Monoprice Monolith 15” THX Ultra Subwoofer Review

Monolith 15" THX Ultra Introduction

After many requests from our readers, and quite few delays, we have at long last brought you our review of Monoprice’s flagship subwoofer, the Monolith 15” THX Ultra. As was promised at the end of our Monolith 10” and 12” THX subwoofers review, we  finally got a chance to review Monoprice’s full Monolith THX subwoofer line. But for those who are new to the subwoofer market and haven’t been keeping track of which sub has or hasn’t been reviewed, let’s give the Monolith 15” THX Ultra a proper introduction. In late 2017, Monoprice, an internet retailer that had been more previously associated with electronics accessories, launched a line of subwoofers in a price and quality bracket that was a step up from anything they had done previously. These subwoofers were a part of their “Monolith” product series of high-performance audio gear that, until then, was comprised of speakers, amplifiers, and headphones. The Monolith subwoofers entered the market sporting THX certification and and very respectable set of performance measurements that made people take notice of their seriousness of being a very real competitor in this fierce market.

Monoprice managed to create a buzz about their subwoofers, and we at Audioholics were regularly asked when we were going to review these subs. Earlier this summer we dealt with the 10” and 12” subwoofers in the aforementioned review, but now it’s time to turn our attention to the heavy-hitter, the Monolith 15” THX Ultra. The 10” and 12” impressed us with their no-nonsense, heavy-duty designs, and we expect the same but even more so from the 15”. Can the Monolith 15” THX Ultra deliver on the promises made by its smaller brothers? Let’s find out...

Unpacking and Appearance

A subwoofer as heavy-duty as the Monolith 15” THX Ultra is going to need some very serious packing to keep it safe in transit. Monoprice provides such packing; they are double-boxed with sturdy grade of cardboard, and both boxes have built-in handles There are large foam blocks capping the top and bottom of the sub and edge-protecting foam pieces to safeguard the side edges. The subwoofer itself is wrapped in a nice drawstring black-cotton bag to protect it against moisture and scuffs. Monoprice has also included two white cotton gloves to handle the 15” THX Ultra, which is nice but a bit overkill, since it doesn’t have the kind of finish that would easily show fingerprints.

Once out of the drawstring bag, the Monolith 15” THX Ultra reveals itself to be a formidable-looking piece of audio equipment. First of all, it’s big; it is one of the largest single driver 15” subwoofers on the market. Ported subwoofers striving for strong deep-bass output must be physically large by necessity, but they can take a wide range of shapes and aren’t as restricted in that regard. The Monolith 15” THX Ultra has exchanged width for height and is nearly 27” tall. This was a good idea so that it could fit in more locations since height does not increase the footprint of the subwoofer, but it does make it look visually imposing. In fact, the Monolith 15” THX Ultra, more than any other product from the Monolith series, really lives up to its name; it looks monolithic (dictionary definition of Monolith: a singular, massive object).

The Monolith 15” THX Ultra is finished in a nice black oak finish, but it isn’t a high-end or luxurious finish. The black oak gives it more personality than plain matte black though. The cabinet is essentially a large block with the lengthwise edges sheared off in a hard bevel. As I said in the review of the Monolith 10” and 12” THX subs, this gives it an almost “muscle car” aesthetic. The beveled edges make it look brawny; rounded edges would have perhaps softened its look and simple square edges would have looked a bit ‘Ikea’ minimalist. Without the grille, this subwoofer is daunting, since that 15” woofer and those three ports look ready for the business of big bass. With the grille on, the Monolith 15” becomes more subdued, but it is still such a big object that, as a big, black, faceless cabinet, it is still a bit intimidating. This is not a unit that will make for a natural fit in every living room. It should not present a problem in a dedicated home theater room, but I can see how some household inhabitants might be put off by its inclusion in a more common living space.

