Hsu Research STF-2 Test & Review
By Ed Mullen
1) Introduction:
The STF series of subwoofers from Hsu Research are fixed tune units with the tuning frequency carefully chosen to provide an optimal balance of extension and output. These subs were designed for enthusiast who doesn’t need the more expensive variable tuning option offered on the VTF series. Hsu Research offers three STF models: The STF-3 (12” woofer), the STF-2 (10” woofer), and the STF-1 (8” woofer).
The STF-2 represents Hsu’s entry into the $400 subwoofer class, and is essentially a fixed tune version of the venerable VTF-2 operating in Maximum Extension mode. The rated extension of the STF-2 is 25 Hz +/-2 dB. Venting is accomplished with one flared (both ends) port, with an inner diameter of 3 inches and an outer flare diameter of 4-3/8 inches.
The STF-2 came with unpacking instructions, a well-written Owner’s Manual, a Quick Set-Up Guide (with plenty of diagrams) for HT applications. Warranty is 7 years on the subwoofer, and 2 years on electronics. For review purposes only, Dr. Hsu also provided me with a demo music CD, and a demo HT DVD.
The Hsu R&D and general headquarters is located in Anaheim, California. The STF-2 was designed and engineered in the United States, and built in China. The website is located at
www.hsuresearch.com.
2) Size, Appearance, Fit & Finish:
The STF-2’s external dimensions are roughly 14" wide, 19" tall, and 18" deep, including the amplifier controls and the spiked plastic feet. The sub weighs about 40 pounds uncrated.
The plastic spiked feet on the STF-2 can vibrate on hard smooth surfaces (like my laminate flooring), so I used a rubberized welcome mat to prevent this from occurring. Owners could also use rubber furniture cups under the spiked feet. Either method is approved by Hsu Research.
The STF-2 cabinet has an external ridge on the top and bottom caps, and comes finished in a finely textured satin black vacuum-sealed vinyl coating. The coating application was uniform with no detectable surface defects. The vertical seam in the vinyl discretely runs along the backside of the subwoofer. The amp, woofer, and vent were well fitted.
3) Amplifier Feature Set:
The rear flush-mounted digital BASH amp is rated at 200 watts continuous. Standard amp features include:
- Off/Auto-On/On
- Single Low Level Input
- Phase Control (0/180 switch)
- CV LPF (30-90 Hz 4th order with defeat switch)
- Replaceable Fuse
- Detachable Power Cord
4) Ground Plane Frequency Response Measurements:
In order to determine the quasi-anechoic frequency response, the STF-2 was placed outside, away from any reflective structures, and measured using ground plane techniques. Per the recommendation of Dr. Hsu, the STF-2 was laid on its side, with the woofer and vent equidistant from the mic. Measurements were conducted at 2 meters from the centerline of the subwoofer enclosure, with the low pass filter disabled, and the phase control set to 0 degrees.
FR sweeps were conducted at progressively increasing sound pressure levels until dynamic compression was noted. Dynamic compression is caused by a combination of thermal and mechanical driver limitations and port flow restriction.
Using 104 dB as a midpoint, the 2M GP FR curve measured an outstanding +/- 2 dB from 25-100 Hz. The response was –9 dB (95 dB) at 20 Hz. With respect to dynamic compression, the curve stayed linear up to 102-103 dB in the 25-40 Hz bandwidth.
5) Ground Plane Total Harmonic Distortion (THD) Testing:
Harmonic distortion occurs when harmonics (multiples) the fundamental signal are produced due to non-linear behavior of the electrical, magnetic, or mechanical mechanism of the driver. A subwoofer with low THD will sound clean and distinct. At the lowest frequencies, high THD means the listener will feel less of the true fundamental note, and hear more of the false harmonics.
Measurements were conducted at 2 meters from the centerline of the subwoofer enclosure. To calculate the 1 meter test values, add 6 dB to the 2 meter reading. THD was limited to 10% unless otherwise noted. With the exception of 22 Hz, test frequency spacing was at 1/3 octave intervals.
Equipment reviewer Tom Nousaine directly averages his dB values, providing equal weighting to all sound pressures. This method correlates well with our subjective impressions of subwoofer performance, and was therefore used in this review.
