Monoprice Monolith 16” THX Ultra Subwoofer: Brutal Output, Audiophile Performance

Monoprice knocked us out of our socks in our review of their new Monolith 13” THX sub, and we were very intrigued by its bigger brother, the 16” THX Ultra. Unfortunately, the 16” was just too large and heavy for us to do a full review (hey, you try lugging a 170 lbs subwoofer up and down a steep flight of stairs multiple times!) Happily, Monoprice was able to coordinate a sample for testing, so we can get a close look at its performance even if we aren’t able to do any listening in our home. The results of our testing are the subject of this performance analysis. Before we dig into the test results, let’s briefly go over the design of the Monolith 16” THX Ultra.

Design Comparison

The Monolith 16” THX Ultra shares many of the same characteristics and features as the Monolith 13” THX Ultra, and we have covered that pretty extensively in the 13” THX Ultra review, so we will mostly just touch on the differences. One major difference, as the product name implies, is the cone diameter; the Monolith 16” THX Ultra uses a large 16” cone. Cone diameter is one thing, but without adequate excursion, the largest cones are for naught, and here the 16” cone is backed by a great deal of excursion. This is evidenced by its absolutely massive surround. The motor is largely the same, which is good because the 13” THX Ultra motor was gigantic and should easily be enough to throw around the extra cone mass of the 16” cone. Outside of the larger cone and the measures taken in the surround to accommodate the increased cone diameter, the driver remains very similar to the 13” THX Ultra.

The amplifier is pretty much the same as the 13” THX Ultra as well. Of course, different filters and DSP settings were used to achieve target responses given the larger cone and enclosure, but physically, it’s the same 1,800-watt amp. The enclosure is significantly larger and uses four 3 ¼” diameter ports instead of three. That should give the 16” THX Ultra a significant advantage in port-generated frequencies over the 13” THX Ultra, and the 13” THX Ultra was quite powerful as it was.

 

The comparison raises the question: how much greater performance can be had from simply increasing certain physical dimensions such as cone size and enclosure size? Because that is mostly what this sub is doing. There isn’t any greater physical force than what the 13” THX Ultra already has. The amp is the same and the driver’s motor is the same, so we have the same electrical and magnetic force moving the cone back and forth. How much can cone size, port volume, and enclosure size raise performance? The answer is, as we will see below, a lot...

Measurements and Analysis

The Monolith 16” THX Ultra was tested using ground plane measurements with the microphone scaled to 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 microphone. The temperature was recorded at 65F degrees with 80% humidity. The subwoofer’s gain was set to maximum, phase was set to 0, and the low pass filters were left off.

 

The above graphs show the measured frequency responses for the Monolith 16” THX Ultra subwoofer. This family of curves is beautifully flat and neutral, as we have come to expect from Monoprice’s Monolith line, with the exception of the THX EQ when all ports are open. With the THX EQ engaged and all ports open, more emphasis is placed around the 20Hz to 30Hz range in the response. This may create a ‘heavier’ or more weighty bass sound, and some people who like to watch movies with deep bass may quite enjoy this kind of response. In the Extended EQ, sealing one of the four ports does gain a few hertz of extension that trades a couple of dB of headroom in port-generated frequencies. Given the nature of this sub, that is a worthwhile trade-off, and that is the configuration I would recommend. It has so much output that giving up a little bit of headroom is worth it for bass that digs into infrasonic realms below 20Hz. For those who are wondering which port to seal for this configuration, the answer is that it doesn’t matter.

Monoprice includes ports plugs for all four ports, and we do show the response for a sealed mode with all ports plugged, but there isn’t much sense in running a huge sub like this in a sealed configuration. Take advantage of this subwoofer’s capability and run it in a ported configuration. All modes stretch to 200Hz without losing any power, so despite the large driver diameter and tremendous excursion ability, the Monolith 16” can still be used to support upper-bass frequencies in systems that have that requirement. The sealed response makes the stretch to above 250Hz without a significant shortfall in the response, and that is because it isn’t seeing the pipe resonance just above 200Hz that occurs with ports open.

It isn’t recommended to run the Monolith 16” with just one or two ports open, and that is not an officially supported operating mode, because the sub wouldn’t have enough port volume to cope with the amount of air being moved. It would be prone to turbulence with only one or two ports open. Theoretically, it would lower the resonant frequency of the ports, but the Extended EQ filter would probably shave off most of the lowered bass extension in that kind of configuration, so there is no advantage to doing that.

Note (7/26/23): Our initial CEA-2010 measurements were about 2-3dB too high due to a microphone calibration issue that was later discovered. We have since then revised the data table accordingly to reflect the actual output capability of this subwoofer more accurately.

