Outlaw Audio Ultra-X13 Ported Subwoofer Review

Ultra-X13 Subwoofer Introduction

It has been some time since the North American subwoofer market has seen  a THX certified sub, but the omission of the THX logo certainly hasn’t slowed down interest in ever larger and louder subs. In the face of this race to produce the loudest subwoofer, Outlaw Audio thought it would be a good idea to revisit the principles of THX in presenting the sound as the artists intended rather than only getting as loud as possible. This is Outlaw Audio’s rationale for their newest subwoofer, the Ultra-X13; a subwoofer that can get loud but never at the expense of fidelity to the source content. For THX, raw output ability is just one parameter of performance. There are other parameters that must be considered for a subwoofer to be up to THX’s standard of fidelity such as distortion quantities, linearity in frequency response, conformity to THX’s bass management requirements, compression behavior, and time-related functions like group delay. In fact, THX conducts over 400 tests on subwoofers to insure that they meet certification standards.

THX tested the Ultra-X13 and awarded it their Ultra certification as a single unit. Most subwoofer systems that qualified for the equivalent of the Ultra certification did so as two separate units within one system, so they were basically two subwoofer setups (the Ultra certification used to be named ‘Ultra2’ before THX recently simplified their certification classes). At the time of this writing, there is only one other single subwoofer in the world that is Ultra certified, but that product is only available in Europe. We can see that THX does not hand out their certification easily, and Ultra is a difficult standard of performance to meet.

So the THX Ultra certification is an assurance that the product has cleared a high bar of performance, but what does that mean more specifically? According to THX, a subwoofer with ‘Ultra’ certification should be able to reach 115 dB with low distortion at the listening position of 12 feet or further from the screen in room sizes up to 3,000 cubic feet. It has to be able to do all of this down to the very low frequency of 20 Hz. THX will not release more specific performance criteria than that in order to protect their brand. The THX Ultra2 output requirements do seem to be in pretty close alignment with our Extreme Bassaholic room size rating.

Outside of a desire for a THX Ultra certified subwoofer, Outlaw Audio wanted a new flagship subwoofer that offers a major performance increase over their existing subs, and a single subwoofer that could satisfy a major craving for uncompromised bass so listeners did not have to resort to using multiple subs to hit their performance targets. They wanted to achieve this in a subwoofer that wasn’t tremendously larger than their existing top-of-the-line sub, the Ultra-X12; what is the point in producing a high-performance subwoofer if few people would tolerate having it in their living rooms? It had to be acceptable to other inhabitants of the home. A large performance gain without a large size gain necessitates much more powerful components, and this is exactly what the Ultra-X13 brings to the game. Of course, much higher quality components mean a much higher price, and at $1,499, the Ultra-X13 is more than twice as expensive as the Ultra-X12. The question that faces us now is what kind of subwoofer is Outlaw Audio offering us for $1.5k?

Unpacking and Setup

The Outlaw Audio Ultra-X13 arrived on my doorstep in a large box. I would have thought something as large as its packaging would have required freight shipping via a pallet, but it seems that it falls just under the limit for that shipping method. However, it is robustly packed, so it should be protected from harm under normal shipment conditions. It comes double boxed with heavy-duty cardboard edge protectors, and the subwoofer comes sandwiched between two very large, thick polyethylene foam pieces. The subwoofer was wrapped in a large, black cotton sack with a drawstring, which suggested a high-end item. Another classy touch is the white cotton gloves that come with it, to prevent fingerprints from marring the satin black finish during placement. The packing was quite good, and users would be advised to hold onto it in case the Ultra-X13 ever needed to be shipped again or relocated to another home.

The Ultra-X13 is a large sub, and, at 105 lbs., fairly heavy, so it takes a two-person team to safely move it, unless you are using furniture sliders. It has spiked feet with rubber tips, so it needs to be lifted to be moved, otherwise it will leave streaks if dragged. For those who will be interested in experimenting with the different operating modes it has available, I wouldn’t pack it away too far into a corner, because you will want to have access the bottom of it to adjust operating modes. The included manual has a fair amount of depth but is still accessible to those who are new to subwoofers. The different operating modes and their effects in the response is detailed well-explained. All-in-all, the Ultra-X13 is not tremendously complicated to set up and get going, but it might take a little bit of a workout to get it into place, and that is typical for subwoofers of its size.

