Hsu Research VTF-2 mk5 Subwoofer Review

VTF-2 mk5 Introduction

It’s not easy being a middle child. The older sibling gets all the glory, and the younger sibling gets all the affection, but the middle child has to struggle just to get noticed. The same can be true of product lines from almost any manufacturer. The entry-level product is the most accessible since it is the most affordable, the top-of-the-line product provokes the most excitement since it is usually the most powerful and/or most feature-packed, but the middle product often just elicits a shrug. Hsu has recently updated the middle child of their VTF subwoofers, the VTF-2, to its fifth iteration, the mk5. In this review, we will determine whether the VTF-2 mk5 is a standout child or simply a middle rung in the ladder of Hsu’s VTF subwoofers.

The VTF-2 has been a part of Hsu Research’s VTF subwoofer line for 17 years and has developed considerably in that time through a number of iterations. The ‘VTF’ stands for ‘Variable Tuning Frequency’ and is Hsu’s line of ported subwoofers that use multiple ports that adjust the tuning point to the user’s preference. The first VTF-2 used a down-firing 10” driver and a 150-watt regular Class AB amplifier in a 2.9 cubic foot cabinet. Through every version, it has steadily grown and improved, and the newest VTF-2 sports a front-firing 12” driver, a 350 watt BASH amplifier, and 3.9 cubic foot enclosure. Happily, one aspect about the VTF-2 which has not substantially increased over the years is the price: the original sold for $499 whereas the mk5 sells for $539, and this is likely due to a combination of Hsu Research’s maturation as a manufacturer and also the lowering of cost of technology manufacturing in that time.

Unpacking and Set Up

The VTF-2 mk5 arrives in a large but not unmanageable box emblazoned with the Hsu logo. Like all Hsu subwoofers that are shipped in the continental USA, there is a demo CD in the packing envelope featuring a handful of orchestral tracks and a variety of test tones. The box is sealed with both heavy-duty staples and packing tape, and opening it reveals a thick foam top piece that looks very adequate for the task of protecting the subwoofer during rugged shipping. The VTF-2 is sandwiched by these large foam pieces, and a plastic bag covers the subwoofer to protect it from moisture. Altogether the packing is very practical and effective, and it would take particularly brutal shipment to damage this subwoofer.

The owner’s guide, which is also available on the company website product page, is appropriately accessible for novice subwoofer owners and contains good advice for fine-tuning and placement even for more experienced subwoofer owners. It’s more than a perfunctory guide and is a worthwhile read for new owners. 

Appearance

At 20.5” high and 22” deep, the VTF-2 mk5 is probably not many people’s idea of a small subwoofer. That is not to say it is gigantic but perhaps on the large side. It has a rugged vinyl black finish that is not quite as fine as the satin black on Hsu’s higher end subwoofers, but it does present a clean appearance and would not be as easily scratched. The VTF-2 mk5 will not dazzle anyone with beauty, but, with its rounded edges, it has a clean simplicity that would be innocuous in a room’s corner. It is a perfect height and shape to serve as an end table for those who want some near-field punch and can accommodate that kind of placement. With grille on, it is simple and smooth black box, but with grille off, a gleaming cone and two sleek ports exudes a readiness to rock. I prefer the VTF-2 mk5 with the grille off!

Design Overview

It is difficult to pin down the foremost feature of the VTF-2 mk5 since it is an evenly balanced design. To begin discussion of its design, we will start with the feature that most distinguishes it from many competing subs, namely the dual-port variable tuning. Hsu’s variable tuning subwoofers have been around for years now, and for those not acquainted with the idea, here’s a brief explanation. The function of ports on speakers and subwoofers is to produce sound more efficiently in 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 VTF-2 mk5’s variable tuning system allows the user to have either bass extension down to 18 Hz or more output at 25 Hz and above. The exact amount of trade-off of deep bass extension vs. efficiency at higher bass frequencies can be seen in our measurements and analysis page. The user can also seal both ports, which substantially reduces deep bass output. This can be handy in small rooms that can boost deep bass to excessive levels. Simply plugging or unplugging a port is not all there is to the VTF system. A control switch on the amplifier plate changes the frequency of a low frequency filter since the driver needs to be protected below the tuning frequency of any ported system. Below the port tuning frequency, there is not enough resistance from internal air pressure on the back of the woofer to stop the driver from flying too freely and easily bottoming out which can potentially damage it. When both ports are open, the filter switch labeled ‘operating mode’ should always be set to ‘EQ 2’, but any other combination of operating mode settings and port closures will be safe configurations to use

HSU VTF-2 mk5 Amp (left pic) and cabinet (right pic)

For the VTF-2 mk5, Hsu has upgraded the BASH amplifier to a 350-watt unit, up from the 250-watt amp of the mk4. 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-2’s amplifier, a Q control is used to manipulate the slope of the low-end rolloff. 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 90 Hz to 30 Hz gives users control over the subwoofer’s bandwidth for setups that lack bass management. A voltage input switch makes the subwoofer useable in countries with different electrical standards.

