Hsu Research VTF-1 mk3 Ported Subwoofer Review

We started our Hsu Research VTF-2 mk5 review off by noting its relatively long history and evolution since its initial appearance. Hsu’s VTF-1 hasn’t been around as long as a model name, but its inception is so closely related to the original VTF-2 that it has to be considered an extension of that product line. The VTF-2 line started with 10” drivers and very similar cabinet dimensions and tuning points as the VTF-1, but the VTF-2 name was promoted to a line of medium sized 12” subwoofers, and the 10” variable tuning subwoofers were given the name ‘VTF-1.’ Essentially, the VTF-1 carries the form that the VTF-2 started with. In evolutionary terms, this would be called common descent: two different organisms that share a common ancestor.

The VTF-1 mk3 makes a more radical change to its form than the mk2, which only added a Q control from the mk1. The mk3 rearranges the cabinet by placing the driver and ports in front. It also gives the amplifier a boost from 200 watts to 250 and makes some alterations to the driver. Rubber feet replace spiked feet, and the satin black finish is now a smooth vinyl finish. Hsu also claims a very significant performance increase moving up to the mk3 along with a substantial price drop. This brief history of the VTF-1 has led us here with the mk3 in our hands, and we will see how these many layers of refinements over the years add up as a subwoofer in our review.

Unpacking and Appearance

The VTF-1 mk3 arrived sandwiched between eight thick foam blocks on every corner that kept the subwoofer away from the box sides and protected it from bumps and shocks. It was bagged in a soft foam wrapping to protect it from scuffs, and a thick plastic bag around the foam bag to protect it from moisture. Since it is not a massive or heavy unit, these efforts should be more than sufficient to keep the it safe during shipping.

     

Out of packing, its looks like a fairly typical subwoofer. The vinyl finish has a smooth sheen that is nice, although, as with the VTF-2 mk5, it seems to be somewhat of a downgrade from the satin black finish of the previous mk2. The rounded edges do soften its appearance, especially with the grille on. It is not a very large sub, at least in comparison to the subwoofers I normally review. It should be able to disappear in a dimly lit corner without problem if the user does not want it to stick out. Without the grille, it does look functional with the driver, ports, and grille guides on display. I do like the shiny inverted dustcap of the cone. The VTF-1 mk3 looks like a smaller VTF-2 mk5.

As with other variable-tuning subwoofers, new owners are advised to read the manual to operate them properly, even more so than with regular subwoofers. They are not quite as simple as plug-and-play if you want to get the most out of them. Owners would do well to understand how sealing the ports combined with onboard equalization mode will affect the system.

Design Overview

The VTF-1 mk3 looks like a smaller VTF-2 mk5, because, in many ways, it is a smaller VTF-2 mk5. It shares the same amplifier technology, same cabinet construction and design; only the sizes are different. As with the VTF-2 mk5, the cabinet paneling of the VTF-1 mk3 uses 3/4” MDF, except for the front baffle which is 1.5” thick. There is a piece of cross-bracing supporting the two 12.5” long ports. Each port has a 3.5” diameter. The cabinet is lined with polyfill. The rubber feet are the same, as well as the grille construction that uses rounded metal pins for the grille guide sockets.

Inside view of VTF-1 mk3 Subwoofer

The ‘VTF’ in the VTF-1 stands for Variable Tuned Frequency, which is the system that allows the user to adjust the extension of deep bass. For those who are not familiar with how this system works, we will quote from our VTF-2 mk5 review:

“The function of ports on speakers and subwoofers is to produce sound more efficiently at low frequencies than the drivers are able to. They do this by using the backward motion of the woofer into the cabinet to resonate the air mass within the port. The port’s length and width affect the frequencies at which the port generates sound; the longer a port is with respect to its width, the deeper frequencies it will produce. However, the wider a port is, the more output it will generate at its resonant frequency. Placing multiple ports in a speaker or subwoofer will act like one port that has the width of all the ports combined. In systems such as Hsu’s VTF subwoofers, the user can exchange output headroom at higher frequencies for lower frequency output by plugging one of its ports, thereby increasing the length of the ports with respect to their collective width. This enables the subwoofer to play deeper frequencies far more effectively but at the cost of output potential at some frequencies above that point.”