Appearance

     

Compared to many other subwoofers, the Ultra-X13 looks positively ‘stately.’ The driver and ports are hidden on the bottom. It doesn’t have many visible features and relies on negative space for the overall thrust of its aesthetic. ‘Clean’ is the operative word, with its satin black finish, curved side edges and beveled top edges. There is a smoked plexiglass inset panel on the top that can serve as a tasteful tabletop. The overall look reminds me of the monolith from ‘2001: A Space Odyssey’ but with different dimensions. Despite its large size, it should be able to disappear in a living room or family room due to its clean, uncluttered appearance and satin black finish acts as a light sink in low light conditions. It can be used as an end table and can be further hidden by end table covers if so desired. For a large subwoofer, it does have an understated and handsome appearance that shouldn’t draw too much attention to itself, so long as it is given an out-of-the-way placement. Its a good style for people who do not mind the appearance of nice-looking speaker equipment and also for those who would rather their audio equipment not be the centerpiece of the room.

Design Overview

The most basic design description of the Ultra-X13 is that it is a variable-tuned ported subwoofer. What that means is the user can adjust how low in frequency they want the subwoofer to play, but in exchange, they give up low-frequency output, so it can be made to play either very low frequencies or not-as-low frequencies at louder levels. This is done by sealing or opening one of the Ultra-X13’s two down-firing ports, and also selecting the mode on the ‘EQ Switch.’ Outlaw Audio specifies the low-end responses to be 15 Hz when the Ultra-X13 is set with one port open and the EQ switch is set to ‘EQ1’, and 20 Hz when both ports are open with EQ switch is set to ‘EQ2’. It is reasonable to assume, from these specs, that the user can choose between low-frequency extension down to 15 Hz, or louder deep bass at 20 Hz and above. The higher output advantage of having all ports open usually only makes a significant difference in port-generated frequencies, meaning it will affect the lower end of the sub’s playback range the most and not so much mid-bass and upper-bass frequencies. In the case of a very deeply tuned subwoofer like the Ultra-X13, I wouldn’t expect the different operating modes to affect performance very much above 60 Hz or so, but we will see a detailed comparison of the performance differences when we reach the measurements and analysis section of this review.

The Ultra-X13 also supports a sealed mode for those users who would prefer that the ports not generate any output at all, and this greatly diminishes its low-frequency abilities, although it should improve some time-related performance aspects, even if only slightly.

The Ultra-X13 is only THX Ultra certified when configured with both ports open and EQ switch set to ‘EQ2’. This mode should grant the user the most output with the least distortion. Setting the Ultra-X13 to lower tuning points reduces the port-generated output and also forces the driver into greater excursions that more easily push it into distortion, thereby disqualifying it from THX’s Ultra certification. This is not to say it sounds bad or performs poorly in these lower tuning points. It is only saying that it doesn’t quite meet the high bar of Ultra certification.

      

In order to get low distortion and high output in anything but a gigantic cabinet, a very long-throw yet highly-linear driver is needed. The 13” Ultra-X13 driver would seem to foot that bill. The cone body is a long-fiber pulp laminated to a partial glass fiber body. The cone is connected to the cast aluminum basket by a Nitrile Rubber surround. The surround uses a double roll shape in order to allow for more cone surface area without hindering excursion or reducing the linearity of its throw. Supposedly, this geometry is better behaved at the extremes of excursion as well. The spider is made from two layers of woven Nomex that are heat-pressed together. The second layer is a thinner material used in the innermost corrugations where the stresses are the most severe. Reportedly it took six iterations of suspension design before the Ultra-X13’s driver engineer, Dan Roemer, was satisfied with its linearity.