The cabinet is constructed from ¾” thick MDF all around except for the front baffle, which is a bit thicker. There is a piece of bracing midway in the cabinet that supports the ports. The feet are firm rubber rings that may help damp any mechanical vibrations from the enclosure to the floor. The grille guides use metal prongs instead of cheap plastic tabs, so the grille should not break easily. The cabinet is generously lined with polyfill that helps damp enclosure resonance. Two 3” flared ports protrude far back into the cabinet. Overall, the enclosure is a simple and sturdy design. More bracing might be welcome, but the thick paneling should compensate for that to an extent. I never felt any sidewall vibration or detected panel resonance in my time with the VTF-2 mk5. One note about the port and included port plugs is it can be a nuisance to remove the plugs if they are pushed in too far; make sure there is an edge of the port plug available from which to grab them to pull them out. They do not have to be shoved in very far to create an air-tight seal.

HSU VTF-2 mk5 Driver

Examining the driver, the first thing that jumps out about it is the beefy magnet in the motor; a stack of two ¾” iron pieces with a 6.25” diameter which form the bulk of the 16 lbs. weight of the driver. The basket is made from stamped steel of a decent thickness with a rib on the spokes for additional rigidity. A butyl foam surround holds a treated paper cone that meets a nomex spider at what appears to be a 2” diameter former. A bumped-out back plate has a pole vent for thermal management. The VTF-2 mk5 driver uses shorting rings to reduce flux modulation. Altogether, this driver looks to wisely allocate the bulk of the cost to the motor.

Hsu Research VTF-2 mk5 Subwoofer Listening Tests

The VTF-2 mk5 was placed in the location in my room where a single subwoofer has the least worst frequency response at listening position. The response is not great, but there are no peaks and dips worse than a 6 dB swing, and a narrow cancellation dip at 60 Hz. Readers should keep in mind, therefore, that in a reviewer’s listening experience with a subwoofer the room acoustics and subwoofer placement are being auditioned as much as the subwoofer itself, and this is true of any subwoofer review. Getting a reasonably flat in-room response in low frequencies almost always requires multiple subwoofers and intelligent placement. Equipment used was a Pioneer Elite SC-55 receiver and Fluance Signature Series H-Fi floor-standing speakers, and crossover was set at 100 Hz.

Music Listening

A disc said to be good for demonstrating subwoofer performance is “Mussorgsky: Pictures at an Exhibition; Stravinsky: 3 Dances from Petrouchka” (CD - Dorian #90117) performed on the pipe organ by Jean Guillou. I purchased a copy and gave it a spin. This turned out to be a terrific recording and was given weight and a sense of authority by the VTF-2 mk5. The powerful fundamentals of these compositions as played on the majestic pipe organ at the Tonhalle Concert Hall in Zurich extended down to 20 Hz and were accompanied by a stack of harmonics, which were all executed with an excellent sense of definition. The resounding polyphonic depth in the track “Catacombae” became a palpable experience that seemed to go beyond mere sound. The Tonehalle Concert Hall was given its due sense of grandeur by the VTF-2 mk5, and, although I have not been there myself, I would like to think that the VTF-2, abetted by the rest of the sound system, brought the Tonehalle Concert Hall to me.

Movie Listening

One of the first films viewed with the VTF-2 mk5 was the fourth film in the Jason Borne series titled “The Borne Legacy”, also known as “The One Without Jason Borne”. While many action films are mixed with an abundance of low frequency content, the nice thing about this particular film is that the bass is not so overdone that it swamps out the rest of the soundtrack. The bass in “The Borne Legacy” is sharp, punchy, and sporadic; it is used sparingly enough so that it fits in with the rest of the sound mix instead of dominating it, and this quality allows the subwoofer to blend in better with the rest of the system instead of merely contributing a continuous rumble of varying loudness as other sound mixes do. The VTF-2 mk5 uses this challenge as an opportunity. I could detect no overhang; the thuds of bullets and fisticuffs do not linger a moment longer than is called for. Complex content were kept as separate sounds by the VTF-2 mk5, particularly in the motorcycle chase finale, where the drone of traffic is distinct from the buzzing growl of the motorcycles. “The Borne Legacy” may not be my favorite film in the Borne series, but the VTF-2 mk5 helped to make it a fun viewing experience nonetheless.

