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Hsu Research VTF-1 mk3 Subwoofer Measurements and Analysis



The Hsu Research VTF-1 mk3 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 and port side facing the microphone. The subwoofer’s gain was set to maximum, phase was set to 0, and the low pass filter was left off. Weather was recorded at 70°F and 43% humidity.

 VTF1_1port_response.jpg     VTF1_2ports_response.jpg

VTF1_sealed_response.jpg     VTF1_response_comparison.jpg 

frequency responses of the VTF-1 mk3 for different configurations 

The above graphs show the frequency responses of the VTF-1 mk3 in various configurations. In its vented configurations, we can see the VTF-1 does sport a nicely flat response. Of course, the single ported mode does have a more extended response that is flat out to 30 Hz as opposed to both ports open which is flat down to 40 Hz. Our measured responses do not quite meet the stated window of responses by Hsu Research, but it is close. It is so close that the difference could easily be due to slightly different measurement methods. One feature to note is the extended high frequency response; this is a subwoofer that could feasibly be used with a crossover point as high as 300 Hz if the user wanted. While that is not practical in the vast majority of setups, it certainly doesn’t hurt anything and might be fun to experiment with for those who are curious.

Editorial Note on the Benefit of Wide Bandwidth Subwoofers:

by Gene DellaSala

We've found it particularly important for getting the best blend between small speakers with anemic bass response to mate them with subwoofers that are flat at least an octave beyond their usable bass extension limit. In such cases, dual subwoofers could be deployed in close proximity to each of the front left/right speakers and run as a stereo pair at a higher crossover frequency (ie. 120-150Hz) to create a more seamless blend while also preserving stereo bass above 80Hz.

One important point that can be seen in these graphs is the sheer difference of extension between having both ports open with the VTF-1 set to ‘2EQ’ versus having one port open and set to ‘EQ1.’ The -3 dB point between them are the mid 20 Hz region and the mid 30 Hz region. This is by far the most significant difference I have yet seen on a variable-tuned subwoofer. Most variable-tuned subwoofers have their tuning points at the low 20’s and the mid teens, and that is not a big audible difference. That sort of difference usually only a subtle one in actual content that takes advantage of such deep frequencies. However, the difference between 25 hz and 35 Hz is very audible and can significantly alter the listening experience (providing that the content takes advantage of frequencies that deep).

This leads us to the question of why would anyone want to use the VTF-1 mk3 with two ports open when it chops off so much of the deep bass? The answer to that is it can be worthwhile trade-off if the VTF-1 mk3 offers substantially more output at 40 Hz and above with both ports open. The reason is that while modern movies can routinely take advantage of frequencies below 40 Hz, most music does not. Rock music rarely digs down as deep as 40 Hz, and even dubstep and hip-hop music seldom dig lower than 40 Hz, but they often do have a lot of energy in the range of 40 Hz to 80 Hz as the fundamental in the bass line and tuning of the kick drum. Opening both ports allows more headroom for bass in music ranges, while closing a port exchanges that mid-bass headroom for deep bass extension.

In the VTF-1 mk3, this exchange of deeper bass for louder bass is far more useful than in typical variable tuned subwoofers, since there is only so much headroom that can had from a smaller subwoofer built from modest components. With the ability to reallocate its strength towards frequency bands that suite specific content, the user has the choice between a punchier sub for music content or a deeper digging subwoofer that can catch low-frequency film content like earthquakes and rocket launches.