The motor is an overhung design using an aluminum voice coil. Two 6” diameter magnets with a 1” thickness make the bulk of the motor. That should provide plenty of flux for a strong motor force.  A bumped backplate allows a much greater throw without danger of bottoming out the voice coil former. Outlaw claims the driver has 80 mm of peak-to-peak excursion, so it will need as much room for throw as it can get. Three aluminum shorting rings are used to reduce induction, thereby giving the driver a wider band of operation and less even-order harmonic distortion. FEA analysis and Klippel testing was heavily involved in many stages of design for optimizing both the motor and the suspension. The driver was produced specifically for this subwoofer and is one of the first in THX’s “approved by THX” component certification program, where individual components can get THX certification. While this certification looks to be an assurance of quality, one can see by merely examining the driver and learning about its design that it is certain to be a high-performance part.

 

The Ultra-X13 uses a 1,000-watt digitally-controlled amplifier which is composed of two independent class-D 500 watt modules, each with its own power supply. The two amps are bridged across the X13 woofer with the added benefit of two power supplies working in unison with the differential amplifier outputs to yield twice the power that 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. In addition to a pair of RCA inputs, for LFE or for a stereo input pair, there is also an XLR input and output (XLR balanced inputs are much quieter for long runs than traditional RCA inputs). This makes it easier to incorporate into pro-audio equipment and also allows the user to easily daisy-chain multiple subs for cleaner wiring. There is a variable low-pass filter running from 40 Hz to 160 Hz, and also a variable phase control instead of a simple phase switch.

The THX Ultra certified operation requires the settings to be at certain values in order to let the controlling software aboard the THX processor have precise control over the subwoofer. It must be remembered that only one component of a THX system does not grant you a THX level reference system. In order to have a full THX setup, the entire system must be THX certified: subwoofer, speakers, amplifier, processor, etc. Everything must be setup correctly as well, and the room acoustics must be addressed too. A full-scale THX setup is a comprehensive system, and addresses nearly every technical aspect that affects the experience. That being said, even having only one or two components with the THX badge would be improved over those components not being able to meet THX certification, so THX certified products can still be helpful even if incorporated into a non-THX system. The principles of THX still apply to any part of a sound or video system and can be a marked improvement over lesser performing component, and this is especially true of the subwoofer, hence the raison d’etre of the Ultra-X13.

     

The cabinet is a sturdy construction. The side-panels are made from 1” MDF with the lower panel holding the driver and ports having a 2” thickness. There are two window braces that bisect the cabinet at perpendicular angles, with the horizontal brace being 1” thick and the vertical brace using ¾” thickness. The woofer is slightly recessed in the cabinet to allow room for its high excursion without touching the floor. That, in combination with the 2” height of the feet, should give the driver plenty of clearance for its throw. The internal enclosure walls are lined with polyurethane foam to reduce panel resonances. The two ports have an approximate 4” diameter and 20” length including the flared opening. Such large ports are needed for a vented sub to produce lots of deep bass cleanly. Overall the build quality is very good. It is sensibly solid without being overbuilt to impractical ends. 

Outlaw Audio Ultra-X13 Subwoofer Listening Tests

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. Receiver used was a Pioneer Elite SC-55 with a variety of different speakers, and the crossover was used mostly at 80 Hz and also 100 Hz at times. 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 this subwoofer in this review, but for any subwoofer in any review. Most listening was done with both ports open and the EQ switch set to ‘EQ 2.’

