Hsu Research ULS-15 mk2 Subwoofer Review

After feedback from customers who wanted a subwoofer that was friendlier to living room decor than the behemoth ported boxes and tall cylinder subs Hsu had been producing, Hsu Research released their first sealed subwoofer in 2008- The ULS-15. Hsu had already built a solid reputation for deep-digging, accurate, and affordable subwoofers. The caveat had always been size per Hoffman’s Iron Law which states that you can have two of the following, but never all three: sensitivity, small enclosure size, and deep bass. Low sensitivity necessitates a powerful amp and heavy-duty driver, which raises costs considerably. So, in an effort to make their subwoofers affordable, Hsu had traditionally decided to compromise on size instead. However, a strong demand for subwoofers with a more attractive size and shape prompted Hsu to break with their tradition in the ULS-15. While the ULS-15 went on to successfully fill its niche in the Hsu line-up, the years went by and new technologies made a compelling case for an overhaul, which brings us to the ULS-15 mk2.

The ULS-15 mk2 shares some of the same design features of the mk1, but also some important differences. However, before we get into design discussion, let’s begin where most users would with the ULS-15 mk2: unpacking.

Unpacking and Setup Guide

   

The ULS-15 mk2 showed up in a large box proudly sporting the Hsu logo. The external packaging is serious, with both packing tape and heavy-duty staples holding the box shut. There is certainly no danger of the subwoofer falling out, as I had to use pliers to pull out the staples in order to open the box.

      

Inside the box, removing a light foam cover reveals an inner box and some heavy cardboard corner protection pieces. Inside the inner box we see a large heavy-duty foam top and bottom piece neatly sandwiching the subwoofer. The subwoofer itself is wrapped in an internal layer of a soft foam-ish plastic cover and an external layer of a heavier plastic covering, presumably to protect it from liquids and moisture. The packaging overall consists of double-boxing, double foam insert shock-absorbtion, both tape and staple sealing, and double-bagging. This is the best packaging I can recall seen on a subwoofer. I guess Hsu really hates dealing with shipping damage claims.

The included Owner’s Guide is well done if not exhaustive. It has a lengthy trouble-shooting section and contains lots of tips and advice about optimizing subwoofer performance as well as basic instructions for subwoofer novices. It is also available on the ULS-15 mk2 website product page as PDF file. One thing that might be easy to miss for those who are unpacking the ULS-15 mk2 is the Hsu demo disc that is shipped in the packing list envelope. This CD contains a handful of tracks of classical music that makes for excellent demo material, and also a number of different subwoofer test tones. The demo disc is a nice touch, and the first track, the Saint-Saëns Organ Symphony excerpt which contains a powerful 16 Hz fundamental from a pipe organ, is a real corker for subwoofers.

Appearance 

The ULS-15 mk2 does not look dramatically different from the original ULS-15,  which sported a conventional and tasteful appearance. The mk2 cabinet is the same, and the only obvious visual difference is the cone. With grille on, they look identical, except for markings on the plate amplifier. The ULS-15 mk2 comes in satin black finish and also a real rosewood veneer for an additional $150. The satin black of this review unit is a very smooth black that is not polished-level shiny but certainly not a matte-black light sink either. It reflects light but in a diffuse manner. The cabinet itself is essentially an 18” cube with rounded longitudinal edges; this symmetry lend it a stylish simplicity that would make for a great fit in modern decor, but its conservative demeanor lets it blend in with more traditional interiors, especially in the Rosewood veneer. For those who want minimalism, the grille makes the ULS-15 mk2 look nearly featureless. However I prefer it with the exposed cone; the glossy woofer center and beefy surround make the mk2 look poised to do some serious rocking.

 

Design Overview

The 18” cube cabinet is constructed with 3/4” MDF side-walls and a 1.5” front baffle. A ¾” window brace helps support the driver while reinforcing the side-walls at the center length of the cabinet. The sidewalls are neatly lined with egg-crate type foam for stuffing, which is a departure from the mk1 which used fiberglass wall insulation for stuffing. The foam stuffing looks like it would work well, while staying well out of the way of the driver and amplifier. The feet are sturdy rubber rings that do well to protect the cabinet when setting it down, although they make it awkward to move the sub by sliding.  These feet look like they would make use of a subwoofer isolation pad a moot point, much like SVS’s SoundPath Isolation feet. Altogether the ULS-15 mk2 sports a simple and sensible enclosure for a sealed subwoofer of this size. A higher-end and more expensive subwoofer might sport more bracing and thicker walls, but that would only add weight and expense for negligible returns. The ULS-15 mk2 cabinet construction is orderly and very pragmatic.