Design Overview

The Monoprice Monolith 15” THX Ultra is a large, variable-tuned, THX-certified subwoofer. For those who don’t know, let’s briefly go over the meaning of these descriptive phrases. “Variable-tuned” means that the subwoofer can be made to change how low in frequency that the sub is able to play by sealing one of its ports. The trade-off for extending the ability to play deeper frequencies is that there will not be quite as much headroom in deep bass frequencies overall. In most variable-tuned subwoofers, the difference is usually a subtle one but still audible and can change the experience of the content. Typically, the difference can be experienced in movies that has significant deep frequency content; the deeper bass can make the scene a bit more immersive. According to Monoprice’s specifications of the different tuning configuration responses, the -6dB point between the ported modes can be switched from 20 Hz to 14 Hz. That might not seem like a lot, but, physically speaking, that is a very substantial difference: over a half-octave, and a 40% greater amount of air displacement.

“THX-certified” means that this product has been certified by THX Ltd. to meet a minimum standard of quality and behavior that it can be used in a THX environment. That is a bit of a tautology, so let’s expand on what a “THX environment” is a bit further. Here is an aspect of THX certification that is still so widely misunderstood. Many people hold the impression that THX certification for subwoofers and loudspeakers is a performance standard which goes something like: “speaker/subwoofer X is able to achieve a certain SPL level with low distortion.”  That’s true, but there is a lot more to THX certification than that. Yes, a THX certified product has to meet some minimum performance metrics for whatever level of certification it is qualifying for, but THX is more about maintaining control over the reproduction of the content from the mixing stage to the end user’s experience. Maintaining low distortion for a given SPL level in the playback stage is just one part of that. What THX is striving for is that the end user should hear what the production engineers and artists intend for them to hear. They are trying to make it so that whatever is done in the production will have a predictable effect in end use consumption. So the THX badge should be regarded as insurance that, in the right environment and correctly set up, the system will deliver what the artist intends.

The Monolith 15” THX Ultra holds THX’s “Ultra” performance class certification, which means it should be able to help to present a THX experience in a 3,000 cubic foot room with a 12’ viewing distance from the screen. That is a big space to fill, and the laws of physics dictate that a large sub is needed for that task (or at least a whole lot of small subs). The 15” THX Ultra is such a behemoth that, at a glance, it does look like it might be up to the task, but the THX certification is verified proof, so no guessing is needed. However, the 15” THX Ultra is only certified for that level of performance in one of its operating modes: all ports open and EQ switch set to “THX.” It isn’t that other operating modes will perform significantly worse, it’s just that they don’t have very specific behavior needed to act totally predictable by the THX controlling processor otherwise.

To start the discussion of the components that make up the 15” THX Ultra, we will begin by talking about the massive cabinet.  The Monolith subs use high-density fiberboard (HDF) as opposed to medium-density fiberboard that is the standard for speakers and subs at this price point. Monoprice choose HDF because they claim it makes for a more sonically inert cabinet. The front baffle is 2” thick with the other panels being 1” thick. There is one 1” thick window brace at the midsection of the cabinet, and two perpendicular braces, one of which helps to support the ports. The cabinet walls are lined with stuffing for the sake of damping and isothermal conversion (which helps to effectively make the enclosure volume behave larger than it really is). The 15” THX Ultra uses three 3.5” ports that need to be so long that they make a right turn within the cabinet. This is necessary because long ports are needed for extremely deep bass tuning frequencies, at least if the deep bass is to have loudness on parity with the output produced by the driver directly. This is a large cabinet, and it has to be. As we mentioned before, ported subwoofers that are attempting serious deep bass output must be large by the laws of physics.