                                            Hsu VTF-1 mk3 CEA-2010 Burst Test Measurements

                            1 Port Open Mode                              2 Ports Open Mode                           Sealed

Frequency (Hz) SPL (dB) THD + N (%) Threshold limit SPL (dB) THD + N (%) Harmonic limit SPL (dB) THD + N (%) Harmonic limit
16 79.2 23.5 3rd, 5th - - - 81.5 14.1 5th, 7th
20 93.0 24.0 4rth 90.5 15.7
84.2 15.2 5th
25 101.0 22.1 3rd 93.2 31.5 3rd 90.7 16.2 5th
31.5 105.3 12.6
105.6 29.4 3rd, 4rth 96.6 17.0 5th
40 106.7 12.7
110.8 12.4
102.1 18.7 3rd, 5th
50 109.9 10.1
112.0 9.7
106.7 11.0
63 110.0 11.2
112.2 12.8
108.3 13.3
80 109.1 11.4
110.6 10.7
108.1 15.9
100 107.3 12.9
108.7 12.7
107.3 15.6
125 106.6 9.8
107.4 13.0
107.4 10.7

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.

In the table of CEA-2010 measurements, we can see how much is lost and how much is gained between the operating modes. One example is measured output at 25 Hz; with one port open set to EQ1, the VTF-1 mk3 has over twice as much output as opposed to two ports open. On the other hand, at 40 Hz, two ports open has sixty percent more output available versus one port open mode. In music ranges of 40 hz and above, two ports open mode has a consistent advantage, as would be expected. At 31.5 Hz and below, one port open mode has the advantage. One might think that since the recorded score at 31.5 Hz  for two ports open mode of 105.6 dB is greater than that of one port open mode’s measurement of 105.3 dB, two port open would have greater performance, but this isn’t really the case. Two ports open mode has over three times as much distortion for roughly the same output level at this frequency; it is maxing out the allowable distortion to pass this testing, whereas, in one port open mode, the VTF-1 mk3 can only achieve 12.6% distortion at the worst, which is reasonably good for being pushed as hard as it can possibly perform.

Sealed mode doesn’t really offer any advantage here. Normally a sealed operating mode can help the subwoofer achieve a smoother low end response in smaller rooms that can overboost low-frequency output through an acoustic phenomenon known as pressure vessel gain, however the VTF-1 mk3 has so precious little low-frequency output in its sealed operating mode that gain is unlikely to become a problem. In situations where the user wants to temper the deeper frequency output, I would suggest that they use it with the Q control set at 0.3, the EQ control set at ‘EQ2’, and with a port sealed. The ability to run it in sealed mode might come in useful in situation like apartments were deeper frequencies travel through walls more easily and can bother neighbors, since sealed reduces deep bass output so dramatically. 


Frequency Breakdown of CEA-2010 Burst Measurements for the Hsu Research VTF-1 mk3 

The above graphs show the measured frequency spectrum of the increasing CEA-2010 burst tests. 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 no more output was to be had from the subwoofer. 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.

The above graphs are of measurements of the VTF-1 mk3 with one port open. This can be seen by comparing the 20 Hz measurements to the 25 Hz measurements; the 25 Hz performance exhibits for more control and linearity. The 10 Hz through 16 Hz graphs show the output is dominated by distortion, since, of course, this frequency range is below the subwoofer’s intended operational range. For those who are worried about hearing a lot of distortion, the good news is that output down here is very low, since the VTF-1’s limiter doesn’t permit much sound to be generated in this range at all. The overall performance shown here is quite good until the last dB where the subwoofer is pushed to the very edge of its mechanical limits, but even then distortion does not become so severe as to overwhelm the fundamental.

What is seen here is that the VTF-1 mk3 produces clean bass for the great majority of its dynamic range of operation. It is a well-balanced system that will not be pushed into high levels of distortion above 30 Hz no matter how hard it is pushed. 25 Hz does allow for a greater levels of distortion than 30 Hz and above, but that is the bottom of its response knee. To be sure, the bass seen here is not quite as clean as much larger and more expensive subwoofers we have tested in the past, but, when not even considering its size, specifications, and price, it is still good. However, when considering its size, specifications, and price, it is exceptionally good.

