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Monoprice Monolith 13” THX Ultra Subwoofer Conclusion


 13 outdoor testing.jpg


The Monolith 13” THX Ultra was tested using ground plane measurements with the microphone at a 2-meter distance in an open setting with well over 100 feet from the nearest large structure. The subs were tested with woofer facing the microphone. The temperature was recorded at 45F degrees with 77% humidity. The subwoofers’ gain was set to maximum, phase was set to 0, and the low pass filters were left off.

13 frequency responses.jpg 

The above graphs show the measured frequency responses for the Monolith 13” THX Ultra subwoofer. As with other Monolith subwoofers, we see a beautifully flat response down to 20Hz in the ‘THX’ EQ mode. Sealing a port and setting the EQ mode to ‘Extended’ gives the user a few more hertz of extension but at the cost of a neutral response. Sealing all ports naturally reduces low-frequency output. While the Monolith 13” enables the user to do this, there is not much of a reason to buy such a huge sub only to run it in a sealed mode; that defeats much of the purpose of this subwoofer. Any of the modes deliver a strong response to nearly 200Hz, and the sealed mode and THX mode with all ports open can even get up to 300Hz. This makes experimentation with higher frequency crossover points possible, and that can help address more room modes through placement rather than equalization as well as use more of the subwoofer’s dynamic range in mid-bass frequencies rather than the loudspeaker.

Monolith 13.jpg

Monolith 13" THX Subwoofer CEA 2010 Measurement data at 2-Meter RMS

Go ahead and crank the volume to the max; 112dB with only 7.6% THD is a nearly supernatural sound.

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. The Monolith 13” produces a very strong set of measurements here, and that is what I was expecting based on its pedigree as well as the specs of the sub. This thing is ridiculously powerful all the way down to 16Hz. What is more is that its distortion numbers are terrific even when being pushed as hard as possible. It cannot be pushed into any significant distortion above 16Hz no matter what. Go ahead and crank the volume to the max; 112dB with only 7.6% THD is a nearly supernatural sound. 118dB+ mid-bass is nothing to scoff at either. Most subs and speakers have to make a trade-off of very wide mid-bass dynamic range or low deep bass extension, but the Monolith 13” does both fantastically.   

13 compression sweeps.jpg 

The Monolith 13” will not change its sound character very much based on loudness...

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 the subwoofer is capable of. These tests were run with all ports open but with the subwoofer set to the ‘Extended’ EQ mode in order to maximize output. The Monolith can maintain a steady 115dB from 30Hz and above when driven to the edge of its performance envelope, and that is a staggering performance level. This is another terrific set of measurements, and we see the response undergoes very little change until the final sweep. Most subs will alter the response shape well before this point. What that means is that the Monolith 13” will not change its sound character very much based on loudness, so long as it does not encounter very much nonlinear distortion. So now let’s look at nonlinear distortion…

13 THD levels.jpg

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. What we see here is that the Monolith 13” maintains very good distortion levels that only get pushed up somewhat at the highest drive level, and even then only just above port tuning which is where we would expect the most driver motion. Below the highest drive level, distortion stays below 10% where it should be inaudible. At nominal drive levels, it hovers around 1% THD. Until you max out the volume, this is a very clean and high-fidelity subwoofer, and even at maximum output levels it is still mostly well behaved except around 25Hz. That notch of distortion is unlikely to intrude in any realistic listening situation and probably wouldn’t be apparent unless you were listening closely to sine wave tones.

13 2nd order distortion.jpg  13 3rd order distortion.jpg

The above graphs depict measurements of the constituent harmonics from the long-term output sweeps and are 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 tend to be much further down as a percentage of distortion compared to the second and third.