In order to reproduce the bass frequency spectrum in a linear manner, the driver has to be a solid design, and the 15” THX Ultra driver displays unmistakable signs of being such a driver. The cone is a paper/fiber glass composite that combines low weight with very high rigidity. A large rubber surround affixes the cone to a very stout-looking cast-aluminum basket. Monoprice claims that the surround was made from a type of Nitrile Rubber that was formulated after months of modeling for optimal performance. An 8” Nomex spider serves as the other half of the suspension for the moving mass of the driver. The motor system is quite beefy: the permanent magnet is composed of a stack of three 7” diameter, ¾” thick plates. Venting is done through slots underneath the spider. The motor uses two aluminum shorting rings to reduce induction effects thereby lowering even-order harmonic distortion. The backplate is bumped out to allow for larger excursion. All in all, this is an impressive-looking driver that is certainly plausibly believable of being the central engine of a THX Ultra-certified bass system.

Monolith Subwoofer Amplifier

Of course, a driver can do nothing without current, and Monoprice has supplied the driver with a brute of an amplifier that is specified for 1 kilowatt RMS output. The 15” THX Ultra amplifier looks to be using two of the same class-D modules used in the 10” and 12” Monolith subs in a bridged configuration. Two modules have the added benefit of two power supplies working in unison with the differential amplifier outputs to yield 2X the power, which would ordinarily require a much larger power supply in a single amp configuration. Other than the obvious power doubling, the amplifier now has more energy storage and improved low frequency stability. The amp has fairly basic controls, but there is an EQ switch that can adjust it for the THX mode or the lower extension mode when a port is plugged. There is also a variable phase knob which gives the sub a bit more flexibility when dialing it into a sound system. Connectivity includes a pair of RCA inputs and an XLR input and output. The XLR output can be handy for those who want to run multiple subs without turning the wiring into a spaghetti tangle of cabling, since they can simply daisy-chain subwoofers on a single run of cable. Balanced cabling like XLR do a better job of rejecting noise, and XLR connectors are a much more secure connection than traditional unbalanced connectors as well. 

At this point, I do want to voice one small complaint about the design of this subwoofer. This subwoofer is using the same feet as the SVS PB-4000, and, as I noted in that review, they were simply too short to safely be set down when carried with fingers on the bottom of the sub. It can problematic, especially for those with dense carpeting, in that it creates a pinch-point. Like I suggested to SVS, I would encourage Monoprice to look at taller feet that allows for more clearance for fingers.

Listening Sessions

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 handful of different speakers, and the crossover was used mostly at 80 Hz. 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 the subwoofer in this review, but for any subwoofer in any review. Most listening was done with one port sealed in the ‘Extended Mode’ configuration.

Music Listening

To see how the Monolith 15” THX handled music from conventional instrumentation, I threw in the music soundtrack from the film ‘Sicario.’ While the music from ‘Sicario’ uses conventional instrumentation, it is not all all a conventional film score. The music score for ‘Sicario’ is a key component is setting up the tension in the movie, and a key component in much of the music itself is the use of bass. Low-pitched drums, distorted percussion, and deep omninous cellos comprise the bass in the music here. The music itself is so evocative that it hardly needs to be paired with the visuals of the movie to develop tension. For a miscalibrated system or a sloppy subwoofer, cello playback can sound poor if some part the bass becomes too dominant. It can sound unnatural if the fundamental and lower harmonics are too loud with respect to the upper harmonics of the instrument’s sound. While room acoustics is a big factor here, another major contributor can be a subwoofer with a peakish or uneven response. Thankfully, that isn’t the case with the Monolith 15” THX where the bass was certainly present but not overbearing. With some dialing-in, the sub blended in nicely with the rest of the system and disappeared in the sound. That is exactly how subwoofers are supposed to behave. The Monolith 15” THX added the weight of deep bass to the sound without announcing itself. It proved to be a terrific subwoofer for orchestral music.    