 

The biggest point of departure of the mk2 from the original ULS-15 is the driver.  The first ULS-15 used an underhung XBL^2 motor. A design goal of the XBL^2 design was greater linear excursion due to a more uniform magnetic field for the voice coil to travel in (for those who want to get into the details of the XBL^2 technology, here is a good starting point). Indeed, the ‘ULS’ stands for Ultra Linear Sealed, denoting the extraordinarily long linear stroke delivered by the XBL^2 design. Another characteristic of the XBL^2 design is a wider-band driver than conventional topologies, since not as much voice coil mass is needed, thereby greatly reducing induction. However, a disadvantage of the XBL^2 design was a hefty penalty in sensitivity. Hsu dispensed with the XBL^2 design in favor of a more traditional overhung design in the mk2. While the XBL^2 design was an interesting approach in the mk1 driver, the mk2 driver is no slouch. A stack of two hefty 1” x 6.75” magnets comprise the bulk of the motor.  The pole piece is undercut for a better magnetic field symmetry, and multiple shorting rings reduce induction effects which reduces even-order distortion. The mk2 uses a 12 spoke aluminum basket and a Butyl foam surround holds a fiberglass-impregnated cone in place.  

The amplifier is a 600-watt continuous BASH amplifier. BASH amplifiers have been around for a while and combine the efficiency of Class D amps with the fidelity of class AB amplifiers (although that is somewhat of an over-simplification). The ULS-15 mk2 amplifier has a host of noteworthy features. Balanced XLR inputs make it a good choice for higher-end and pro-audio setups. Speaker-level inputs make it a good choice for setups with no line-level outputs such as older receivers and integrated amplifiers. An operating mode switch can set the frequency response flat down to 20 Hz in the ‘EQ1’ mode or a more typical roll-off to sealed subwoofers with a shallow slope starting at 50 hz in ‘EQ2’ mode. A ‘Q control’ knob changes the slope of the response; lower Q settings will make for a steeper rolloff and mid bass will be accentuated more. There are also the more traditional features on the amplifier such as a 0°-180° phase switch and a 30 Hz to 90 Hz variable low-pass filter. One nice touch is the 120V~60Hz/240V~50Hz voltage selector which makes for easy use in electrical standards around the world.

Hsu Research ULS-15 mk2 Subwoofer Measurements and Analysis

Outdoor ground plane testing of the Hsu ULS-15 mk2 

The Hsu ULS-15 mk2 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, mode was set to EQ2, Q control was set to 0.7, and the low-pass filter was switched off. Weather for testing was recorded at 74° F with humidity at 50%.

 

Hsu ULS-15 mk2 Frequency Response 

Looking at the frequency response, we can see that the ULS-15 mk2 has an extraordinarily flat response. The website product page for the mk2 states a 20-200 Hz, +/- 1dB, and this is essentially correct, at least for the EQ1 mode; this is an especially remarkable response for the fact that it accomplished with an analog amplifier. The EQ1 response uses a significant boost in deep frequencies to achieve such an impeccable response, and we can see a 12 dB lift centered at 23 Hz over the EQ2 response. It then falls off at a very steep rate, presumably to protect the driver from over-excursion. The EQ1 curve mimics that of a ported subwoofer, and, while this can be advantageous in some situations, it can present some problems in other situations. The subwoofer can only sustain that response as long as it has enough linear throw, and, as you turn up the volume, it can run out of excursion very quickly in deep frequencies, so this EQ1 curve is only useful up to a certain point. After that point, the driver is pushed past its linear excursion, and heavy distortion starts to set in, so the EQ1 mode is not where you want the subwoofer set if you intend to listen to content with deep bass at loud levels.