   

The driver looks adequate for the task but not heavy-duty. It is largely assisted by the generous cabinet size and port area. It uses a stamped steel basket, a decent sized Nomex spider for a 10”, a 2” voice coil, and a single 5” x ¾ magnetic slug. We can see that the backplate has been bumped out to make room for higher excursions, and there is a hole in the pole piece for ventilation. Hsu informed me that the driver uses a shorting ring. On the whole, the driver looks adequate for the task at hand.

The VTF-1 mk3 amplifier is similarly a scaled version of the VTF-2 mk5’s amp. For the VTF-1 mk3, Hsu has upgraded the BASH amplifier to a 250-watt unit, up from the 200-watt amp of the mk2. The BASH (Bridged Amplifier Switching Hybrid) amplifier technology combines the sound quality of AB amplifier topologies with the high efficiency of Class D amplifiers. Among the features of the VTF-1’s amplifier, a Q control is used to manipulate the slope of the low-end roll-off. Speaker-level inputs allow the subwoofer to be used with systems that do not have an LFE output or a line-level output. An adjustable low-pass filter ranging from 30 Hz to 90 Hz gives users control over the subwoofer’s bandwidth for setups that lack bass management. A voltage input switch makes the subwoofer usable in countries with different electrical standards.

Hsu Research VTF-1 MK3 Subwoofer Listening Tests

The best placement for a single sub in my room gives me a relatively flat response for an un-EQ’d single subwoofer, with a window of +/- 4 dB from 25 Hz to 100 Hz with no broad dips in important ranges. This location trades low-end room gain for a relatively flat response, a worthwhile trade for my tastes. The receiver used was a Pioneer Elite SC-55 with a variety of different speakers, and the crossover was used mostly at 80 Hz and also 100 Hz at times. Since room acoustics have a huge effect on low frequencies, the way this sub sounds in my room at my listening position is not necessarily going to be the way it sounds anywhere else for anyone else, so readers would do well to keep that in mind, and not just for this subwoofer in this review, but for any subwoofer in any review.

It should be mentioned here that the subwoofer placement for the flattest response in my room is directly behind my sofa. This is advantageous for a smaller subwoofer like the VTF-1 mk3, since my main listening position is going to get hit with a lot of its direct acoustic energy. This type of placement, called near-field placement, provides a much more tactile experience than what would normally be had from a subwoofer of this caliber. Typical placements available for most users, such as the front corner of the room, will not have this effect, and the reader should bear that in mind. However, deliberately placing the VTF-1 in a location with an inferior frequency response would not be fair for any product under review, so I decided to stick with the very advantageous near-field placement.

Music Listening

The pipe organ is famous for its powerful deep bass abilities, and one series of recordings that makes extensive use of this is the Davies Symphony Hall recitals by Michael Murray from the Telarc label. The Louise M. Davies Symphony Hall in San Francisco is home to a formidable pipe organ, and Mr. Murray’s choices of music and performance really demonstrates its potential. In ‘The Ruffatti Organ in Davies Symphony Hall: A Recital of Works by Bach, Messiaen, Dupre, Widor, & Franck,’ Mr. Murray does not shy away from very low frequencies, and strong bass digging down to the low 20 Hz region can be heard at times in this recording. Subwoofers were invented for this type of content since so few full-range speakers have significant power below 30 Hz.

The VTF-1 mk3’s performance on this album put me on notice: do not be fooled by the modest 10” driver diameter. The Davies Symphony Hall is one of the largest, if not the largest, concert halls in North America, and the VTF-1 mk3 brought about that mammoth size to my living room. The Fratelli Ruffatti Pipe Organ’s visceral power was also brought to fore by the VTF-1. It may be a subwoofer of modest size and specs, but it was able to replicate the sound of a monster pipe organ rather convincingly, at least for the healthy volume levels I was listening at. While I would guess that the VTF-1 mk3 would not be able to match the sheer power of a live performance of a huge pipe organ (a feat that few subwoofers would be capable of), I do think it can reproduce it at loud enough levels to satisfy most people in a medium to small room.