For an even deeper and darker dive into the far realms of music with heavy doses of low frequencies, I queued up an album called ‘Sternklang’ by a musical artist named Tholen. I don’t know much about this album or artist except that Tholen hails from Germany, and the album was released in 2007 on the Cyclic Law label which specializes in dark and industrial ambient music. This album consists of a single 71-minute track that sounds like a soundtrack to some dark science-fiction film; it is an ominous cosmic journey into deep space that has an expansive yet forbidding atmosphere. It is largely electronic music with sounds like bells drenched in reverb, heavily distorted synthetic vocal pads that sound like alien communiques, portentous drones, and a distant, rhythmic thunderous thumping that sustains a feeling of unease even though this is an ambient work (not all ambient music is relaxing or pleasent). Subwoofers are kept busy throughout the duration of this album, sometimes only being driven softly but at other times pushed to some pretty serious low-frequency output. These low-frequency noises are not all the same, however, and they provide foundations to different sounds throughout this album. Muddy playback of the bass here would blend all of the low-frequency sound into an indistinct rumble. As mentioned before, room acoustics is often a major culprit here, but the subwoofer needs good linearity to avoid this effect as well. A THX-certified sub should fit that description, and the Monolith 15” THX certainly does reproduce ‘Sternklang’ without confusion and it provides good delineation of the various bass noises. This kind of subwoofer serves an album like ‘Sternklang’ well. It captures the different textures of low-frequency sound while keeping them distinct from each other while not creating an audible distinction between the subwoofer-generated sound and main speaker-generated sound.

For musical bass of a far less subtle nature, I listened to the album ‘Eskmo’ by the artist Eskmo. While this now-classic 2010 dubstep album certainly lands squarely in the ‘dubstep’ genre, it does not sound like other dubstep music. Eskmo’s music has a clear and unmistakable flavor in a genre where there can often be a sameness that leads to a near-anonymity. Eskmo’s glitchy use of percussion, more cinematic melodies, and far more inventive use of bass separates his music from many of his peers. The bass line in many normal dubstep tracks is often just a repeating tone at 40 Hz or so with “wobblies” overlayed on it to give the bass note itself some kind of varying rhythm, but the bass line is just a single note played over and over again. Eskmo uses far more variation in bass notation by sliding and swinging the bass line up and down a wide range of notation, and his music is much more fun and interesting for it. It’s also a lot more fun than hammering the same note over and over again for a subwoofer that has good pitch definition. A subwoofer that is engineered for accuracy should be able to clearly follow the bass notation of Eskmo’s recordings while also giving the kick drum a tactile thump. The Monolith 15” THX does precisely that. This is powerful, musical bass that needs a sub with high output capability that plays low frequencies evenly, and that is a fitting description the Monolith 15” THX. Eskmo’s playful bass melodies were rendered with verve and lucidity on the Monolith 15” THX, and that did not change no matter how loudly I cranked it. This sub does not change the character of the bass when amplitude is changed. There is no audible distortion that is a cue of loud levels which can occur with other subs, since the Monolith 15” THX remains clean no matter how hard it is pushed. The bottom line is that this sub is great for this type of music, since it gets loud, can’t be overdriven, and accurately reproduces any low-frequency sound within its operational range. 