Output Linearity for a given bandwidth is calculated by dividing the average SPL by the maximum SPL and expressing the results as a percentage. A score of 100% means the subwoofer exhibits perfect output linearity across the given bandwidth.
STF-2 10% THD Ground Plane 2 Meters (add 6 dB to obtain 1 meter values):
20 Hz: 74.7 dB
22 Hz: 80.9 dB
25 Hz: 91.7 dB
32 Hz: 99.5 dB
40 Hz: 98.8 dB
50 Hz: 104.8 dB
63 Hz: 106.9 dB
80 Hz: 104.7 dB
20-80 Hz: Average 95.3 dB; Output Linearity 89%
22-80 Hz: Average 98.2 dB; Output Linearity 92%
25-80 Hz: Average 101.1 dB; Output Linearity 95%
6) In-Room Frequency Response Measurements:
The in-room frequency response of a subwoofer is affected by things like room acoustics, the location of the listener, interaction with other speakers, and even the bass management circuit of the AVR. Experimentation with subwoofer placement and different listening positions is encouraged in order to obtain the best results in your own room.
To simulate a typical user set-up, the STF-2 was placed in the front corner of my 2,000 ft3 home theater room. For bass management, all speakers were set to small with a crossover of 80 Hz, and the STF-2 low pass filter was disabled. The phase was set to 0 degrees (this provided better results than 180 degrees). The in-room FR sweeps were processed through the AVR with only the main speakers and the STF-2 operating.
The in-room FR was measured at three popular listening positions, all about 12 feet from the STF-2. The three curves were then combined to create an average response curve. The three curve average is +/- 5 dB from 100-30 Hz, with good extension to about 23 Hz. There is some room gain in the bottom octave, which can help compensate for the human ear’s lack of sensitivity to very deep bass.
7) In-Room Maximum Output Levels:
The maximum in-room output from a subwoofer is strongly influenced by variables like room size and acoustics, boundary integrity, sub placement, the distance to the listening positions, and room gain.
To assess the maximum dynamic output from the STF-2, I conducted frequency response sweeps at the primary listening position at progressively increasing sound pressure levels until dynamic compression was noted. The STF-2 stayed linear up to about 108 dB. I also measured the distortion limited (10%) output at the listening position, and obtained 91 dB @ 20 Hz, 100 dB @ 25 Hz, and 103 dB @ 30 Hz.
Obviously, these test results are only valid in my particular room, but they should provide some indication of what to expect from the STF-2 in terms of dynamic output and clean deep bass levels in a mid-size room.
Equipment reviewer Don Keele has developed some guidelines for estimating maximum clean output from a subwoofer in a typical in-room environment. He takes the ground plane 10% THD output figures, and adds the following:
25 Hz + 8 dB
32 Hz + 6 dB
40 Hz + 5 dB
50 Hz + 4 dB
Whether the enthusiast should use the 2 meter or 1 meter ground plane data in Keele’s formula really depends on the room size and how far the subwoofer will be placed from the listening position. In my room, using the 2 meter ground plane data in the formula correlated well with my actual in-room measurements. In a smaller room with the subwoofer closer to the listener, using the 1 meter ground plane data in the formula might be more appropriate.
8) Subjective Impressions and SPL Levels For Home Theater DVD Movies:
To provide a real world indication of how loud the STF-2 can play in my mid-size HT room, I measured some sound pressure peaks from a few action-oriented DVDs.
I used an SPL meter (set to C-weighted Fast) at the listening position. The sound pressures listed are straight meter reads, with no correction factor applied. This is something the average HT enthusiast can try at home with a Radio Shack SPL meter.
For a given DVD, the playback volume was increased until minor distortion and compression artifacts were occasionally noted on the deepest and most challenging scenes. The average enthusiast (without any test gear) can rely on these audible warning signs that the subwoofer should not be pushed any harder.
If higher playback levels are desired, the STF-2 can be moved closer to the listening position (be sure to recalibrate). If moving the subwoofer closer is not a viable option, the enthusiast should consider the larger STF-3 or VTF-3.