For heavily textured low-frequency content, I picked the album ‘Of Darkness and Recreation’ by the Norwegian artist Svartsinn. ‘Of Darkness and Recreation’ is an album of dark ambient music released in 2003 and sounds like the aural incarnation of foreboding and desolation. It is not a ‘fun’ album in the traditional sense of the word, but there is not much traditional about this recording at all. This is an album that uses a variety of bass sounds as a part of its sonic palette, from low-key grinding noises, ominous drones, pulsating rumbles, and the quiet roar of some distant, monstrous clamor. The sound is cavernous, and the bass is multi-layered at times and can dip to positively subterranean frequencies. It all adds up to a very gloomy, cinematic soundscape.

The VTF-1 mk3 was able to convey the various textures of bass throughout Of Darkness and Recreation, and it did so with authority. Most of the low-frequency content in Of Darkness and Recreation is a bit recessed, but it does have moments of tremendous bass, and the VTF-1 handled both elements of subtlety and high-energy with aplomb. A larger sub might have brought a bit more of the deepest frequencies out of this album, but the Hsu subwoofer acquitted itself admirably and helped bring life to the bleak scenery described by these compositions.

For something percussive and with a very wide dynamic range, I found the album ‘Heartbeat - Kodo 25th Anniversary’ to be a great demonstration of those aspects, and so I gave it a go on the VTF-1 mk3. This album is by the renowned Japanese Taiko drum group Kodo. The high-quality recording of the enormous Odaiko drum in action can give any subwoofer a chance to flex its muscles. Their primary Odaiko drum looks to have its fundamental at 60 Hz but with plenty of subharmonics below that, with many of the other drums having subharmonic content into deep frequencies as well. Kodo has long been a source of high-quality percussion recordings, and ‘Heartbeat - Kodo 25th Anniversary’ is certainly no exception. It is one of their albums that does use subwoofer-band frequencies extensively.

There are moments in ‘Heartbeat’ when the bass is simply thunderous, and the VTF-1 mk3 was able to execute that effect without any loss of poise. On passages using the mighty Odaiko drum especially, the listener is pummeled with low-frequency blows. Other drums feel more like jabs and hooks because of their rapid decays and higher frequency composition, but the Odaiko feels like a succession of powerful body blows due to its long decay and deeper bass. The VTF-1 managed each of the variety of drums with a great sense of control when they had spectral components that fell into the subwoofer range. At no point did I feel muddiness or confusion in low-frequency playback even on passages that were dense with percussion. ‘Heartbeat - Kodo 25th Anniversary’ is a fun album made all the more enjoyable with the competent bass reproduction from a subwoofer like the VTF-1 mk3. 

I also wanted to try something more contemporary and reflective of modern rock and pop recordings, and toward that end, I queued up the album ‘You Need Therapy’ by Technical Itch. ‘You Need Therapy’ is a mix of hard drum’n’bass, a genre of electronic music that makes very heavy use of bass frequencies, as its name would suggest. The bassline is nearly continuous and contains the bulk of the available energy from the dynamic range. Percussion is dense and also heavy in low-frequency energy, and it flies non-stop at a high tempo throughout the duration of the album’s running time. This type of music is unrelenting on subwoofers; at any significant volume level at all, the subs are always being pushed. It would constitute abuse to run any subwoofer at high volumes for the entirety of this 80-minute album, and that is exactly what I did with the VTF-1 mk3.

On ‘You Need Therapy’, the VTF-1 mk3 was able to punch above its weight class. I ran the LFE channel 6 dB hot, and the sheer quantity of bass this Hsu subwoofer could produce was impressive, and not just for a sub of its size and specs. Of course, larger and much more powerful subwoofers can hit harder, but the VTF-1 mk3 was delivering a true sense of weight to the music. It was more than merely loud as well; every different kick drum and bassline had its own character, and that was conveyed by the VTF-1 mk3. ‘You Need Therapy’ is a test of subwoofer stamina, and the VTF-1 was able to keep up with this brutal album without losing composure. Even though I had an idea of the capabilities of the VTF-1 mk3 beforehand, experiencing what it can do at this level of output was still a surprise. 