Movie Listening

In my previous subwoofer review, I watched the remake of ‘Clash of the Titans,’ so I thought why not tie up the series and watch its concluding sequel for this review? I threw in 2012’s ‘Wrath of the Titans,’ since this sequel promised more of the same low-frequency carnage that heavy-duty subwoofers excel in recreating. Its predecessor was met with a mixed critical reception, but ‘Wrath’ was received with considerably worse fanfare. However, I went forth undaunted, since I wasn’t viewing this movie for its emotionally stirring character arcs to begin with. Bring on the mythological monster mayhem! Surprisingly, I found myself enjoying ‘Wrath of the Titans’ quite a bit. And as for low-frequency content, it had as much, if not more, than its predecessor. There were earthquakes, giant, two-headed, fire-breathing chimeras, cycloptic giants, the enormous, shifting stone Labyrinth of Tartarus with its minotaur, and the gigantic father of the gods Kronos who was encased in an underground mountain. In other words, there was plenty of deep bass here to keep the subwoofer busy. The Monolith 15” THX Ultra had no troubles keeping up with the abundance of bass in ‘Wrath of the Titans.’ I could feel my sofa trembling as the 15” THX Ultra pounded it with pressure waves. The rumble produced by this sub was felt as much as it was heard. The 15” THX Ultra was seemingly untaxed by the massive setpieces of the movie, no matter how loud I turned up the volume. While the 15” THX Ultra does have limits to how loud it can get, it isn’t easy to sense those limits, since it doesn’t produce audible distortion no matter how hard it is pushed. It doesn’t lose control of itself, and this is mandated by THX certification. THX needs tight control over every aspect of operation to ensure the experience of one THX system is repeatable to any other. For this reason, the distortion quantities allowed is very low. It’s reassuring to know that non-linear behavior is not going to affect whatever low-frequency sound that is heard in a movie like ‘Wrath of the Titans.’ The 15” THX Ultra simply reproduces whatever is in the recording, adding nothing and subtracting nothing.

Another movie that I watched which made the Monolith 15” THX stretch its legs was ‘Dawn of the Planet of the Apes,” the 2014 sequel to ‘Rise of the Planet of the Apes.’ I wasn’t terribly impressed by the primatological illiteracy of its predecessor, but with the bar already having been set so low, I decided to sit back and try to enjoy the sequel for what it is, not for what I wish it could have been. I have to say, at the moment when this movie featured a chimpanzee leading a warcharge on horseback while firing machine guns in both hands, ‘Dawn of the Planet of the Apes’ had won me over. ‘Dawn’ isn’t as bass-heavy as ‘Wrath of the Titans,’ but it still draws out the subwoofer in a bunch of scenes, such as a stampede of deer, an intense military battle, and the action-packed climax which has its fair share low-frequency commotion. Low frequencies are used more sparingly here, so when they do kick in, they have more impact than movies where the bass is much more copious. When the sound mix called for it, the 15” THX Ultra could punch and punch hard, yet not do so in a manner that makes its existence as a subwoofer blatant. In other words, it blended in with the sound system so well that no one would have known if a sub was in use or just full-range speakers. One attribute that could help to explain that is, again, the extremely low distortion. Nonlinear distortion could bring a lot more attention to subwoofer-band frequencies, since the harmonic distortion components lay in a much more audible band than the fundamentals themselves. That means the subwoofer becomes a lot louder than it is supposed to be when distortorting, even if it was properly gain-matched during calibration. When viewing ‘Dawn of the Planet of the Apes,’ the 15” THX Ultra did what it was called to do and nothing else, which is what a subwoofer should do. Anything else would interfere with the immersion of the experience.

Monolith 15” THX Ultra Subwoofer Measurements and Analysis

The Monolith 15” THX Ultra was tested using ground plane measurements with microphone at a 2 meter distance in an open setting with 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 68°F and 71% humidity.

Frequency responses for the THX EQ mode of the Monolith 15” THX Ultra: all ports open and sealed 

The above graph depicts the base frequency responses for the 15” THX Ultra with its THX EQ switch engaged for its various port configurations. The responses exhibited here are superbly neutral, especially with two ports open, where the sub has a response window smaller than +/-0.5dB from 30 Hz to 200 Hz! Sealed and 3 ports open still give a very flat response, but not with quite in such an extraordinarily tight window. If we overlook the slight pipe resonance dip just above 200 Hz, this sub reaches to over 300 Hz with no problem. Needless to say, that is an overall outstanding degree of linearity. This sub reproduces the signal without coloration, period. It “tells it like it is,” to use that cliched phrase but nonetheless apt description of this performance.