My HT system is calibrated to Dolby Reference Level at master volume 0.0 with the Avia Guide To Home Theater DVD. Master volume settings are listed for reference purposes only.
a) Underworld (DD 5.1), Master Volume -15
Most of the bass in Underworld is in the 28-35 Hz region. The STF-2 can play loud and clean in this bandwidth, and felt very dynamic on this DVD. I was particularly impressed with its dynamics at 1:26:24, marking the end of a Sonja flashback scene.
Watery Grave (0:30:40-0:30:50): 105 dB
Sonja Condemned To Death (1:25:05): 101 dB
Sonja Condemned To Death (1:26:13-1:26:24): 105-108 dB
Sonja Condemned To Death (1:26:40): 104 dB
Endangered New Species (1:46:33): 102-103 dB
b) Matrix Revolutions (DD 5.1), Master Volume -7
Revolutions contains significantly deeper bass than Underworld, and the STF-2 occasionally missed infrasonics and lacked deep impact in certain scenes, but otherwise did a convincing job with this demanding DVD. At anything higher than Master Volume –7, the STF-2 started to sound uncomfortable on the APU stomp scene.
APU Stomp (0:59:02): 101 dB
Rumbling Before Dock Breach (1:02:42): 99-101 dB
Driller Hits Ground (1:03:20): 100 dB
APU's Firing (1:03:49): 97-98 dB (sustained)
Driller OutRigger Stomp (1:05:03): 101 dB
Kid Fires APU (1:19:17): 100 dB
c) LOTR-ROTK (DD-EX 5.1), Master Volume -9
The STF-2 delivered decent impact on the battering ram and spike ball scenes. It sounded a bit strained on the deep oliphant collision. The STF-2 partially captured the body and weight of Frodo's ultra deep heartbeat, but omitted the infrasonics and started to compress toward the end of this challenging scene.
Battering Ram Hits Door (1:46:45): 103 dB
Oliphant Stomping (2:04:20-2:04:23): 101 dB
Oliphants Collide (2:06:05): 102 dB
Witch King Spike Ball Hits Ground (2:10:10): 102 dB
Frodo's Beating Heart - Mount Doom (2:40:33-2:40:57): 97-99 dB
9) Subjective Music Evaluation:
a) Los Lonely Boys – Or Music, LLC / Sony Music Entertainment, 2003
There is plenty of aggressive electric bass guitar work on this Latino rock CD. The STF-2 sounded rich and resonant, with good distinction of notes and solid extension on low E (about 40 Hz).
b) Gladiator soundtrack - Hans Zimmer, Decca Records, 2000
This soundtrack remains one of my favorites for evaluating the musical capabilities of a subwoofer. Virtually the entire soundtrack is filled with deep, rich, and highly textured bass. The STF-2 handled Gladiator very well, playing all the varied bass notes with clarity, texture, and definition.
c) Led Zeppelin II - Atlantic Records, 1969 (1994 Digitally Remastered)
Moby Dick contains one of the best drum solos ever recorded. The STF-2 displayed excellent attack and impact on the drum strikes, sounding tympanic with just the right amount of decay and no detectable overhang.
d) Poem of Chinese Drum – Poems Of Thunder, Yim Hok-Man
This was a song from the music CD provided by Hsu Research. This mesmerizing cut contains dozens of deep and powerful drum strikes which the STF-2 handled easily, always sounding realistic and natural, without any boom or slop. The transition from the mains speakers was smooth and the STF-2 filled in the bottom octave nicely.
10) Summary And Conclusion:
The STF-2 is a really nice sounding subwoofer. One glance at the commendably flat quasi-anechoic frequency response will tell you that accuracy was Dr. Hsu’s first priority when he designed the STF-2.
The STF-2 can certainly play loud enough for any reasonable music application. On demanding and deep action-oriented DVDs, I reached its dynamic limits in a mid-size room at moderately loud playback levels (refer to sections 7 & 8 for details). If louder playback levels are desired, the STF-2 can be moved closer to the listening position, or a larger model like the STF-3 can be considered.
When operated within its clean limits, the STF-2 will reward the listener with smooth, well-defined bass and good in-room extension to 23 Hz, and represents an excellent value in the $400 subwoofer price class.
Being an STF-2 owner myself, I really enjoyed reading it!