The EQ2 response more closely resembles that of a traditional sealed subwoofer and is also quite flat, achieving a +/- 1 dB window from 40 Hz to 200 Hz. I would recommend this mode for small rooms since room gain will naturally shore up the deep frequency response. I also recommend this mode for those occasions when the sub will be played loud with deep frequencies present in the material, since the driver is more restricted in deep bass, which will allow for more headroom. The response of either mode stretches up to nearly 200 Hz, allowing for a high crossover point if desired. This can be handy for those who want more dynamic range than can be had from their main speakers in mid and upper bass frequencies above the conventional 80 Hz crossover point, although it is recommended to have multiple subwoofers to do this in order to avoid localization of the subwoofer. Personally, I thought the system sounded much fuller with the use of a 100 Hz crossover, since the ULS-15 mk2 is so much more potent in that area than my tower speakers. Very few speakers, even large towers, will have mid bass dynamics of the ULS-15 mk2.

  

Hsu ULS-15 mk2 Q Control Effect on Frequency Response 

The measured effect of the Q control is not dramatic but it does change the response. By de-emphasizing deep frequencies, bass can be made to sound tighter, since deep bass isn’t as readily perceived as higher frequencies. The Q control may also be helpful in adjusting the amount of room gain to desired levels.

 

Hsu ULS-15 mk2 Internal crossover response effects 

 

The internal crossover is based on a simple Linkwitz-Riley filter and uses a 24 dB/octave slope. Internal crossovers in subwoofers still have their place in systems without bass management, such as systems using integrated amps, or simple computer audio systems that use the computer as the pre-amp for active speakers. Since the crossover in the ULS-15 mk2 seems to affect the gain, level matching will have to be done after the internal crossover is set. Note the ULS-15 mk2 does lose its ruler-flat response with the crossover engaged in EQ2 mode.

Hsu ULS-15 mk2 CEA2010 Maximum Clean Output Measurements, referenced to 2 meters ground plane RMS
Test Frequency (Hz)	Max Passing Measurement (dB)	Total Harmonic Distortion (%)	Harmonic Threshold Limiting
10	No Passing Result
12.5	90.7	15.0	3rd, 9th Harmonic
16	95.0	16.1	3rd Harmonic
20	99.4	18.7	3rd Harmonic
25	104.7	19.2	3rd Harmonic
31.5	110.4	19.6	3rd Harmonic
40	115.3	9.0
50	117.8	13.2
63	118.2	13.1
80	118.1	15.3
100	117.7	13.5
125	117.7	5.3

 

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 ULS-15 mk2 is able to post some excellent measurements. They are fairly close to Hsu’s own measurements. Hsu’s own measurements should be regarded as conservative, since their own tests are conducted near a freeway thereby raising their noise floor. Of note is the passing measurement at 12.5 Hz; 90 dB at such a low frequency is no small task, especially with only 15% THD (of 2nd - 5th harmonics). Also notable is the tremendous mid bass headroom, averaging around 118 dB at 50 Hz and above. It is no wonder Hsu has discontinued their mid-bass module, the MBM-12; the ULS-15 mk2 has substantially more mid-bass power than it. The third harmonic is the chief offender in nearly every measurement, and it is what prevents the subwoofer from achieving even higher SPL readings at frequencies where the amplifier has enough juice to push the driver out of its comfort zone. However, with a driver this formidable, its ‘comfort zone’ occupies a fairly wide dynamic range, and significant distortion does not set in until relatively loud levels. 

Frequency Breakdown of CEA-2010 Burst Measurements for the ULS-15 mk

The above graphs show the frequency spectrum of the increasing CEA-2010 bursts as reproduced by the ULS-15 mk2. 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.

Some noteworthy features emerge from these graphs. We can see that there are levels of 10 Hz output that are not completely swamped by harmonic distortion, so the ULS-15 mk2 does permit some relatively clean 10 Hz output, to a point. This is even more true of 12.5 Hz, where there is relatively clean output until just past 90 dB, where the third harmonic rapidly increases. When pushed to large excursions, the driver ends up producing a lot of third order harmonic distortion. Since higher frequencies don’t need as much throw, we see much less distortion at nominal levels as the test frequency rises. One characteristic that can be seen are the different ways 2nd and 3rd harmonics rise. 2nd order harmonics maintain a more consistent increase that almost correlates with the increase in gain in the fundamental. The 3rd harmonic tends to erupt after a point, and this would be an indicator that the driver is reaching the end of its linear excursion.

It should be said here that anyone looking at all of these ridges of distortion and drawing the conclusion that this is somehow a highly-distorted sound or poor performance showing would be wrong. For the vast majority of the dynamic range of the ULS-15 mk2, distortion is inaudible. Until the loudest levels, there is a very large power delta between the fundamental and distortion products. Just because distortion products can be seen in these graphs does not mean it can be heard. Most of this distortion will be totally masked by the fundamental; for those readers who want to know more about the audibility of distortion at bass frequencies, we point you to this Audioholics article. 

