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Monolith 15” THX Ultra Subwoofer Measurements and Analysis



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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.

Monolith Operating Modes

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.

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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.

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The 15" THX Ultra does not give up accuracy for gains in output.

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.

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 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. 

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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.

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Component harmonics of the Monoprice Monolith 15” THX Ultra for each operating mode 

Top notch engineering is evident with the Monolith 15" THX subwoofer.

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.

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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.


I can’t really say the Monolith 15” TH15 front.jpgX 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” TH15 hero3.jpgX 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.

Bassaholics_extreme.jpgLet’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 subwoofer is virtually indestructible in normal use.

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 mention15 mono cone graphic.jpg 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 15 angle6.jpglike 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

  • StarStarStarStarStar — Excellent
  • StarStarStarStar — Very Good
  • StarStarStar — Good
  • StarStar — Fair
  • Star — Poor
Bass ExtensionStarStarStarStarStar
Bass AccuracyStarStarStarStarStar
Build QualityStarStarStarStarStar
Ergonomics & UsabilityStarStarStar
Dynamic RangeStarStarStarStarStar
Fit and FinishStarStarStarStar
Attached Files
About the author:

James Larson is Audioholics' primary loudspeaker and subwoofer reviewer on account of his deep knowledge of loudspeaker functioning and performance and also his overall enthusiasm toward moving the state of audio science forward.

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