be pushed hard without distress. A lot of dedicated music subs will bottom out
with a loud "crack," distort badly, or compress dynamic range to such an extent
that explosions become anemic. Most home-theater subs are designed to play loud but lack
the necessary refinement for music. Music subs require low distortion, flat extended bass
and good transient response.
As far as subwoofers and [full-range] speakers go, I feel
[subwoofers are] even more important for music. Most of the time, main speakers are
positioned for best imaging, which places them in locations that are sub-optimal for bass
reproduction. Hence, even floorstanders that could and should have good bass often end up
without it. A good subwoofer placed optimally for bass reproduction will almost always add
greatly to most music systems. A very effective way to determine the best placement is to
position the subwoofer in your listening seat location. Walk the various room positions
you have available for the sub and check for the most even and loudest response.
If I were buying a system from scratch, I would not go for
floor-standing main speakers at all. With a good subwoofer, there is really no need for
the main speakers to have good bass response. Save the money and buy a good minimonitor
instead. They tend to image better than large tower speakers do.
SE: How low should a minimonitor go to make a
good match with a sub? At what frequency does a sub become omni-directional to avoid
localization?
PH: The localization threshold varies from one
person to another. In general, I find that 90Hz with a 24dB/octave filter is about the
upper limit. Hence, a minimonitor should have wide dynamic range down to at least 90Hz.
SE: I've noted that many subwoofers with good
reputations for music applications use down-firing rather than front-firing drivers. Is
this just coincidence or does it reflect a particular design philosophy?
PH: I feel that's just a coincidence. In the case of
the VTF-2, the down-firing design was chosen to save money. A down-firing woofer doesn't
need a grill to protect it.  The
upcoming VTF-3 will have a forward-firing woofer because the box is larger. We feel it
would look too plain and large without a grille.
SE: We just reviewed your second generation VTF-2
(shown right). You changed the low-pass crossover network from a second- to fourth-order
slope. Could you explain why?
PH: We changed the slope and also lowered the
low-pass frequency range to 30Hz to get the subwoofer out of the way more quickly, to make
it harder to localize and to allow it to be successfully mated to relatively full-range
tower speakers.
SE: Hsu subwoofers enjoy a reputation for
unusually high output levels at low frequency where their size is concerned. How do you
arrive at that performance?
PH: (Laughs) A good understanding of
the laws of physics.
SE: There's got to be more to it than that!
PH: All designs involve trade-offs. You have to give
up in some other area(s) to get good bass extension and high output levels in the low
bass. A good understanding of the laws of physics allows me to achieve the bass extension
and high output with the fewest compromises in other areas.
SE: What is the best placement position for a
subwoofer -- the corner, between the speakers, or some other place?
PH: With a reasonably low crossover frequency and a
steep crossover slope, corner placement usually yields the widest dynamic range, lowest
distortion and deepest extension. Note that listener position is also critical. As a
general rule, don't expect much deep bass if you have to sit in the middle of the room.
The middle is where the greatest bass suck-outs or nulls occur.
SE: If one does have to sit in the middle, what
would the best subwoofer position be?
PH: There is no one single answer. If the room were
deep, say 40 feet from front to back, I would put the sub close to the listener like a
coffee table. This places the sub in the near field and augments its output. If the room
were shallow, maybe 12-15 feet, I'd place the sub in whatever front corner gives the most
solid bass.
SE: If the main speakers don't require filtering,
what is the best way to connect a subwoofer?
PH: I would drive the sub at line level from the
preamp. A continuously adjustable low-pass filter will work better for such applications.
However, note that even main speakers that do not require filtering may still benefit from
it. You will always obtain lower distortion and wider dynamic range when you minimize the
amount of bass the main speakers are asked to reproduce.
SE: If you could pick only two traits that you
feel differentiate Hsu subwoofers from the competition, what would those be?
PH: Better performance for every dollar spent, and
the ability to excel in both music and home-theater applications.