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 16” puts out some monster numbers here, greatly surpassing the 13”. In deep bass at 31Hz and below, the difference is roughly 3dB, but in mid-bass at 50hz and above, the difference seems to average about 4dB. These are huge numbers, and they tell us that the Monolith 16” can produce more than 50% more output than the Monolith 13” while only having a 25% cost increase; this is well worth the premium if you can handle the larger size and weight (this sub is gigantic). Needless to say, the Monolith 16” easily earns our Bassaholic “Extreme” Room Rating, meaning it is sufficient to charge a room of 5,000 cubic feet with powerful reference level bass.

 

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 in Extended mode with three ports open. The long-term sweeps of the Monolith 16” essentially tell the same story as the burst numbers, that being a significant performance advantage over the Monolith 13”. We do see a curious dip centered around 60Hz at the very highest sweep level, but I doubt anyone listening to such extreme loudness levels would notice that in real-world listening.  At 25Hz and above, the sub can output over 115dB continuously. That is just an abstraction until you hear it in person where it is as much a physical sensation as it is an aural one.

 

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. This is terrifically clean performance exhibited by the Monoprice 16” and can’t be pushed above 10% THD until well below 20Hz. It can be pushed to extraordinarily loud levels without breaking a sweat. At nominal levels at the 95dB sweep and below, it can barely even reach 2% THD above 20Hz. Here is evidence of superlative engineering; it isn’t too hard to get huge output from an extreme long-throw driver with a large cone, 1,800 watts, and an enormous ported enclosure, but to do so with such a great degree of accuracy is the tricky part. For all the brute force on display, it is not without precise application of power. This isn’t just a fast car in a straight line car; it can corner nicely as well.

 

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. Since there is not much total harmonic distortion from the Monolith 16”, we don’t see much distortion from either the second or third harmonics, at least until down to 16Hz. However, output is falling rapidly under 16Hz, so there is very little output at all, let alone distortion, and distorted output only occurs at higher drive levels as well. Lots of liner throw given lots of power adds up to lots of clean, undistorted output.

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. This is an interesting set of results from the Monolith 16”. The results are all very good, and they all manage to stay under 1 cycle, save for the ported THX EQ configuration. This is similar to what we have seen before from Monoprice’s Monolith series; ported subwoofers that somehow keep group delay under 1 cycle! There has to be some kind of DSP trick that manipulates phase to do this. Port output is normally a cycle of phase behind the driver output, and this shows up in group delay measurements similar to what we see in the THX EQ curve, and in most subs, it’s quite a bit higher than even what is shown there. Somehow Monoprice has found a way to keep port output below a one-cycle lag. This is abnormally good among ported subs but par for the course for Monolith’s THX subs. Group delay stays under 10ms above at 40Hz and above where it would be most audible. Delay doesn’t surpass 20ms until below 30Hz, not including the ported THX EQ configuration. This is better time-domain performance than many sealed subs I have tested!

Conclusion

With the Monolith 16”, our test results signify a subwoofer of not only tremendous power but also linearity. Extremely high output, low distortion, excellent time-domain behavior, and solid extension down to 16Hz add up to one heck of a sub. It isn’t an inexpensive subwoofer, but the performance-per-dollar ratio is outstanding. However, there is another price, and that is dealing with the size and weight of such a behemoth. It eats up a lot of floor space and weighs about as much as the average American adult, but performance on this level can not be had from a small subwoofer. Physics will not allow high levels of deep bass to come from a small object; think of all the things that produce audible deep bass in life, and they tend to be very large objects. So you need big speakers to produce a big sound, and this is most certainly a big speaker.

The Monolith 16” is an ideal sub for a dedicated home theater room where people are usually willing to allocate lots of floor space to audio equipment. But don’t just write it off as a home theater sub, because it is a very agile and accurate subwoofer as well and would nicely complement a hi-fi two-channel rig for those who don’t mind its size, especially for those who have large rooms. Its measured sound quality is superior to most of the so-called “audiophile” brand subwoofers out there. It is so good that its THX Certified Ultra mode is the lower-performing configuration among its operating modes. It has to be able to adhere to certain THX performance targets in order to secure THX certification, but if you switch it to the Extended EQ mode, you get deeper bass, a flatter response, and less group delay. Most subwoofers wouldn’t be able to qualify for THX certification, but here is a sub that is actually held back by THX performance requirements; thankfully, they have provided other operating modes to take fuller advantage of the sub’s capabilities. Monoprice’s Monolith 16” THX Ultra subwoofer is sure to make a big impact on the high-performance subwoofer market. Those who are hearty enough to handle this beast are going to have an enviable sound system.

Unless otherwise indicated, this is a preview article for the featured product. A formal review may or may not follow in the future.