Movie Listening

One movie I watched using the VTF-1 mk3 is an underrated action film from 2008 called ‘Doomsday.’’ Doomsday is set in the near-future UK and is about a military officer who must venture into the savage wasteland of Scotland to retrieve an antidote for a deadly virus outbreak in London. The action in this movie is nearly constant and takes place in a variety of venues, and set pieces ranging from car chases, gunfights, swordfights, and even a gladiator tournament. The music score has variety as well, with pounding orchestral music to go with action scenes, vintage analog synthesizer music used to build tension, and 80’s pop music punctuating the film at opportune moments. This is a rowdy movie that will keep any subwoofer busy.

I played the movie loud, and the VTF-1 mk3 gave a good showing. Other larger subs are able to shake my house more, but it sounded tight and punchy and was able to give gunshots and explosions the necessary roar to underpin an exciting scene. Car and motorcycles engines growled and demonstrated how aggressive the VTF-1 could sound for loud action scenes. The music was given a healthy foundation of bass by the VTF-1, even amidst the din and clamor of action. Doomsday is a rough and tumble movie that needs a capable audio system to do the experience justice, and I can report that the VTF-1 mk3 is up to the task.

Another movie I watched was the now classic 1999 film ‘Fight Club,’ which certainly needs no introduction at this point. David Fincher’s brawler has many passages with strong low-frequency content and makes for a great exhibition of effective use of bass. Car crashes, plane collisions, explosions, and the meaty thuds of the fist fights populate the soundtrack of fight club, along with the Dust Brothers’ beat heavy music score, but while bass is plentiful, it is not overused. Some films overuse bass to the point that the sound mix becomes one long rumble. In Fight Club, it serves as punctuation in a sound track, as opposed to the constant use of bass which, I would say, is analogous to typing in all capital letters, which can be inuring.

The VTF-1 mk3 gave a terrific foundation to the sound mix of the Fight Club. Punches, explosions, and kick drums were given their due weight. The bass was tight and never confused different low-frequency sounds, for example, when the bass-laden percussion of the music played during a fight scene with lots of bass-heavy pounding of fist cuffs. The VTF-1 mk3 may not be able to dig quite as deep as larger subwoofers, but, to be honest, I didn’t miss whatever minor bit of ultra low-frequency content that escaped the VTF-1’s capabilities. The VTF-1 mk3 can catch sound down to the mid 20’s Hz range, and that will encompass the vast majority of bass in most movie sound mixes. Maybe if I did an A/B comparison of the VTF-1 mk3 against a much deeper digging sub during sections of Fight Club, that would answer the question of how much the experience is changed, but as it is, to my ears, the Fight Club soundtrack felt complete with the VTF-1 mk3.

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.

     

      

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.

      

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.

     

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. 

     

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.

Hsu Research VTF-1 mk3 Subwoofer Conclusion

Normally in this part of the review, to sum up a product I briefly go over some of its pros and cons, but the VTF-1 mk3 is a modestly-priced, modestly-sized subwoofer, and compromises have to be made within that context, so it seems absurd to nitpick small flaws when looking at its overall performance. This is a $400 subwoofer that can competently dig down to the mid 20’s Hz range with good linearity. I don’t know of any equally priced subwoofer that can dig as deep while keeping a neutral response. It can hit surprising loudness levels while doing so as well.

However, if you crank it really hard, it does lose some of its composure at the low end, mostly below its tuning point in one port mode. Port turbulence can become fairly audible when the VTF-1 mk3 is pushed hard below the mid 20’s Hz range in one port mode. If you crank a movie that has very hot deep bass, it might complain a bit. While it can be made to make some flutter noise and port chuffing, something I was not able to induce was a hard-bottoming noise of the voice coil former hitting the back plate. That should help instill peace of mind since that is the one noise above all others that I would rather a sub not make because it can potentially mean the voice coil former is being damaged which will destroy the driver.