Music Listening

One album that I listened with the Ultra-X13 was ‘Flight of the Cosmic Hippo’ by Bela Fleck and the Flecktones. This eclectic instrumental album is a fusion of bluegrass, bebop, and jazz and has superb bass guitar playing from bassist Victor Wooten. It is popular among audiophiles for demonstrating the bass prowess of their speakers, not just for Wooten’s deft bass wizardry but also for some of the beefy electronic percussion from their percussionist “Future Man.” This is an album that can make near constant use of a subwoofer, although usually the usage is subtle and is not the flashy boom and rumble that other subwoofer demo material will get into. This album is a great way to demonstrate the subtlety that a well-integrated subwoofer can add to a music-listening experience, simply by switching the subwoofer off in middle of a song. The subwoofer doesn’t seem to be contributing much to the experience until it is turned off, at which point it is as though the foundation of the music is pulled out from under it. A good, well-integrated subwoofer is mostly invisible on playback but is still crucial to its experience, at least if the main speakers are not truly full-range. There are moments where subwoofer use is clear, such as the title track that has a lot of energy in the mid 30 Hz and 40 Hz range, and is such a ‘bottom dweller’ of a recording, in the sense of having so much low-frequency energy, that it is obvious that the sub is doing most of the heavy lifting.

On “Flight of the Cosmic Hippo”, the Ultra-X13 blended the bass of the subwoofer band with the main speakers very well. After a few tracks I even ran the sub a few dB hot just to see where I could get it to ‘reveal’ itself on this album, and it still maintained its invisibility aside from giving the kick drums a bit more punch. But when I turned it off, the bottom of the sound falls out and a big chunk of the music is lost. The Ultra-X13 followed the fundamentals of the tuneful and complex bass lines with precision. Percussion has the room-filling, low-end punch called for by the sound mix. Some people might look at a large ported, design like the Ultra-X13 and assume such a subwoofer is made primarily for movie explosions, but that is definitely not the case. The Ultra-X13 is made to reproduce the source content without adding or subtracting anything that the sound engineer intended, and this philosophy was very evident in the time I spent listening to “Flight of the Cosmic Hippo.”

Another album I tried on the Ultra-X13 was a recording of Symphony No.12 ‘Lousto’ by the Finnish composer Kalevi Aho. This composition and recording is truly epic. It uses two orchestras: a chamber orchestra and a full orchestra, each on opposite sides of the audience. On all sides of the listener are different types of horns and percussion, and to get the full effect, this recording should be listened to in its surround sound SACD format with a corresponding setup. It may well have the best use of surround sound for music recording I have heard yet. It is no slouch in subwoofer frequencies either, with tremendous use of bass drums in the first and last movements. In Symphony No.12, the bass drum playing revolves all around the listener, not because the sound engineer is panning the sound, but because the percussionists are situated all around the listener. This colossal performance requires 120 musicians sprawled all around the audience, and the end result is a towering, monumental sound.

The Ultra-X13 gave the bass drums weight and a sharp attack. The thump of the attack could be felt as well as heard when the drumming was at its most intense. This album has a very wide dynamic range, so the drumming ranges from soft to very loud. If played at reference levels, they can get strikingly loud, and this album has to be played back loud to do its dynamic range justice. I cranked the volume and the Ultra-X13 kept pace. I could feel my sofa taking a beating from the sound pressure waves generated by the subwoofer. Throughout the recording, the bass sound of the Ultra-X13 was cohesive with the main speakers, and with the exception of the vibration of my sofa, I never felt it to be separate; the sound was unified. I do think that the full might of this Symphony No.12 would require more than a single Ultra-X13 is capable of, not because the X13 is in any way weak, but because this recording is extremely demanding in dynamic range to reproduce at realistic levels. I am not sure anything but the most powerful commercial subwoofers would be able to tackle Symphony No.12 single-handedly. I think, however, that a single Ultra-X13 would suffice for the loudest levels at which most people would ever care to listen to ‘Lousto’.     

An album that I thought would make for a great test of a subwoofer’s ability for nuance as well as force is Lustmord’s “Carbon / Core,” released in 2004. This album would fall into the genre of dark ambient although there is a bit more drama here than one would find in typical atmospheric ambient music. To me it sounds like the music score for a slow-burning but wicked science fiction/ horror film. Sometimes it is purely atmospheric but in some moments it can be aggressively forward. Sounds of distant drones are superseded by dissonant bells, swarms of mechanical creatures, subterranean thuds, and other anxiety-inducing sounds. They are all artfully arranged and would make a perfect accompaniment to a gallery exhibition of H.R. Giger landscapes. The bass can hang back subtly in recessed hums or creep forward in looming drones, but at some moments the bass can burst forward in grinding, guttural blocks of aural assault. Lustmord is not afraid to dig deep for his bass sounds, and frequencies below 30 Hz are often present on this album.

