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Hsu Research VTF-2 mk5 Subwoofer Measurements and Analysis


VTF2 outdoor testing.jpg

The Hsu VTF-2 mk5 was tested using ground plane measurements with microphone at a 2 meter distance in an open setting with well over 100 feet from the nearest large structure. The sub was tested with woofer facing the mic. The subwoofer’s volume knob was set to maximum, Q control was set to 0.7, and the low-pass filter was switched off. Weather for testing was recorded at 62° F with humidity at 40%.

 VTF2 freq responses.jpg

Frequency responses of the main operating modes of the VTF-2 mk5

From the frequency response sweeps we see that the VTF-2 mk5 is impressively flat, especially in 1 Port Open mode, easily meeting Hsu’s claimed +/- 2 dB from 18 to 200 Hz response. 2 Ports Open mode sees a rise at the lower end of its response with the Q Control at 0.7, but when turned to 0.3, it also falls well into Hsu’s claimed response of +/- 2 dB from 22 Hz to 200 Hz. Even the sealed mode holds an impressively flat response down to 20 Hz, although the shape of the response is very different from the ported modes. In any mode the VTF-2 mk5 has enough linear upper frequency bandwidth to be used well above 200 Hz if so desired.

VTF2 1 port Q diff.jpg

VTF2 2 port Q diff.jpg

VTF2 sealed Q diff.jpg

Hsu VTF-2 mk5 differences in response to Q settings changes

The Q control allows the low end of the VTF-2 mk5 to roll off at a different rate. In addition to adjusting the Q, the user can set the EQ mode switch to roll the low end off at higher frequencies by setting the EQ switch to EQ2 with one port open or all ports sealed; this would be an ‘overdamped’ mode of operation. With a combination of Q settings, port configurations, and EQ mode settings, the low end of the VTF-2 mk5 can be made to have dozens of different response shapes.


Hsu VTF-2 mk5 CEA-2010 Burst Test Measurements


1 Port Open Mode

2 Ports Open Mode


Frequency (Hz)

SPL (dB)

THD + N (%)

Threshold limit

SPL (dB)

THD + N (%)

Harmonic limit

SPL (dB)

THD + N (%)

Harmonic limit










3rd, 5th




3rd, 5th



3rd, 9th






























































































HSU VTF-2 mk5 CEA-2010 2 meter RMS Data

The above CEA-2010 measurements show the subwoofer’s clean peak SPL before heavy distortion sets in. Our measurements have been referenced to 2 meter RMS, which is 9 dB down from the standard requirement for the measurements to be shown at 1 meter peak. However most publically available CEA-2010 measurements are shown at 2 meter RMS, so we followed that convention.

The Hsu VTF-2 mk5 sports a terrific set of CEA-2010 measurements considering it size and cost. In fact, it tracks very closely with the measurements made from Audioholics’ review of the Outlaw Audio LFM-1 EX made 5 years ago, and in a handful of instances matching the test frequencies output exactly. However, the VTF-2 mk5 is less expensive and a bit smaller; this is what they mean about the march of progress. One minor note: we did manage—surprisingly—to get a passing 10 Hz measurement of 81.1 dB, and that might have been due to a fleeting low noise floor moment. We weren’t able to repeat that result and so decided to not include it in our table above, but we feel it’s worth mentioning since in a lower noise-floor environment we do think we could achieve that again.

CEA-10-20x1000c.jpg CEA-25-50x1000c.jpg


Frequency Breakdown of CEA-2010 Burst Measurements for the Hsu VTF-2 mk5

The above graphs show the frequency spectrum of the increasing CEA-2010 bursts as reproduced by the VTF-2 mk5. Essentially, it depicts the behavior of the subwoofer reproducing short burst tones at successively louder levels, with each test tone raised by boosting the input gain by 1 dB until either no more output was to be had from the subwoofer or the subwoofer started making too many mechanical stress noises for the tests to be safely continued. The frequency marked above the graphs note the fundamental tone being tested, and this can also usually (but not always) be discerned in the graphs by the horizontal axis frequency point of the “main ridge,” the highest levels on the vertical axis. The noise below the fundamental (that random spikiness to the left of the main ridge) should be ignored. What should be looked at are the smaller ridges to the right of the fundamental. These are the distortion products of the fundamental, and it is here where we see how cleanly the subwoofer handles a given output level. These are mostly harmonics: whole number multiples of the fundamental.