Note:  In my testing of the Monolith 15” THX Ultra subwoofer, an ‘operator error’ marred the responses of its ‘Extension’ EQ mode measurements. We are going to retest this sub and update these measurements to include a more complete examination of its performance capabilities in the near future. This oversight has only affected response sweeps and not burst testing measurements, so the CEA-2010 burst measurements are ALL valid.

Monolith 15" THX CEA-2010 Tabulated Measurements

The above CEA-2010 measurements are short-term bursts that 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 publicly available CEA-2010 measurements are shown at 2-meter RMS, so we followed that convention.

This is a terrific measurement set, as expected. The 15” THX Ultra’s performance roughly scales up evenly with that of the Monolith 10” THX Select and 12” THX Ultra subwoofers when price is considered, everything is averaged out. In some respects it has a major performance lead over its less expensive siblings, but in other respects its performance lead is relatively modest. The 15” driver definitely has a lot more displacement capability, as can be seen in the low-frequency sealed measurements. As we have seen with other THX certified subwoofers, the 15” THX Ultra has exceptionally low distortion behavior, especially at 25 Hz and above. Again, we see that THX performance targets do not seem to allow much more than 5% THD above 25 Hz. Our measurements line up fairly close to that which was posted by Monoprice on the product page for this subwoofer. In fact, the minute differences are in line with the differences that have been observed between the testing software that we use versus the software Monoprice used to gather those numbers, so in reality these measurements are in even closer agreement then a glance would suggest. Overall, we see an excellent balance of headroom throughout the subwoofer frequency spectrum.

The above graphs show the measured frequency spectrum of the increasing CEA-2010 burst tests. Essentially, they depict 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. These measurements were done with the 15” THX Ultra with three ports open with the THX EQ switch engaged.

These graphs show the vanishingly low distortion of the 15” THX Ultra. We even get some recognizably clean 12.5 Hz output where distortion products hover around 15 dB below the fundamental. At 16 Hz and above, distortion products are totally insignificant until the sub is pushed to its maximum drive level, where it rises from totally insignificant to highly insignificant. Once above extremely deep frequencies, this sub just does not make noises that it is not supposed to make.  Even when pushed to maximum output levels, these levels of distortion would still be well below thresholds of human audibility. The 15” THX Ultra is as averse to non-linear distortion as it is to linear distortion. It does not give up accuracy for gains in output.

 Monoprice monolith 15” THX Ultra long-term output compression for all ports open THX EQ and sealed THX EQ

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 meters from the microphone. We then conduct further 20-second sweeps by raising the gain by 5 dB until no more output could be wrung out of the subwoofer. These tests show us the long-term continuous headroom that this subwoofer is capable of. The 15” THX Ultra doesn’t change its response shape until the last few dB of output is eeked out of it, but even them the difference from its response at lower drive levels is not large. Aside from a bit of extra bass centering on 40 Hz at the highest drive level with all ports open, the response shape remains very neutral throughout. The maximum output bass bump around 40 Hz is a response deformation certainly, but not a harmful one at those frequencies and output levels.The highest drive level in sealed mode even flattens the response to a +/- 1 dB window from 30 Hz to 200 Hz. Maximum output seems to average between 110 dB and 115 dB for the operating modes shown in these graphs, with all ports open exceeding 115 dB in low bass at maximum output. The story of the burst testing is repeated in these long term tests: lots of output without sacrificing much accuracy. 

Monoprice Monolith 15” THX Ultra 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. With all ports open, the 15” THX Ultra is very reluctant to produce any distortion regardless of the output level. If you would argue, “What about the rise in distortion below 20 Hz?” it must be remembered that the output is very rapidly decreasing at this point, so the subwoofer is not making much sound in that region. It is below the tuning point of the subwoofer for that configuration and thus outside of the range of its intended operation. Above that range, the 15” THX Ultra never hits double digit distortion percentages. Only when pushed to the absolute maximum drive level can it even be made to surpass 5% THD. In our measurements at 2m, for drive levels at 100 dB and under, it barely exceeds 2% THD. Running the 15” THX Ultra in sealed mode does incur greater quantities of distortion as would be expected, since the driver no longer has any assistance from the ports, but we still get very clean bass in this configuration. At moderate loudness levels in sealed mode, there is, again, a total absence of any meaningful distortion above 25 Hz. Even at the maximum output level at 25 Hz, total harmonic distortion only hits 10%, which is excellent behavior from a sealed subwoofer. Below 25 Hz at high drive levels, THD does rise, but the sub still maintains good composure until maximum output. For a sealed subwoofer at these kind of output levels, this is extremely clean bass.