VTF1_compression_sweep_1portc.jpg       VTF1_compression_sweep_2portsc.jpg


Long term output sweeps of the Hsu Research VTF-1 mk3 in different operating modes 

Testing for long-term output compression was done by first conducting a 20 second 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 thing we can see here is a more explicit view of the difference in headroom that the different ported modes make. Again, we see that 2 ports open mode has a big advantage from the mid 30 Hz range and above, while the single port open mode has a big advantage at the lower 30’s and below.

 In its 1 port open operating mode and sealed operating mode, the VTF-1 mk3 sees only mild compression affecting its frequency response at the highest drive level. With 2 ports open, compression does take a more significant toll at 70 hz and above, but for those using an 80 Hz crossover point, that isn’t going to make a big impact on the sound. From 40 Hz to almost 70 Hz, the VTF-1 mk3 is hitting 110 dB continuously. In that frequency band with both ports open, that is the kind of performance one would expect to see from a decent 12”, not a 10” subwoofer with its modest specification set. This design is squeezing the absolute most performance that can be wrought from these components. Altogether this is a heavily optimized system, and that shouldn’t be so surprising considering that this is essentially the fifth generation of this subwoofer when the VTF-2 mk1 and mk2 are considered in its lineage.

VTF1_THD_1portc.jpg      VTF1_THD_2portsc.jpg


Hsu Research VTF-1 mk3 Total Harmonic Distortion per operating mode and output level

The above graphs show the corresponding total harmonic distortion to the long-term output graphs. Essentially they depict how linear the subwoofer remains for the corresponding drive level seen in the long-term sweeps. The quantity being measured is how much of the subwoofer’s output is distortion and is shown here as a percentage. For those who are familiar with our reviews of variable-tuned subwoofers, there will be no surprises here. One thing worth mentioning is that at the 90 dB sweep, we can see a bit more distortion in the low frequencies than what is seen at the next higher sweep; this is because there is so little output in that region that environmental noise becomes a significant fraction of what is recorded, and anything not recorded as signal is seen as distortion by the test equipment. The reality is there is not much output produced by the VTF-1 mk3 in those frequency bands at all. Lower output should be kept in mind for all the regions where distortion is seen to be skyrocketing; it is spiking up rapidly as the drive level increases, but the subwoofer’s output is rapidly declining as well. This is more true for the ported operating modes than the sealed operating mode which rolls off at a shallower rate.

A t nominal drivel levels of 90 dB and 95 dB, distortion is extremely low in the various modes’ intended range of operation; non-existent for practical purposes. At the 100 dB sweep, THD rises to roughly 5% across the board. Here we can see mechanical stress is having an effect, but 5% is still very unlikely to be audible. The 105 dB sweep brings THD closer to 10% and the 110 dB sweep, where the VTF-1 mk3 is being pushed to its absolute limit, is mostly north of 10% THD. The bottom line is that this is a very clean subwoofer until it gets pushed hard, but even here it still maintains reasonably good performance. We can see it brush up against its mechanical limits when it is being throttled, but it never loses control completely. 

VTF1_2nd_3rd_harmonicC.jpg      VTF1_4rth_5th_harmonicC.jpg

Component harmonics of the Hsu Research VTF-1 mk3 for each operating mode 

The above graphs depict measurements of the constituent harmonics from the long-term output sweeps and is what the total harmonic distortion measurements are composed of up to the fifth harmonic. The individual harmonics can give us a clue as to what might be the cause of some quirk or non-linearity. There is not much surprising in the VTF-1 mk3’s measurement set here, but one thing we learn is that most of the distortion above deep bass frequencies is primarily composed of the second harmonic. This is very likely the result of induction, where the motion of the voice coil around the permanent magnet’s field causes a counter-current with its own corresponding magnetic field. This counter magnetic field interferes with the desired magnetic field between the voice coil and magnet. Its effects can be seen by higher levels of 2nd order harmonic distortion and also a reduction in higher frequency output. Both of these effects are present in the VTF-1 mk3, but neither is severe thanks to the shorting ring in the driver which helps to short out the counter current.

