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Monoprice Monolith THX-365T and THX-365C Measurements & Conclusion


The Monoprice Monolith THX speakers were measured in free-air at a height of 7.5 feet at a 2-meter distance from the microphone, and the measurements were gated at a 9-millisecond delay. In this time window, some resolution is lost below 250 Hz and accuracy is completely lost below 110 Hz. Measurements have been smoothed at a 1/12 octave resolution.

THX center outdoor testing.jpg

Outdoor Measurement Platform with Monolith THX-365C Speaker

Minitower SpinOrama.jpg THX Center SpinOrama.jpg

The above graph shows the direct-axis frequency response and other curves that describe the speakers’ amplitude response in a number of ways. For more information about the meaning of these curves, please refer to our article Understanding Loudspeaker Measurements Part 1. While the responses that I measured don’t exactly match the responses that are seen on the product pages for these speakers, they are nonetheless good overall. The responses are fairly neutral, but they do have a very slight downward slope; that will be their chief audible distinction from perfect neutrality. The THX-365T Mini-Towers do have a minor peak centered around 9 kHz, but that won’t be very audible. These speakers may have a slightly warm or laid back sound compared to many other speakers because there is a bit more energy in the woofer and midrange dome bands than the tweeter’s band. We can see that these are both relatively wide dispersion loudspeakers because the early reflections curve tracks the on-axis response and listening window response very closely until upper treble frequencies. This can also be seen in the directivity indexes; the dispersion of these speakers will begin to narrow above 5 kHz. For the most part, however, both speakers provide a very good response and should sound great to most ears except maybe for those listeners who are looking for a bright sound.

 Minitower waterfall response 3D.jpg THX center waterfall response 3D.jpg

Minitower waterfall response 2D.jpg THX center waterfall response 2D.jpg

The above graphs depict the Monolith THX speakers’ lateral responses out to 100 degrees in five-degree increments. More information about how to interpret these graphs can be read in this article: Understanding Loudspeaker Review Measurements Part II. Once again, we see how well behaved these speakers are, but these graphs show us how well the horizontal off-axis response correlates with the on-axis response in greater detail. As the directivity indexes suggested, these speakers hold a nicely uniform axial response up to 5 kHz where the tweeter does roll-off quite a bit more at farther off-axis angles. We do see a slight amount of off-axis cancellation from the woofers in the center speaker, but it is extremely mild compared to what occurs in most two-way center speakers. This response from this center speaker is very good and places it among the best center speakers we have measured to date.

 Minitower Polar Map.jpg

THX center polar map.jpg

The above polar map graphs show the same information that the preceding graphs do but depict it in a way that can offer new insight regarding these speakers’ behavior. Instead of using individual raised lines to illustrate amplitude, these polar maps use color to portray amplitude and this allows the use of a purely angle/frequency axis perspective. The advantage of these graphs is they can let us see broader trends of the speaker’s behavior more easily. For more information about the meaning of these graphs, we again refer the reader to Understanding Loudspeaker Review Measurements Part II. In these graphs, we can better see the breadth of dispersion of the Monolith THX speakers. Again, we see a very wide dispersion pattern out to 80-degrees that extends up to 5 kHz, but here we also get a better look at the width of dispersion at the top end of the treble response. Listening is best done within a 30-degree angle from the front axis, but that is pretty much within the range of most people’s normal listening positions anyway. There are some directivity mismatching issues with the center speaker at around 1.5 kHz and below that are inevitable byproducts of aligning two woofers on a horizontal plane but then spacing them apart, but it is mild in this design compared to most other center speakers. 

Minitower vertical waterfall response.jpg  THX center vertical waterfall.jpg

The above graph shows the Monolith THX speakers’ response behavior along its vertical axis where zero degrees is directly in front of the tweeter, negative degree values are below the tweeter, and positive degree values are above the tweeter. It should be said here that the vertical response isn’t nearly as critical as the horizontal response, so an imperfect vertical dispersion is much less of a problem. The responses seen here are unusually good for this type of speaker. There are lobing nulls caused by conflicting drivers off-axis, but they are very moderate compared to what is normally seen. These speakers actually have a relatively wide vertical angle off-axis where no severe nulls occur. They will probably never be listened to at an odd vertical angle, but it doesn’t hurt to have a good off-axis vertical response due to the consistency in sound from the acoustic reflections from the floor and ceiling.

Monolith THX low frequency responses.jpg 

The above graphs show the Monolith THX speakers’ low-frequency responses that I captured using groundplane measurements (where the speaker and microphone are on the ground in a wide-open area). While I included the responses of both the Mini-Tower and Center Channel Speaker, I didn’t need to since they both measure almost exactly the same. That is not surprising given the similarities in design for low-frequency output. The low end here is pretty much textbook THX bass response with a 12 dB/octave roll-off occurring just above 80 Hz. As was mentioned before, the sealed enclosure provides for a much more predictable phase response in this region which makes it a lot easier for a THX processor to blend the subwoofer with the speakers at the crossover. The bass response here is beautifully flat out to 400 Hz. This speaker is behaving exactly how one would hope for and expect in this range.