SE: Do you have any tips on how to best evaluate
subwoofer performance in a retail store -- any favorite bass test tracks for either music
or movies?
PH: I would select a variety of music and DVD to
evaluate bass tightness, integration, boominess, extension and dynamic range capability.
I'd start off with nicely recorded male vocals to check for boominess, chestiness and
integration. Can you hear the sub's location? If the sub is of high quality and set up
properly, you should not be able to locate its placement position. I like Aaron Neville
for this test.
Next, I would play a piece with lots of tight, clean mid
and upper bass to test bass tautness, distortion and dynamic range. Sheffield's Drum
& Track Record [Sheffield Lab CD-14/20] and Bela Fleck's Flight of the Cosmic
Hippo [Warner Bros. 26562-2] are excellent tests. Then use a good organ piece to test
very low bass. The Virgil Fox CD that comes with our subwoofers [Laserlight 15313] and
Reference Recording's Pomp & Pipes [RR-58] are good examples. For DVD, I would
recommend chapter 19 of Titan AE and the depth charges from U-571.
SE: What are the most important features a buyer
should be looking for in a subwoofer?
PH: I can only say what I would look for, which of
course is a biased opinion! I would look for a subwoofer that is musical, tight, not
boomy, that can be integrated seamlessly and has great dynamic range capability down to
very low frequencies. Where features are concerned, I prefer high-order low-pass filters
to minimize the overlap of shared frequencies between subwoofer and main speakers. If
you're using a surround-sound processor, a crossover defeat switch on the subwoofer is
good to have. And a good sub shouldn't cost an arm and a leg.
For more bass evaluation tracks, Dr. Hsu sent me the
recommended list he always includes for all of his customers. (For home-theater demos,
Dr. Hsu reminds us that most DVDs automatically cause surround-sound processors to default
to Dolby Pro Logic. For best bass performance, you must reset the processor or receiver to
5.1 mode.)
Recommended DVDs:
- Event Horizon
- Titanic [chapter 7]
- True Lies [chapter 31, the helicopter scene]
- The Fifth Element [chapter 3]
- Godzilla
- Air Force One
- Armageddon
- The Haunting
- Saving Private Ryan.
Recommended CDs:
- Bass Outlaws: Illegal Bass [Newtown NTN2210]. Track
12 features ultra-loud bass in the low to sub 20Hz range.
- Enya: Watermark [Geffen 242332-3]. Track 10,
"The Longships," features bass from 20Hz to the low 30s.
- Hindemith: Organ Works [Argo 417 159-2]. Track 5 at
4:45 features a 19Hz note that is 6dB louder than the highest level anywhere else on the
disc.
- Mendelssohn: Organ Works [Argo 414 420-2]. Extremely
loud 19Hz tones on track 8 -- proceed with caution!
- Mussorgsky: Pictures at an Exhibition [Dorian DOR
90117]. Tracks 5 and 15 feature the best bass.
- Prokofiev: Alexander Nevsky [London 410 164-2]. 25Hz
bass from the lowest-tuned conventionally-sized bass drum.
- Reference Recordings HDCD Sampler[RR-S3CD]. Tracks 1
and 3 feature tight and gut-shaking drum strokes without overhang.
- Respighi: Pines of Rome [London 410 145-2]. Track 4
at 1:22 and 2:29 features a pure 22Hz tone.
- Saint-Saens: Organ Symphony [Philips 412 619-2]. Nine
minutes into track 2 features deep bass down to16Hz.
- Star Tracks II [Telarc CD80146]. Track 2 features
extremely deep bass about one minute into the track.
- Thom Rotella Band [DMP CD-650]. One minute into track
12 features a bass guitar solo that illustrates superb bass transients.
- Jennifer Warnes: The Hunter [Private Music
01005-82089-2]. Tracks 8 and 9 feature loud, tight mid to low bass.
- The Ruffatti Organ in Davis Symphony Hall [Telarc
CD80097]. Tracks 1 and 11 feature sub-20Hz bass.
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