The Ultra-X13 handled “Carbon / Core” with the same finesse that it reproduced the more earthly music that I had given to it heretofore. Passages of clean low frequency content were easily distinguishable from passages that were rich in harmonics and therefore thick with richly textured sounds. Deep rumbles, thunderous growling, and undulating buzzing noises were all distinct on the Ultra-X13 as opposed to a single booming noise that a low-quality subwoofer often reduces those different sounds to. The Ultra-X13 only made its presence known when the music pushed it into low-frequency playback of such depth and magnitude that was clearly beyond the capabilities of the main speakers. Outside of those moments, it made for a seamless partnership with the main speakers. “Carbon / Core” is a nightmare world of music, and the Ultra-X13’s fidelity helped to realize that world with a level of vividness that I imagine would give anyone a sense of apprehension.

For something that makes far less subtle more and more explicit use of subwoofers, I turned to the genre of “Drum and Bass” music, where low-frequency content is nearly maximized. “Drum and Bass” is a rather literal name for this genre, since it serves as a descriptor as well, being a music made largely from bass and percussion. I choose the 2005 album “Us Against the World,” a mix by ‘D’n’B’ duo Evol Intent. This album is on the harder end of Drum and Bass, and the bass here is brutal, heavily compressed, and unrelenting. It’s rough stuff, and the subwoofer gets near constant use.

The Ultra-X13 proved to be as adept with monster loads of raw low-frequency bass as it was with merely providing a subtle foundation for much more restrained types of music. The percussion and bass were utterly visceral. The bass lines gave me a buzzing sensation along my spine, and I felt the kick drums in my chest. The Ultra-X13 was hitting so hard I could even feel the toms. No doubt that my seating in such close proximity to the sub was a major factor in these tactile sensations, but the point was made that, in the right conditions, the Ultra-X13 can be an utterly bruising subwoofer. I decided to have some fun and run the bass super hot for a while to see how much of a beating the Ultra-X13 could take and also to see how much of a beating I could take. Who would break first, me or the sub? It turned out to be a draw; the album finished before I gave in or the sub gave up. I have to admit to feeling slightly battered afterward, but the subwoofer didn’t seem to be any worse for wear. I can report that the Ultra-X13 is a great choice for bass-heavy electronic music.

Movie Listening

One movie I listened to with the Ultra-X13 was the 2014 Tom Cruise film “Edge of Tomorrow.” This science fiction opus is notorious for the low frequency content in its opening credits, with strong tones starting at 30 Hz and stepping down in 5 Hz increments to 10 Hz where it nearly saturates the dynamic range to full scale in the sound mix. Among connoisseurs of infrasonic content (and yes, such people exist), it is known as one of the toughest scenes of any sound mix to fully reproduce. It is generally not a good idea to crank this scene with just any subwoofer, because if there was ever a scene that could make a subwoofer bottom out, this is it. Yes, I cranked the opening scene of “Edge of Tomorrow” to reference level, because I wanted to see how well protected The Ultra-X13 was. To be sure, that is something I would be hesitant to do with my own subs, and if the Ultra-X13 didn’t survive, I could just weasel out of responsibility by telling Outlaw Audio that the sub needed a bit more work. Happily, such evasion wasn’t needed, as the Ultra-X13 barreled through the scene without any sounds of distress. 10 Hz is well below the tuning point of the sub, and it wouldn’t have been able to produce a lot of sound at that frequency, but the point of the exercise was to see how well protected the Ultra-X13 was, and it passed with no problems.

The rest of the film was reproduced with the force and precision that was standard operating procedure for the Ultra-X13 by that point. Explosions, crashes, gunfire and other warfare sounds mixed in science fiction sounds such as aliens burrowing underground and ambushing soldiers on the Edge of Tomorrow sound mix, and it all sounded quite convincing with the Ultra-X13. Given what I understood of its design, none of that came as a surprise to me. High linearity means that the subwoofer will be able to handle any kind of content with equanimity.