The third and fifth order harmonics in the sealed configuration lucidly show the effects of high-excursion stress on the driver. It is not a high-excursion driver, so it does run into a great deal of distortion in deep frequencies in its sealed mode at high drive levels, but the limiter can be seen bringing it under control again by the time it reaches down to 10 Hz. Something else than can also be seen more clearly is the effects of the noise floor in lower level sweeps, which shows up here erroneously as output from the subwoofer. The reader should keep in mind that at lower drive levels, speakers and subwoofers almost always have lower quantities of distortion, not higher quantities, and the VTF-1 mk3 would be no exception.  


Hsu Research VTF-1 mk3 group delay per operating mode 

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 VTF-1 mk3 has very good control of group delay across all three of its operating modes. Nothing here would be close to being audible, except that blip at 160 Hz, which theoretically might be audible in laboratory conditions, but since few VTF-1 mk3 owners are going to cross their speakers to their subwoofer at such a high frequency, let alone in a carefully controlled laboratory, that is nothing to be concerned about. Any of the larger peaks of group delay below 30 Hz is much too deep in frequency to be audible. As usual for variable-tuned subwoofers, the sealed mode has the lowest amount of group delay, but since nothing in the ported modes would be audible, that makes little difference. Overall, what is seen here is a reflection of all the other metrics that we have seen from the VTF-1 mk3; it is a heavily-optimized system with well-controlled behavior.


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Recent Forum Posts:

Bucknekked posts on October 12, 2017 19:01
I may be the odd man out here, but, I use the near field set up today for my little subwoofer and if I replace it with a Hsu I would still do that. Because I'm in a near field listening setup for music, the choice to measure it smack dab behind the listening position makes me smile ! I liked the completeness of the review. Reviews like this give me confidence in making a future selection. Reviews like this and I can call Pogre up and have him come over and make recommendations in person. That does the trick.
shadyJ posts on September 28, 2017 18:39
William Lemmerhirt, post: 1212839, member: 81215
Nice review, but I think the near field review should be accompanied by another one with placement more akin to where most people will use it. As you said shady, it wouldn’t be fair to intentionally place the little 10 in a spot to a disadvantage but conversely, a near field review is equally unfair. (Most people won’t be able to see through the disclaimer early in the review about typical placement behavior).
All of my review subs get placed in that near-field position, because it offers the flattest response for a single sub in my room for my listening position, not because it provides an higher tactile sensation. So the VTF-1 is on a fair footing with the other subs in my reviews, at least for the subjective experiences. If I placed it anywhere else, it would not be fair to it with respect to other subs I have reviewed.

We just have to encourage prospective subwoofer buyers to take the time to place the sub in an optimal position to get good results. Great subwoofers can be made to sound terrible with poor placement, and mediocre subs can be made to sound passable with good placement. It is such a huge factor in how a subwoofer will sound in room that it can not be stressed enough.
William Lemmerhirt posts on September 28, 2017 12:58
Nice review, but I think the near field review should be accompanied by another one with placement more akin to where most people will use it. As you said shady, it wouldn’t be fair to intentionally place the little 10 in a spot to a disadvantage but conversely, a near field review is equally unfair. (Most people won’t be able to see through the disclaimer early in the review about typical placement behavior).
BoredSysAdmin posts on September 28, 2017 09:07
another excellent review James
gene posts on September 28, 2017 01:38
Hsu's latest subwoofer, the VTF-1 mk3, is a little powerhouse on paper with a 10“ driver, 250 watt amplifier, and dual 3.5” diameter ports. We have been interested in seeing what it can do ever since it was announced late last year. Now that we have finally had a chance to give it a few laps around our test track, we are ready to report what is delivered from Hsu's $400 subwoofer (price not including shipping). Read our review of the VTF-1 mk3 to learn about this point of entry to high-fidelity subwoofage.


Read: Hsu Research VTF-1 mk3 Ported Subwoofer Review
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