These results are all quite good for a sub with the VTF-2 mk5’s specifications.

A few interesting but unsurprising results can be seen in these tests. What is seen is pretty much textbook behavior for a subwoofer of this type. As the sub approaches its limits, distortion grows rapidly. A good illustration of how long excursions affect driver behavior can be seen in the 12.5 Hz graph, where we have angled the graph for a better view; odd-order harmonics dominate, which is what is expected when it is reaching past the edge of its linear throw. The stiffening suspension or the reduction of the magnetic field at the outside of the throw is likely the culprit, since those affect both directions of travel. Compare that to the upper frequency measurements where the even-order harmonics are dominant; the excursion does not reach far, so other elements becomes the cause of the leading nonlinearities, elements which are only affecting one side of the woofer’s travel.

Theory is fun and all, but how well did the VTF-2 mk5 perform? These results are all quite good for a sub with the VTF-2 mk5’s specifications. Distortion is very heavy at frequencies below the tuning point (18 Hz as tested here), but that is expected since it is outside the subwoofer’s intended range of operation. Above the tuning frequency, distortion only becomes significant at the very highest drive levels. Don’t think that just because you can see the distortion in these graphs that they can be heard; much of the distortion through these graphs are 20 to 30 dB beneath the fundamental which is well below audibility (we have an article discussing the audibility of distortion in bass frequencies here). What is apparent is a great deal of care was invested in optimizing the performance of the VTF-2 mk5. Distortion is kept under control for most of its operational range, and it stays clean even at very loud levels.

1 port longtermC.jpg  2 port comp sweepC.jpg

sealed compression sweepC.jpg

Long term output sweeps of the VTF-2 mk5 in different operating modes

Testing for long-term output compression was done by first conducting a sweep tone where 50 Hz hit 90 dB with the subwoofer 2 m from the microphone. We then conduct further 20 second sweeps by raising the gain by 5 dB until no more output could be rung out of the subwoofer. One slight oddity we see is that the sealed mode has a tad more mid bass headroom, which is not a result I would have expected. I sent a question what might cause this to the VTF-2’s designer, Dr. Hsu (guess who the company is named after?) He speculated that the woofer’s resonance will be higher in the enclosure with the ports open, and so the impedance in mid bass frequencies will be higher for the ported modes meaning the amplifier would not be able to deliver as much power to the driver versus the sealed mode.

The long-term sweeps depict very good behavior in every operating mode. No compression artifacts surface until the 105 dB sweep below 40 Hz, except for the 2 port mode where no response shape differences surface until the 110 dB sweep. Comparing the burst output to the long-term output, we see that they are very close in deep bass, but that the VTF-2 mk5 has quite a bit more short-term power than long-term power in mid bass frequencies. The high short-term RMS output of the BASH amplifier’s soft power supply is demonstrated more in the upper frequency tone bursts, which are much shorter than the lower frequency bursts.

SealedTHD.jpg     1portOpenTHDc.jpg   2portsOpenTHDc.jpg

Hsu VTF-2 mk5 Total Harmonic Distortion per operating mode

The above graphs show total harmonic distortion, essentially depicting at what drive level and frequency the subwoofer loses linear playback. As with the long-term output sweeps, there is not much surprising here, but there are some noteworthy features. We do see a correspondence of where distortion rapidly rises in these graphs with where the frequency response compresses in the long-term sweeps. We see where the port output alleviates driver motion below the distortion bump above the port tuning point. These are standard characteristics of subwoofer distortion profiles. The VTF-2 mk5 fares very well in these measurements. Distortion does spike heavily when the subwoofer is driven hard, but it mostly occurs below the tuning points of the operating modes, and this is where output is quickly falling off anyway, so while the percentage of distortion might be high, any sound at all produced by the subwoofer will be low.