Examining these graphs, we see that the Monolith 13” runs into more odd-order distortion than even order. The 3rd harmonic spike at 25Hz is indicative of the driver reaching the limits of its linear throw in both directions, and since that comprises the bulk of the distortion between the 2nd to 3rd, it says that the driver is fairly balanced in terms of excursion. There is some 2nd harmonic distortion, and that is not too surprising since the Monolith 13” has such a large voice coil and that will yield some induction effects, although Monoprice has taken substantial efforts to mitigate them. Indeed, the even-order distortion here is so low as to be insignificant. What is impressive is seeing just how far the driver can move at 25Hz yet still be shy of 20% THD. I am sure that we will be seeing Youtube videos of Monolith 13” owners throttling the subs in low frequencies to show off the driver’s tremendous throw. However, the thing to keep in mind is that it is easy to create a long-throw sub; the hard part is not losing linearity in that long throw. The Monolith 13” sustains a great deal of linearity for a tremendous amount of excursion.  

13 group delay.jpg


Group delay is the measurement of how much time it takes for individual frequency bands of an input signal to be produced by the speaker. It can indicate that some frequency components are developing slower than others or are taking longer to decay. It is generally thought that 1.5 sound cycles are needed for group delay to be audible at bass frequencies, although there is an argument that group delay should remain under 20 ms to be completely unnoticeable, but that is likely meant for mid and upper bass frequencies. The Monolith 13” turns in a terrific measurement with group delay not exceeding 20ms until around 25Hz. The time-domain behavior on display here is phenomenal. Delay does ramp up at port tuning, as it does with all ported subwoofers, but it is very well-controlled, and delay at such deep frequencies are very likely inconsequential since human hearing is so insensitive in that range. In the range that matters, virtually the entire mid-bass range, we see some of the best group delay measurements that I have yet tested. Whenever you hear someone say that ported subs are ‘slow,’ remember this chart and laugh at them.


13 hero.jpgI can't help but evaluate the Monolith 13" in light of the Monolith 15". They do have a lot in common. Similar build attributes, similar performance targets, and some aesthetic similarities. But the Monolith 13" kicks everything up a notch. It's a significantly better subwoofer for a few hundred dollars more. In my opinion, it scales well in improvements for the added cost. You get double the amplification, a beefier driver, and a nicer-looking subwoofer. It's like a deluxe version of the Monolith 15" in that it keeps the same basic design and size but improves every other aspect including performance.

Before bringing this review to a close, I will briefly go over the pros and cons of the product under review, and, as always, I will start with the cons. There aren’t many with the Monolith 13," so the cons of it is a short list indeed. The most significant caveat of the Monolith 13" is the size and weight. As has been mentioned repeatedly throughout this review it's a big one as well as heavy at 150 lbs. That is not something everyone can handle, and it will certainly be a presence should you choose to keep it in a nicely furnished living room. Interested buyers will need to plan out the logistics of getting it in place in their home should they choose to purchase one (or more than one). This is not a complaint so much as it is a heads-up for anyone considering buying it. The fact is that its level of performance can't really be achieved in a smaller or lighter package, so I don't hold its size and weight against it.

Monolith logo.jpgThis does touch on something else that I do have a minor gripe about which is the feet: they do not give much clearance for the fingers of people who are lifting these things up or setting them down. As with previous Monolith subwoofers, I wish Monoprice would give these things more substantial feet.

One other nit I would pick is the finish: the black oak veneer is not bad but it doesn't really distinguish the Monolith 13" from other large subs. This is a matter of personal taste, of course, and I am not sure what Monoprice could do differently that wouldn't be off-putting for buyers; these subs tend to be large black boxes and people seem to want it that way. Colors other than black just do not sell that well, so I can't fault Monoprice too much on this point. Nonetheless, these subs are pretty monotonous, visually speaking, and I don't mean just the Monolith 13" but nearly every single sub from every manufacturer within this product category.