Hsu ULS-15 mk2 Long-Term Output Compression

Testing for long-term output compression was done by first conducting a sweep tone where 50 Hz hit 90 dB, and then we conduct further sweeps by raising the gain by 5 dB until no more output could be rung out of the subwoofer. As with the CEA-2010 measurements, the long-term measurements show the ULS-15 mk2 is a force to be reckoned with, roughly averaging 115 dB from 40 Hz to 150 Hz. We do see the natural response compress a bit at the highest two sweeps around 20 to 30 Hz and also from 200 to 400 Hz. The slope of the band of compression would seem to suggest that pushing the gain even more would have resulted in more output at lower frequencies but not anywhere else, and this is also hinted by the 12.5 Hz CEA-2010 burst measurement, where the angle of that compressed slope would have intersected 12 Hz at around 90 dB which correlates with the 90.7 dB CEA-2010 measurement at 12.5 Hz.

Total Harmonic Distortion of the ULS-15 mk2 as a percentage 

The total harmonic distortion measurements in the above graphs correspond to the long-term compression sweeps; they are the harmonic distortion measured during those sweeps. The distortion is presented here as a percentage of the fundamental, or how much of the sound produced by the subwoofer is distortion versus intended test tone reproduction. The graphs show the ULS-15 mk2 to be a very clean subwoofer until pushed to the very edge of its performance envelope, where a spike of distortion erupts from 15 Hz to 25 Hz. Distortion subsides below 15 Hz due to a filter that gives the low end a parabolic slope. The 15-25 Hz spike of distortion is the woofer over-driving the filter, and this can also be seen in the long-term compression sweeps, where the shape of the response changes from curved to flat in that frequency band at that output level. The distortion crest in the deepest frequencies in the lower power 106 dB and 111 dB sweeps is partly caused a lower signal-to-noise ratio at those frequencies and output levels; in more ideal testing conditions it would be lower like the same frequency range in the 115 dB and 116 dB sweeps. Overall a picture emerges of a highly linear subwoofer, especially ideal for music since it can pound out an average of 115 dB output levels from 40 Hz to 150 Hz with less than 10% THD. If you like your bass clean and loud, the ULS-15 mk2 is a solid choice.

Hsu ULS-15 mk2 Harmonic Distortion Components as Percentage

The above graphs break down the composition of the harmonic distortion into individual harmonics. The most notable feature that can be seen is that the dominant distortion is the 3rd harmonic. Odd-order harmonics such as the third, fifth, and so on are typically the result of a symmetric non-linearity that affects both directions of a driver’s throw, so it stands to reason that the heavy spike in third-order distortion is due to the driver running past linear excursion. The shape of the low-end filter prevented the driver from over-extending below 15 Hz in our testing, so the spike of distortion ended there, but if we had pushed it even harder at low frequencies, we could probably have induced more distortion below that point.

Hsu ULS-15 mk2 Group Delay

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. Either way, the ULS-15 mk2 presents nothing to worry about. Group delay stays under 20 ms down to around 30 hz, a frequency low enough that it is certain that group delay would not be discernable at or below it. The rise in group delay as frequencies descend hints at the subsonic filter guarding the low end of the response, but there is nothing measured here that would come close to being audible. Altogether, this is a good showing.

How much of an upgrade is the ULS-15 mk2 over the ULS-15 mk1?

ULS-15 mk1 (left), ULS-15 mk2 (right) 

Since I happened to have a ULS-15 mk1 on hand, I decided to hold a mini-shootout between the mk1 and mk2 to see how much of an improvement, if any, was to be had. On the base specifications, I would not have predicted much of an improvement. The mk1 was a much more expensive subwoofer at $1099 and weighed a good deal more. Most of the weight difference occurred in the driver, where one would expect the additional weight to add up to a performance advantage; the mk1 driver weighs 41 lbs and the mk2 driver weighs 22 lbs. The extra weight of the mk1 seems to come from the massive top plate which is needed for strength and long stroke of the XBL^2 design.

ULS-15 mk2 driver (left), ULS-15 mk1 driver (right) 

So how does the mk2 fare against the mk1?