Component harmonics of the Monoprice Monolith 15” THX Ultra 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 for the 2nd and 3rd harmonics. These individual harmonics can give us a clue as to what might be the cause of some quirk or non-linearity. We are only showing the 2nd and 3rd here because they more or less reflect the higher even-order and odd-order behaviors, although higher-order harmonics will likely not be as abundant in quantity as the 2nd and 3rd harmonics. From these graphs we can see that the 2nd order harmonic grows at a steady rate with the rising output, whereas the 3rd harmonic erupts more suddenly at high drive levels. This suggests that induction is the chief culprit of the even-order harmonics. However, there is certainly not enough to complain about, and clearly this driver has gone to serious lengths to address induction effects. For such a massive driver to exhibit so little adverse effects of induction is an achievement. The graph for 3rd order harmonics in the sealed mode looks like a profile of a mountain range, and here we can see a limiter kick in and rescue the driver from danger as the amplitude gets to be very high in very low frequencies. All in all, there is low total harmonic distortion, so, of course, we don’t see much in the way of individual harmonics. This is a very balanced distortion profile that shows what extensive optimization looks like. In other words, we see top-notch engineering evident here.

 

Monoprice Monolith 15” THX Ultra 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. There is no significant group delay to worry about in these measurements; on the contrary these are among the better group delay measurements I have seen for a ported subwoofer. The sealed mode group delay measurements are better in deep bass but not by much. There is no delayed output over 10 ms until under 50 Hz, and nothing over 20 ms until approaching 30 Hz. There is nothing even close to audible delay here. The 15” THX Ultra will have no overhanging notes or bass sound. This is an excellent measurement set.

Conclusion

I can’t really say the Monolith 15” THX held any surprises for me in my time with it, and that’s a good thing in the case of this subwoofer. My expectations were set high after having dealt with its smaller brothers, the Monolith 10” THX Select and 12” THX Ultra, and it did not let me down. It has the same high build quality and performance, just deployed on a larger scale. As always, before closing this review, I like to briefly go over the high points and also aspects that could use some improvements, and I will start with the latter, although there is little to complain about with this sub.

One aspect that I feel is a shortcoming of the Monolith 15” THX Ultra is the feet, as I had noted in the ‘Design Analysis’ section. This is a minor nitpick, and it says a lot about how well-thought out it is that I only have this one gripe about its design. However, it is a gripe, and I am hoping that Monoprice can change the feet to something more substantial that gives a bit more clearance underneath the subwoofer. It’s a heavy sub, and the ½” feet just do not give fingers enough space under the sub if it has to be lifted by hand. I would advise Monoprice to look into feet that give the 15” THX Ultra 1” or so of clearance.

Another drawback of the Monolith 15” THX Ultra is its heavy weight. This isn’t so much a gripe, because there is no way a subwoofer with this combination of size, build quality, and performance could ever be light. However, its weight should be taken into serious consideration when buying it. If there isn’t a dolly present to move this thing, then two people in good physical shape are required to lift it, and even then something like a shoulder dolly would be needed to move it up and down stairs. Potential buyers will want to work out the logistics of getting this sub into place before pulling the purchase trigger.