 Minitower phase response.jpg

THX Center speaker impedance.jpg

The above graphs show the electrical behavior of the Monolith THX speakers. Again, there is not much point in posting both graphs since they are so similar, but we have done so any way to demonstrate their similarity. Monoprice specs these as 4-ohm speakers, and in that they are correct. The impedance never dips down below 4 ohms, but the steep phase angle at the impedance minima between 100 Hz and 200 Hz may be a tough load for cheaper amps. Budget amplifiers should be OK with this load if it isn’t played loud, but who buys THX speakers to only be played at soft volumes? Outside of a small but important frequency band between 100 Hz and 200 Hz, the electrical load is fairly easygoing. I wouldn’t task an entry-level AVR amp with running these speakers, but a mid-level AVR should be OK.

I measured both the Center Speaker and the Mini-Tower’s sensitivity at 91.0 dB for 2.82v at 1 meter. That is certainly an above-average sensitivity for a bookshelf speaker but not so surprising for large and powerful speakers such as these. While the lower impedance does play a role in that, this higher sensitivity will help alleviate the electrical load that these place on amplifiers. These are relatively sensitive speakers and don’t need a lot of electrical power to get loud.

Atmos Speaker Measurements

Atmos speaker responses.jpg

For those who are curious about the performance of the Atmos speaker, the above image contains a couple of measurements to give an idea about their behavior. These were taken with microphone at a 1-meter distance and cover the vertical axis, but with a coaxial driver like this, the horizontal axis measurements should be very similar. We see a surprisingly well-controlled on and off-axis response out to 5 kHz. The on-axis response does get a bit peakish from 6 kHz to 7 kHz, although the off-axis response holds pretty steady out to 8 kHz. The lack of upper treble is not likely a serious issue here since this speaker banks on reflected sound, and high treble would get the most severely attenuated by reflections anyway. The electrical impedance measurements didn’t contain anything worrisome, and sensitivity was measured at 87.9 dB at 1 meter for 2.83v, which is fine.

Editorial Note On Atmos-enabled Speaker Frequency Response

The Dolby Atmos-enabled speaker is supposed to employ a complex 7-element crossover network to shape its response using a HRTF to enhance the height effect. The peakiness in frequency response noted in the measurements may be a deliberate design attempt to accomplish this goal. Research has found this may NOT be necessary since each individual has their own unique HRTF response and artificially mimicking this in a loudspeaker design can be hit or miss as a result.

For more information, see: Are HRTF really necessary in Dolby Atmos Speakers?

One possible concern that a response like this could cause is that it is a relatively wide dispersion response. The problem is one we mentioned before; in a top-mounted Atmos speaker, the listener ideally shouldn’t be hearing any sound directly from this speaker. The sound heard from this channel should only be arriving at the listener as an acoustic reflection from the ceiling, but, with such a wide dispersion pattern, if the listener is at a slightly high listening angle with respect to the speaker, they could be hearing a significant amount of direct sound from the Atmos speaker which could ruin the intended effect of sound coming from above. To ensure that this doesn’t happen, make sure that the listener’s ear height is below the top front edge of the cabinet. Ideally, the listener’s ears should be level at a height somewhere around the tweeter’s or midrange driver’s elevation. If used at the correct height, the measurements here indicate that this speaker should be able to do its job well as an Atmos height speaker that uses reflections within the limits of such designs.


As always, beforeMinitower grille.jpg wrapping this review up, I will briefly go over the pros and cons of the product under review, and I will start with the cons-- but the Monolith THX speakers are such a well-designed and well-conceived product that the ‘cons’ are few. One con might be that the THX-365T Mini-Towers are essentially very large bookshelf speakers and might be a bit too large in certain instances that a normal bookshelf might work. However, that is the price that must be paid to get this level of performance. A smaller speaker would have a lesser dynamic range. If a smaller speaker is needed but with a similar level of fidelity, Monoprice also has the THX-265B speaker that is THX Select certified. This isn’t really a criticism of the Mini-Towers since what they are doing can’t physically be done in a smaller enclosure; it is just a caveat for those circumstances where size is a consideration. 

Another disadvantage might be their lack of low bass output. However, their low-end response is a deliberate design goal of these speakers, and they would not work as well otherwise. Besides that, if they did have a deeper bass response, that would lower their sensitivity which is not a trade-off worth making for high dynamic range speakers that are intended to be used with subs. So again, this isn’t a criticism so much as it is a notice for those who are looking to their speakers to handle full-range sound: these are not the speakers for that application.

The Monolith THX are some of the most robust bookshelf speakers that I have yet encountered.

Something else that could be said is that the finish of the Monolith THX is not the slickest and that other similarly priced speakers do have higher-end finishes. That’s true, but these speakers are much more about performance for the price and adding a nicer finish would drive the cost up significantly thereby sending these into a price range beyond the reach of many potential buyers. While these don’t look bad in my opinion, the decision to keep costs down by using a serviceable but standard finish was a correct decision. I do wish Monoprice would offer a version of the 365T sans the Atmos driver for those that don't need that feature. This could potentially shave a couple of hundred dollars off the price of a pair.