Hsu ULS-15 mk1 CEA-2010 measurements and difference between the mk2’s measurements 

From 20 Hz to 40 Hz we see the two models are very close to each other in performance, but at 50 Hz and above the mk2 pulls away, as the mk1’s output begins to drop as it enters mid bass frequencies. At 63 Hz and above, the mk2 is more than fully doubling the mk1’s output. This is perhaps due to the split gap in the top plate that is characteristic of the mk1’s XBL^2 design which may lessen sensitivity around the rest position of the voice coil. This would reduce output as the frequencies increase, since less stroke is needed as frequency increases, so the voice coil isn’t moving out as far from its rest position. However, this does not explain the difference at 16 Hz. Furthermore, what isn’t shown is the difference at 10 Hz and 12.5 Hz, where the mk1 was not able to post a passing measurement. In fact, at these deep frequencies, the mk1 produced substantially more distortion than the mk2 for the same output level. This ran contrary to my expectation that the mk1 XBL^2 design would exhibit less distortion than the overhung mk2 at very high excursions.

Overall, the ULS-15 mk2 is a major improvement over the mk1, despite costing much less. It has much more composure at very deep frequencies and far more headroom at higher frequencies. This isn’t to say the original ULS-15 was a bad sub- far from it. Its 20 Hz to 50 Hz performance still rivals and exceeds more expensive sealed subwoofers. The mk2 slightly improved on all of its strengths and massively improved on its weaknesses.

Hsu Research ULS-15 mk2 Subwoofer Conclusion

One way my thoughts on the ULS-15 mk2 can be summed up is that I am sad to have to send my review sample back. If I didn’t already have so much subwoofage, I might have purchased my review unit. There is so little to complain about, and so much to praise with the ULS-15 mk2. Nonetheless, I will get the criticisms out of the way before moving onto the compliments.

The chief criticism I have of the ULS-15 mk2 is the potential for misuse of the EQ1 mode. The reader will recall that the EQ1 mode makes the frequency response flat down to 20 Hz with a steep drop-off below that. That can be a benefit in situations where room gain does not shore up the low end, but the problem is that in large rooms or open areas, and the subwoofer is more likely to be played very loud. This can end up being very taxing on the driver in the boosted frequency ranges in the EQ1 mode, and the sub can quickly reach its limits if played too loudly in this frequency band. We can see the consequence of that in the distortion sweeps in the 15 to 25 Hz band. While the sub can survive that kind of use in short bursts, you definitely do not want to push it that hard very frequently. I would only recommend the EQ1 mode in either near-field use or in large open areas with the stipulation that the subwoofer isn’t going to be played loudly below 30 Hz or so. The EQ2 mode has a lot more headroom, so if you want to blast content that has deep bass, use EQ2 mode. Those who want big-time headroom at 20 Hz would do better by looking at the ULS-15 mk2’s ported siblings, the VTF3 mk5 and VTF15h mk2.

A minor criticism I have of the ULS-15 mk2 is that the slope in EQ1 mode becomes a bit steep at the bottom due to a subsonic filter, and a shallower slope would be able to take greater advantage of room gain. However, this criticism also leads to some praise; the steeper nature of the rolloff helps protect the driver. In all of our testing, we were never able to get the driver to bottom out. We were able to push it past its linear excursion, but we never heard any noises that indicated the sub was in danger in either EQ mode, and our testing procedures pushes the driver very hard, much more so than most ordinary users ever would. I am not saying it’s impossible to bottom out the ULS-15 mk2, but that if it is possible, it is very difficult to do so; you would really have to make a point of doing it. One factor in this might be that since our review sample is new, the suspension might be a bit tighter than a unit that has been in heavy use for a long time, but even so, the ULS-15 mk2 looks to be very well protected against damage from the driver reaching its mechanical limit.

Now let’s briefly go over some of the highlights of the ULS-15 mk2. Its sound quality is superb, and the high fidelity that was heard in the listening sessions were confirmed by the measurements. The ULS-15 mk2 is a very accurate subwoofer and demonstrated highly linear behavior across a slew of different measurements. The output is tremendous for a sealed subwoofer at this price point, and this sub qualifies for Audioholics’ Bassaholic ‘large’ room size rating (in other words it should be able to meet the target 115 dB peak in seating positions in a 3000 ft^3 to 5000 ft^3 room). The tremendous output can be gleaned from the burst tests and compression sweep measurements, but nothing is as convincing as sitting cross-legged on the floor with the ULS right behind you only a couple inches away, and then cranking a hard dubstep tune to reference level on the AVR. Do that and you will know what a punching bag feels like.