13 cone.jpgWith those minor complaints out of the way, let's talk about the high points of the Monolith 13" which are many. The first and foremost strong point of it is the performance. As was mentioned, you really cannot wring much more performance from an enclosure of this size. It's not a small enclosure but it isn't unreasonably sized either, in my opinion. It can produce well over a hundred decibels of deep bass output and hit 120 dB in mid-bass. Those are serious numbers and I don't think that anything else of the same size can quite match its overall performance. It's not just output but also the fantastically low distortion that it manages for these extremes of output. This is not just some home theater explosion maker but a legitimately high-fidelity musically-articulate subwoofer. I doubt very much that most subwoofers from "audiophile" brand speaker manufacturers could match the fidelity of the Monolith 13" despite costing many times more. The Monolith 13" offers a combination of deep, subterranean extension, punchy mid-bass output, and pristine sound quality through it all.

Second to its audio performance is the Monolith 13"s build quality. HDF cabinet construction, a network of internal bracing, massive driver motor, and multiple piece class-D amplifier: it's no wonder that the Monolith 13" packs such a mighty punch for its size. It has a solidity that is normally seen in higher price points. It is a lot of sub for the money.

On top of all of this, it's not a bad-looking sub, and, if tucked away in a corner, it shouldn't call too much attention to itself, especially with the grille on. I know I just griped about how it's another black box as with so many other large subs, but as black boxes go, it is not bad at all and certainly a step up from the other Monolith subwoofers. For a product like this, it is what's on the inside that counts, but the outside does take appreciated efforts to make it more palatable for other household occupants who might not share a deep affection for the appearance of audio equipment.

The Monolith 13" comes with a 5-year warranty as well as a 30-day return window if the buyer chooses not to keep it for any reason (although the buyer will be on the hook for return shipping charges which won't be insignificant for a package of this size). It's not a cheap sub, but even for $1.6k, it is a lot of sub for the money, as has been said. There is a multitude of strong choices for subwoofers in its price range, but it arrives as a very attractive contender. The Monolith 13" is a superb subwoofer, and I think even at its price it will be recognized as a high-value product very quickly.

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
Fit and FinishStarStarStarStar
Ergonomics & UsabilityStarStarStar
Dynamic RangeStarStarStarStarStar
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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Recent Forum Posts:

Danzilla31 posts on September 18, 2021 16:07
Kleinst, post: 1505233, member: 91045
They might be coming out with new towers too which I really want to consider if they look good and depending on dimensions.
It is a good question in the 16 vs the M-215 ( I have the M-215 and like it)
Monolith is coming out with new towers? Oh wow that's cool where did you hear that info?
haraldo posts on September 18, 2021 15:19
Kleinst, post: 1505233, member: 91045
They might be coming out with new towers too which I really want to consider if they look good and depending on dimensions.
It is a good question in the 16 vs the M-215 ( I have the M-215 and like it)

Would be really cool to hear what you get out of the M-215, do you have one single or a stereo pair? … man, those subs look unreal cool

I am thinking myself now to upscale to a stereo pair of dual 15” subs …need more cone area … or ideally quad 15” stereo pair
(I officially disagree with myself now, ref previous posts)
Kleinst posts on September 18, 2021 14:54
They might be coming out with new towers too which I really want to consider if they look good and depending on dimensions.
It is a good question in the 16 vs the M-215 ( I have the M-215 and like it)
Danzilla31 posts on September 18, 2021 12:17
Thread bump the new monoprice 16 ultra that's shipping middle of next month has on there site an option now for the matte black finish. That finish looks niiiiicccce Youthman had one on his review on YouTube of the 13.

The price is like 300 more just 200 under there M-215 monoprice must be pretty confident in the 16 to price it that close to the 215 for that special finish

Man Shady really getting squirrelly on the data for that 16 you got!
haraldo posts on August 28, 2021 16:14
Pogre, post: 1501498, member: 79914
How DARE you sir… describe all of that awesomeness to a bunch of bassaholics and not provide a single pic!

Shame… SHAME on you!

You can check here, happy now?
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