One thing to note is the very low distortion levels in the range of music (40 Hz and above) even at the very highest drive level, where distortion does not rise above 10%. This is all at continuous loudness levels well above 105 dB at 2 meters. This makes the VTF-2 mk5 a great sub to blast music at high volumes, since it remains clean in those frequencies no matter what. And below the highest drive levels, at loud but not enormously loud levels, the VTF-2 mk5 coasts by in this range, for example hovering between 1% and 2% THD at a continuous 95 dB level. This is excellent performance for a subwoofer in this price[1]  range, matching the performance of much more expensive ‘high-end’ subwoofers.

1 port 2nd 3rd HDc.jpg      1 port 4rth 5th HDc.jpg

2 port 2nd 3rd HDc.jpg     2 port 4rth 5th HDc.jpg

sealed 2nd 3rd HDc.jpg   sealed 4rth 5th HDc.jpg

Component Harmonics of the Hsu VTF-2 mk5

The above graphs are the component harmonics of the total harmonic distortion from the VTF-2 mk5. As with the burst test graphs, the chief offender as the sub gets pushed hard is the odd-order harmonics, 3rd and 5th. The even-order harmonics rise at a steadier rate with increased output, instead of precipitously at the highest drive levels. This behavior is in line with what would be expected from this type of subwoofer.

VTF2 group delay.jpg

Group delay is the measurement of how much time it takes for individual frequency bands of an input signal to be produced by the speaker. It can indicate that some frequency components are developing slower than others or are taking longer to decay. It is generally thought that 1.5 sound cycles are needed for group delay to be audible at bass frequencies, although there is an argument that group delay should remain under 20 ms to be completely unnoticeable, but that is likely meant for mid and upper bass frequencies. The group delay of the VTF-2 mk5 steers well away from anything that could be audible. The points where group delay rises above 20 ms or even 1 cycle are too deep to be perceptible. The VTF-2 mk5 exhibits very little overhang or stored energy. The sealed mode naturally makes the best showing here, since there is no lagging wave exiting a vent. In the other operating modes the frequencies in the port’s output range look to have the most group delay, and that is an expected result.

I’d say 1-2% THD at 95dB is excellent for a ported sub at any price, since the residual distortion will always be from the ported driver’s mechanical non-linearities. At modest SPL, under 100dB, a properly-designed sealed sub with the correct driver will have lower THD, since compressed air is essentially perfect and at lower SPL, the air—not the driver’s mechanical suspension—dominates the distortion. At higher SPL, ported subs have lower THD, since this situation is reversed and the air in the port doesn’t distort, while a sealed sub has to provide 4x the excursion for 6dB more output, so the sealed driver’s mechanicals do dominate the distortion at that point.


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Recent Forum Posts:

eralimited57 posts on October 07, 2020 07:56
shadyJ, post: 1423683, member: 20472
For output tests, the Q was set to 0.7. The ‘EQ’ switch was set to ‘2EQ’ for two ports open and sealed, and ‘1EQ’ for one port open.
I set Q to 50, switched to EQ1, 1 port plugged, 1 port open.

One more comment on the SVS subwoofer that I tried out. It sounded flat and boomy with no musicality to it. The HSU sounded just the opposite.
shadyJ posts on October 06, 2020 23:21
paulgyro, post: 1423567, member: 92131
I just got a c-stock VTF-2 mk5 and call me impressed.

Looking at the AH review in detail on this sub, the measurement charts don't say what Q control and EQ settings were used. I know this review is old now but it would be nice to know.
For output tests, the Q was set to 0.7. The ‘EQ’ switch was set to ‘2EQ’ for two ports open and sealed, and ‘1EQ’ for one port open.
paulgyro posts on October 06, 2020 22:39
eralimited57, post: 1423680, member: 84364
That's strange I don't smell any BS here, must be coming from your end.
Thanks for that, I was just wondering what the Audioholics review used for the EQ and Q settings, it only mentioned what ports it used.
eralimited57 posts on October 06, 2020 22:37
eralimited57, post: 1423571, member: 84364
Give Dr. HSU a call or send him an email. He is extremely helpful and responds in a very timely manner.
You can also go to the HSU website and pull up the manual for the subwoofer you're interested in and in there it will explain in detail the functions therein.
eralimited57 posts on October 06, 2020 22:35
That's strange I don't smell any BS here, must be coming from your end.
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