So the VTF-1 mk3 isn’t made to plummet the deepest depths of audio, but it may very well be the deepest digger at its price point. It digs deep enough to catch most of the bass in film and even more bass in music since music does not typically dive below 40 Hz. The difference between its ported tuning modes occur precisely where one would want as a divider between the bass extension in movies and music. As mentioned before, the choice of tuning points in the VTF-1 mk3 makes its variable tuning feature more useful than most variable tuning subwoofers. As a rule of thumb, for conventional music, use it in 2 ports open mode, and for movies, use 1 port open mode. Of course, there is music that digs deep and movies that don’t, so adjust it accordingly. It only takes a moment to swap a port plug and change the EQ switch. The VTF-1 mk3’s size, price, and specifications necessitate performance compromises, but the variable tuning feature allows the user to partly decide where those compromises will be made.

In terms of performance, the bottom line is that the VTF-1 mk3 is exceptionally good for a subwoofer at this price point. In its 1 port mode, it just barely misses our Bassaholics ‘Large’ Room Rating by 1 dB for the frequency band of 31.5 Hz to 63 Hz, so it is awarded our ‘Medium’ Room Rating. In instances in the past, we have awarded a subwoofer a higher room rating when its measurements are very close to technically qualifying where we felt it could handle a large room in actuality. In the case of the VTF-1 mk3, in truth it is not intended for a large room, so we are not going to give it that pass. It is a great sub for a 1,500 ft^3 to 3,000 ft^3 room, but it may struggle in deep frequency reproduction in larger rooms. On the other hand, for content above 40 Hz, in its 2 ports open mode, it may be able to tackle larger rooms, so I would say it depends on the user’s application.  

In terms of appearance, the VTF-1 mk3 isn’t going to win any beauty contests, but it is not really an eyesore either, so it is not bad but not exceptional. Its feature set is above average in its class, but, outside of the variable tuning ability, not extraordinary. One nice thing about these small subwoofers is that they are small and light. They are easy to transport and easy to place. Pretty much any able-bodied human being above the age of ten can cope with this 42 lb. subwoofer. That is a bigger advantage than is generally realized by audio enthusiasts. There are a lot of folks who want a great bass sound but not the daunting prospect of a cabinet that is the size and weight of a large piece of furniture. The VTF-1 mk3 makes that possible, and it also makes multiple subwoofer systems easier to implement since it doesn’t eat as much floor space.

The ease that the VTF-1 mk3 makes in setting up a multiple subwoofer system is a real asset. Multiple subwoofer systems have considerable sound quality advantages over single subwoofer setups beyond merely having more headroom, as we at Audioholics have written about time and time again, such as in article like this: Are Two Subwoofers Better Than One? Hsu encourages this by offering a $25 discount on each VTF-1 mk3 for the purchase of multiples.

    

The VTF-1 mk3 is a relatively inexpensive subwoofer, but it isn’t quite entry level. Most entry-level subwoofers have to give up much more with respect to performance. They cost less, but they typically do not have a flat frequency response nor do they have much deep bass extension. The VTF-1 mk3 is one of the least expensive subs where a flat frequency response can be had, and it goes down to below 30 Hz. Most less expensive subwoofers can produce a bass sound, but they don’t do it very evenly across the low-frequency spectrum. The VTF-1 mk3 is an affordable subwoofer that gives the listener accurate playback. It is an entry to a level of performance where subwoofers become much more linear: an entry-level high-fidelity subwoofer.

The Score Card

The scoring below is based on each piece of equipment doing the duty it is designed for. The numbers are weighed heavily with respect to the individual cost of each unit, thus giving a rating roughly equal to:

Performance × Price Factor/Value = Rating

Audioholics.com note: The ratings indicated below are based on subjective listening and objective testing of the product in question. The rating scale is based on performance/value ratio. If you notice better performing products in future reviews that have lower numbers in certain areas, be aware that the value factor is most likely the culprit. Other Audioholics reviewers may rate products solely based on performance, and each reviewer has his/her own system for ratings.

Audioholics Rating Scale

• — Excellent
• — Very Good
• — Good
• — Fair
• — Poor

Metric	Rating
Bass Extension
Bass Accuracy
Build Quality
Ergonomics & Usability
Features
Dynamic Range
Performance
Value