Something else to note that isn’t a complaint but could be an issue for some people is the 15” THX Ultra isn’t really a visually subtle product. It’s a large and brawny looking subwoofer, and it doesn’t do much to soften that fact. Personally, I like how aesthetically unapologetic it is about its function, however its blunt style is bound to clash with more refined interior decors. Buyers should be aware that it would not be easy to hide this subwoofer or make its presence inconspicuous in-room. There are other subwoofers that are more elegant looking or at least do a better job of disguising their presence.

Let’s now talk about the highlights of the Monolith 15” THX Ultra. First and foremost is its performance. This is a product that was designed with high performance as its first priority. Both burst testing and long-term testing reveal this to have very high output capability while maintaining extraordinarily low distortion. Of course, that is expected, given the design of this sub. Its frequency response remains impressively linear regardless of output level. Its deep bass performance is especially noteworthy; the 15” THX Ultra hits hard at near subsonic frequencies and does so without breaking a sweat. However, its overall performance is so exemplary that it is not easy to pick out one particular quality of sound reproduction above all others.

The Monolith 15" THX Ultra is just an all-around killer low-frequency transducer. It easily qualifies for Audioholic’s Bassaholic ‘Extreme’ room rating in its ported modes. We will also award it the ‘Extreme’ room rating for sealed operation too, since it only just misses the cutoff at 31.5 Hz by 1.5dB, but with only 11% THD, it isn’t being stressed very badly, so it can handle larger rooms without running itself ragged even when sealed.

Something else that is particularly praiseworthy about It can not be over-driven or ‘blown’ by any content no matter how hard it is driven. Go ahead and crank this subwoofer, it can handle whatever you throw at it (well, not physically throw at it, but whatever signal you want to send it). I should add here that it wouldn’t be a good idea to leave it at maximum drive levels at all times, as that would doubtlessly shorten its life by a considerable amount, but for short durations, there is no way this sub can be made to damage itself.

I should also mention here that the build quality of the 15” THX Ultra is outstanding. This subwoofer has a very robust level of construction, although that does incur a heavy weight penalty. An indication of the high-quality of manufacturing is how precisely the amplifier and driver fit in place when I removed and reinstalled them; there was no surplus room, and these components all had an exacting fit. To me, that indicates very low tolerances for error in their production. The cabinet construction, driver build, and amplifier design are all of an extraordinarily high quality level. One aspect that Monoprice did clearly cut corners on is the finish, which isn’t on the level of everything underneath it. A really nice finish befitting of the rest of its construction would have added a substantial hike to its pricing, however, and since this is a performance-first product, Monoprice opted to allocate its manufacturing budget to performance enhancing features rather than prettiness. The irony here is that there are many high-end subwoofers that have a more luxurious finish which suggests the type of build quality that the 15” THX Ultra has but in actuality they are not built with nearly as much as solidity. This sub has high-end build quality and performance, just not a high-end finish. However, that isn’t to say the finish is bad, it just doesn’t seem commensurate with the rest of the subwoofer build.

Those who have read this far will know if the Monolith 15” THX Ultra sounds like a good fit for their circumstances. It is not a good fit for those in small apartments with thin walls. It is not a good fit for rooms with delicately balanced interior design. If you envision having problems dealing with its substantial weight or finding room for its large footprint, there are better alternatives. However, for those after a no-nonsense, high-performance subwoofer, it is a very high bang-for-the-buck choice. It’s a great choice for dedicated home theater rooms or family rooms that have a serious A/V system. It’s also superb choice for high-end two-channel systems on account of its extraordinarily high fidelity. The 15” THX Ultra comes with a 30-day money-back guarantee and FREE shipping, so if you buy one and want to return it for any reason, Monoprice will give you a full refund (minus return shipping) within 30 days of receiving it (assuming it has not been damaged). It also comes with a full 5-year warranty. I enjoyed my time with the 15” THX Ultra and am sad to have to send it back. It does raise the bar for the Monolith product line, and I am excited to see where Monoprice takes the Monolith products next.

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