Another admirable trait is the appearance, a slick satin black or Rosewood veneer with a gleaming concave cone- no way am I hiding that cone behind a grille. The manageable size and weight certainly deserve mention as well; this is a sub that does not take two people to move around. One person of average build can carry the 65 lbs ULS-15 mk2 around without too much of a strain. Since the enclosure is just an 18” cube, it does not eat up a lot of room. Some people might not consider that a small sub, but for the firepower it packs, it is remarkably small. Another point for the ULS-15 mk2 is that its amplifier has a lot of flexibility for different setups, so this is a subwoofer that can be integrated into a variety of systems, and with its adjustable response, its sound can be tailored to suit a variety of tastes.

Outside of the subwoofer itself is Hsu’s reputation for great customer service, a reputation established over many years. Packaging is, as was discussed before, exemplary, so if you ever had to ship it somewhere, you are covered if you hold on to the packaging. The warranty is not bad, but not great; 2 years on the amplifier and 7 years on the driver. An extended 5-year warranty on the amp can be purchased for an additional $90. And that brings us to one of the major high points of the Hsu ULS-15 mk2, the cost: $779 with a $69 shipping fee for shipping within the continental USA. $848 is not inexpensive (to most of us anyway), but if it is compared to many other subs in the Audioholics Bassaholic data compilation, it can be seen to punch way above its price point. When you consider all of the other assets it brings to the table such as its tasteful finish, manageable size and weight, and great customer service from Hsu, it has to be considered one of the top bargains in subwoofers to date.

Hsu Research ULS-15 mk2 Subwoofer Listening Sessions

I adjusted the crossover frequencies between 80 Hz and 100 Hz depending on the material and what aspect of the sound was being evaluated. Most listening was done with the ULS-15 mk2 in the EQ1 mode with a Q setting of 0.7. Equipment used was a Pioneer Elite SC-55 and Infinity Primus speaker set. The subwoofer location in my listening room with the most even overall frequency response for the ULS-15 mk2 still produced a 10 dB dip centered around 40 Hz, and this must be kept in mind while reading any listening impressions for this review. Few rooms support a really good response with a single subwoofer, and for this reason Audioholics stresses the need for multiple subwoofers for a really good bass sound. The point here is that the character of the sound this sub has in my room at my listening position will not be the same as in any other room, and this will be true of any single subwoofer installation in any subwoofer review.

Music Listening

After watching Adam Ben Ezra’s virtuoso double bass performances on Youtube (such as this terrific example), I decided to give his trio’s 2015 debut album, ‘Can’t Stop Running’ a spin. Although the bass doesn’t dig very deep, it is nonetheless an excellent jazz album with which to demo subwoofers because of Ezra’s nuanced playing. The bass is subtle, percussive, multi-layered and bold, but always masterfully performed. The various slides, slaps, and occasionally even polyphonic double bass play are like candy to a capable subwoofer. Kick drums also shine through on this album.

The Hsu ULS-15 mk2 reproduced ‘Can’t Stop Running’ with aplomb. It brought weight and power to the double bass without any sense of exaggeration. The ULS-15 mk2 effortlessly nailed the transient nature of the performance; the sharp pluck attacks of the double bass and the acute starts and stops from the slaps. The mk2 made it difficult not to bob one’s head along with Ezra’s rhythmic bass lines, and it was clear that both the recording and the ULS-15 mk2 deserved better main speakers for greater realism, but the experience was still a delight. 

For a completely different bass experience, I turned to an experimental album from a 1995 entitled ‘A Storm of Drones: The Sombient Trilogy’. This triple-CD release is keyed to gloomy, dissonant sounds in a kind of dark ambience. It is filled with many types of low frequency noises; drones of different textures and intensities, distant rumbling, low-frequency buzzing like a swarm of monstrous bees, electronic whooshes and hums, and foreboding sounds of all manner. It would make an appropriate soundtrack for a nightmare. Much of the bass is subtle, but occasionally it becomes dominant and striking, and a lesser subwoofer can easily make too much of what should only be a hint of low-frequency sound or miss it completely. Happily, the ULS-15 mk2 did neither and gave the low-frequency elements of ‘A Storm of Drones’ a presence without being inordinate. The ULS-15 mk2 rendered the finer details of dread with exquisite care, not overdoing it, and not going MIA either, which would make for a hollow sound. The balancing act in competently reproducing bass on many tracks on ‘A Storm of Drones’ can be seen in a spectrogram, where there is much simultaneous content across many low frequencies, but none of it is recorded at high energy levels. This can become either a muddy, indiscriminate rumble in the hands of an inferior subwoofer or a rich and textured undercurrent in the care of a high-fidelity subwoofer, and the ULS-15 mk2 is firmly in the latter camp.