    

One movie that looked like it could give the subwoofer a workout was “London Has Fallen”, so I decided to give it a go. After viewing the film, I feel a better title for it would be “A Series of Wildly Implausible Events,” but there is no denying it kept the VTF-2 mk5 busy. “London Has Fallen” has a whole lot of explosions, gunshots, and fistfights, none of which were mixed lightly with bass. I also wouldn’t be surprised if this film had one of the highest on-screen body counts for the year. On top of the action effect sounds was the bombastic orchestral score, which uses plenty of bass instruments like tympani and bass violins. I cranked this movie loud, and the VTF-2 mk5 had no trouble coping with the tremendous dynamic range. I did not perceive any sense of strain or overload on the subwoofer during my viewing. However I did sense a great deal of strain on my suspension of disbelief from the movie itself. 

Hsu Research VTF-2 mk5 Subwoofer Measurements and Analysis

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%.

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.

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 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.

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.

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.

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.

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.

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.

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.

Hsu Research VTF-2 mk5 Subwoofer Conclusion

Taking stock of the Hsu VTF-2 mk5, we might judge it in comparison to its similarly priced competition. At its price point, I don’t know of any other subs that can quite match its overall performance. There are some subs of similar cost that do have more mid bass output, but they do not come close to the VTF-2’s deep bass output or flat frequency response. I cannot think of any other subwoofer in this price bracket that can match its deep bass performance. Move the budget up a couple hundred dollars and you can grab a SVS PC-2000, and you can get more output capability under 30 Hz, but you do lose maximum output capability above 40 Hz. There are some subwoofers that have a smaller size and nicer finish for similar cost, but they do not come close to its performance. Then again, not everyone can handle a subwoofer the size of the VTF-2 mk5, and that is a compromise some people must make.

Another yardstick by which to measure the VTF-2 mk5 is how does it compare to its predecessor, the mk4. According to Hsu’s CEA-2010 measurements for both, the mk5 beats the mk4 by roughly 2 dB in burst output below 40 Hz and by about 3 dB at 40 Hz and above. That is a very substantial margin; effectively a 25% to 50% boost in performance for a subwoofer that costs a bit less. However, one factor in those higher measurements may be partly due to the mk5 having both ports and driver on the same surface, which is different from the mk4’s design. This can be advantageous for this type of testing, but it does not nearly account for the full extent of the gains being seen here, not even close. In comparing the mk5 to the mk4, it should be said that the mk4 did have a finer satin black finish than the mk5’s vinyl black, and the hidden driver and ports gave it a sleeker appearance. On the other hand, the mk5 does look brawnier without the grille, which is perhaps not something everyone can appreciate but is an aesthetic that this reviewer digs.

It is difficult to find something reasonable to complain about with this subwoofer. One thing I mentioned to Hsu Research was that the gain knob is a bit sensitive, as it rises at a logarithmic rate. They agreed, and future models will be seeing a more gradual gain structure, so even that tiny nitpick is now moot. Another area of improvement that it would be nice to see addressed in the VTF-2 mk5 is the bracing. While the paneling is sturdy and there is a piece of bracing midway in the sub on the lower half of the cabinet, I wouldn’t mind seeing something that reinforces the cabinet at the midsections on all the free panels. However, this critique is more for peace of mind than anything sensed during actual listening, since, as noted previously, I didn’t hear anything that I thought might be cabinet resonance, and the cabinet does not feel to be flexing when touching it during playback of loud content. It’s worth mentioning here that the protection filters work very well on the VTF-2 mk5; this is a subwoofer that will not overdrive easily. Unless the user turns the gain knob to maximum level, and blasts some 10 hz test tones at full volume, there is no way to bottom out the sub. In other words, the user would have to deliberately be trying to overdrive the subwoofer in order to accomplish that task. 

To bring this review to a close, anyone looking for a subwoofer in the $600 point ($539 + $60 shipping for continental US) would do very well to consider the Hsu VTF-2 mk5. Its overall performance and feature set is one of the best, if not the best, that can be had at this price. It achieves our Bassaholics “Large” room size recommendation in the ported modes (meaning it is suitable for rooms from 3,000^ft3 to 5,000^ft3), except the sealed mode, which nets the “Medium” room size recommendation (room sizes of 1,500^ft3 to 3,000^ft3). The VTF-2 mk5 is the least expensive sub to capture the “Large” room size recommendation, which is a nice new precedent for those looking for the most bang for their buck (more information can be learned about our Bassaholic Room Size Rating.) The VTF-2 mk5 also carries a feature set that is uncommonly good for subwoofers in this price range as well. Along with the hardware, the buyer also gets Hsu’s well-regarded customer service, a 2-year warranty on the amp (upgradable to a 5-year warranty for an additional fee), a 7-year warranty on the driver, swift shipping, good packing, and a subwoofer with manageable size and weight that isn’t an eyesore and doesn’t need a two-man team to move around. I have to give the Hsu VTF-2 mk5 a strong recommend for what it brings to the table for the cost, and if I were shopping for a subwoofer at its price point, it would very likely be my first choice.

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
Fit and Finish
Performance
Value