Minitower and center hero.jpg

Now let’s talk about the strengths of the Monolith THX speakers. First and foremost is their performance; it is exceptional, as one would expect from a THX Ultra certified product. They are speakers with a very linear response, great dynamic range, and very good dispersion characteristics. They sounded terrific with everything that I threw aTHX center.jpgt them over a very wide range of loudness levels. They are a touch warm on the side of total neutrality, and I doubt that many people would have a gripe about their tonality except for those who prefer searing hot treble for the extra ‘detail.’ One aspect I particularly like is that they bring to market an affordable three-way center speaker as an alternative to the many two-way MTM center speaker designs that inevitably have severe off-axis lobing interference. The THX-365C Center Channel Speaker is an affordable, powerful, tonally balanced center speaker that largely avoids the problems associated with that compromised design.

Another strength is the build quality of the Monolith THX speakers; they are very nicely put together and have some very good components with sensible cabinet. They might not cost a fortune to buy, but they are not a ‘cheap’ construction. One does see a little bit of cost-cutting in the finish, but, as was said before, the finish is in line with the goals of this speaker, anMinitower pair12.jpgd it doesn’t look bad. The use of a rounded cabinet does give the speakers a far more attractive appearance than that of an oblong box that they could have had.

The overall story of the Monolith THX speakers is much the same as was told by the Monolith THX subwoofers: here, for maybe the first time, one can get THX Ultra certified components for a very affordable price. These are high-performance speakers for $500/ea (365T) and $400 (365C), respectively. I can’t remember any THX Ultra certified speaker that was priced so low; all of the ones that I can think of were at least four-figure items. It used to be that you needed big bucks to put together a THX home theater system, and in many ways, it still does, but at least the subwoofers and now the speakers can be had without breaking the bank. But beyond THX certification, these are just plain-old good speakers and a bargain at their pricing. While they are intended for home theater, I think anyone shopping in this price range for high-fidelity speaker should give these a hard look so long as the performance attributes are understood, i.e., they do need a subwoofer. Even if their wide dynamic range is not needed, the baseline performance at lower levels is so good that these would also be great for a two-channel music system with a subwoofer (or two—or more). They reproduce sound very well and do so for an extremely reasonable cost given the level of performance.  Highly recommended!  

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
Build QualityStarStarStarStar
Treble ExtensionStarStarStarStar
Treble SmoothnessStarStarStarStarStar
Midrange AccuracyStarStarStarStar
Bass ExtensionStarStarStar
Bass AccuracyStarStarStarStarStar
Dynamic RangeStarStarStarStar
Fit and FinishStarStarStarStar
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:

Space2013 posts on July 25, 2020 11:28
Thanks! That makes sense. I was probably focusing too much on specific dips and peaks and missing the overall trend of the graph. I do want a speaker that is as neutral as possible but has good dynamic range for movies. I'm not seeing a lot of passive speakers that competes with these in this price range for that combination.
shadyJ posts on July 25, 2020 01:45
Space2013, post: 1406414, member: 66533
I’m confused a bit by how the various frequency response graphs compare. The first on-axis graph shows what appears to be a -4 dB drop ambetween 1-2 kHz and a -5dB drop above 5 kHz relative to the frequency response around 400 Hz. However the waterfall graphs the dips don’t look as bad. Is that just because the graph is stretched out more? Aren’t dips like that an issue?
Those graphs are using the same measurements, so yeah, it seems like the scale of the graphs are throwing you a curveball. The thing to look at is overall trends, that will be what is most audible. So what is going to be most audible here versus a perfectly neutral speaker is the downward slope from the bass to midrange. That will give this speaker a warmer sound. While perfect accuracy is preferred, this kind of voicing isn't bad to my ears, although the mid-bass might get too much of a boost if you place them near boundaries like a corner, but even that can be alleviated by some equalization.
Space2013 posts on July 25, 2020 01:09
I’m confused a bit by how the various frequency response graphs compare. The first on-axis graph shows what appears to be a -4 dB drop ambetween 1-2 kHz and a -5dB drop above 5 kHz relative to the frequency response around 400 Hz. However the waterfall graphs the dips don’t look as bad. Is that just because the graph is stretched out more? Aren’t dips like that an issue?
Space2013 posts on July 14, 2020 00:25
shadyJ, post: 1403937, member: 20472
A drop ceiling probably would absorb a bit, especially at lower frequencies. However, there isn't a good way to install in-ceiling speakers in a drop ceiling, so if you want Atmos height effects, Atmos modules aren't such a bad way to get some effect. Make sure the speaker is positioned so that the Atmos speaker is well above your ear level.

I didn’t realize in-ceiling speakers were more challenging to put in a drop ceiling. That’s too bad as that was definitely my eventually. In that case upfiring might be a worth a try.
ryanosaur posts on July 13, 2020 22:17
I suspect the drop panels will attenuate some of the sound as most of those ceilings are supposed to be absorptive/deadening to a certain extent. It's seemingly worth experimenting with if you are interested.
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