    

Enough with subtlety and delicacy, it was time to let the sub rip and a great choice for pummeling a subwoofer with brutal bass is Calyx and Teebee’s now classic 2007 Drum’n’Bass album ‘Anatomy’. High-tempo breakbeats, bruising yet melodic bass lines, cuts of orchestral recordings, and samples of portentous dialogue from various movies is the stuff that ‘Anatomy’ is made of. This is not a recording to which one sips a fine chardonnay while admiring the understated elegance of a Cezanne painting. It is more like the soundtrack for the final battle of the great cyborg vs. android war of the early 25th century. Most of this album’s energy is in the lowest frequencies and rare are the moments in ‘Anatomy’ when the dynamic range isn’t saturated, so it becomes a nearly continuous assault on bass drivers. The ULS-15 mk2 was as comfortable with this material as it was with the more delicate bass on other music recordings. The punch on kick drums was concussion-inducing, and the buzz from the bass lines could be felt in the chest. ‘Anatomy,’ as reproduced by the ULS-15 mk2, is a positively visceral experience at high volumes. ‘Hearing’ is almost too soft of a word for the experience, and a more apt description would be the ‘sensation of pressure,’ since these output levels seem to cause an involuntary muscle tension that doesn’t end until the end of the track. The subwoofer withstood this assault admirably with no noticeable complaints, even at very high excursion levels.

Movies      

One movie I watched with the ULS-15 mk2 was ‘Letters From Iwo Jima’, Clint Eastwood’s highly-regarded 2006 World War 2 drama. While there may be movies out there that have a greater quantity of low-frequency sound content, ‘Letters From Iwo Jima’ has a nice variety of low frequency material that gives the subwoofer plenty to do without overdoing the bass. Another reason why I selected ‘Letters From Iwo Jima’ is because there are so few truly adult dramas that allow the subwoofer to stretch its legs, and ‘Letters’ is one of them. There is much to keep the sub active in ‘Letters From Iwo Jima’, from aerial bombardment, heavy machine guns, ship-mounted artillery fire, mortars, tanks, grenades, and an altogether diverse spread of WW2 sound effects. The ULS-15 mk2 convincingly relayed the sharp thuds of bullet strikes, the rumble of distant explosions, and the subterranean pounding of outside bombardment within a cave network. Thunderous battle sequences were brought to life by the ULS-15 mk2, and I didn’t get the sense at all that these scenes were being hindered by the dynamic range of the subwoofer. I watched the film at a relatively loud level, but it seemed to me that the mk2 still had some muscle left had I wanted to push things harder. In the end, I thought that the precision and power of the ‘Letters’ soundtrack was well-matched by the ULS-15 mk2.

    

Another film I viewed with the ULS-15 mk2 was the fifth movie in the Die Hard series, 2013’s ‘A Good Day to Die Hard’. Yeah, I know, it isn’t on the artistic level of the others, but it still contains some pretty good action scenes and a creditably dynamic soundtrack. The movie features a lengthy, tumultuous car chase, gun fights against military helicopters, dozens of explosions, and a brawny orchestral music score by Marco Beltrami, so the ULS was kept busy. The crashes, booms, and blasts were all vividly reproduced by the ULS-15 mk2, and this is no surprise after having spent some time with it. At loud volumes, the low frequency effects brought out some very high excursions from the driver in EQ1 mode, but there was no sign that the sub was being harmed. High-output in deep frequencies is a lot to ask for from a sealed sub, especially in a medium-large room. Switching the sub to EQ2 mode tamed the large excursions. This is not to say the sub can’t handle that kind of heavy use, it’s just something you don’t want to put your subwoofer through frequently.

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
Fit and Finish
Ergonomics & Usability
Features
Dynamic Range
Performance
Value