Another movie I watched using the Ultra-X13 was the 2015 science fiction survival film “The Martian.” I choose this movie for something that will take advantage of the subwoofer but doesn’t hammer it all the time. When bass is used constantly, it becomes numbing as the listener acclimates to it. Low frequencies can be much more effective when used sparingly. The Martin does not pound the listener into submission like a war film, so when there are some big bass moments such as a rocket launch, it is all the more striking.

In the moments during The Martian where the subwoofer was called upon, the Ultra-X13 was resounding. Martian storms shook the room, and the occasional blast had a concussive punch. Harry Gregson-Williams’ music score gave the sub some work to do as well and was not averse to electronic sounds, so the Ultra-X13 was able to shore up the music during its more tense passages. It may be that a movie sound mix that is bloated with bass can disguise the shortcomings of a lesser performing subwoofer, so it is sound tracks with more restraint like this one that can expose how accurately the subwoofer is reproducing the sound. If so, the Ultra-X13 sounded like it was on point for this sound mix. When I was watching this film with the Ultra-X13, it didn’t sound as if the bass was coming from the subwoofer; it sounded as though it was simply the soundscape of the movie. For a film like this, that is what any speaker should sound like- as though it does not exist, and the sound is emanating from the setting. 

Outlaw Audio Ultra-X13 Subwoofer Measurements and Analysis

The Outlaw Audio Ultra-X13 was tested using ground plane measurements with microphone at a 2 meter distance in an open setting with well 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 70°F and 55% humidity.

Frequency responses for various configurations of the Outlaw Audio Ultra-X13 

The above graphs are the baseline frequency responses for combinations of ported/sealed modes and the EQ switch. Each graph depicts how the EQ switch affects the response for that operating mode of the Ultra-X13. We can see that the EQ switch set to the EQ 2 position will grant us the flattest response, except in the sealed mode, where EQ 1 nets the flattest response in that mode. EQ 1 allows deeper extension, and in ported modes, it looks to give a bump in the deep frequencies. Those who want more low-end ‘oomph’ on movie night can flick the EQ switch to EQ 1 for more deep bass, and when they would rather have a more linear response, they can switch back to EQ 2. The flattest response of all is the THX certified mode of both ports open and EQ switch set to EQ 2. This gives an astonishingly flat response window of +/- 0.5 dB from 30 Hz to 125 Hz. Rolloff in that mode starts at 30 Hz and it is down by 6 dB at 22 Hz which is where is +/- 3 dB window would begin. With one port open and EQ switch set to EQ 1, we lose the near-perfect neutrality of the 2 port / EQ 2 response shape, but we gain a wider +/- 3dB response window of 17 Hz to 170 Hz.

Users in small rooms who see a lot of low-end room gain might prefer the EQ 2 setting in either the sealed or 1 port open configurations, unless they like lots of deep bass, in which case 2 ports open with an EQ 1 setting should give them a tremendous excess of deep bass. The upper end response in any mode is relatively good, and the Ultra-X13 could be used with high crossover points without much problem.

                                                         Outlaw Audio Ultra-X13 CEA-2010 Burst Test Measurements

2 Ports Open, EQ 2

1 Port Open, EQ 1

Sealed, EQ 1

Frequency (Hz)	SPL (dB)	THD + N (%)	Harmonic limit	SPL (dB)	THD + N (%)	Harmonic limit	SPL (dB)	THD + N (%)	Harmonic limit
10	-	-		-	-		74.5	10.2	9th
12.5	-	-		96.9	12.4	5th	87.7	11.8	5th, 9th
16	98.1	22.9	9th	102.3	16.6	6th	95.8	18.0	5th
20	106.8	11.1		105.8	16.1		100.9	18.1	5th
25	111.1	9.9		108.9	9.2		103.3	11.8	6th
31.5	114.1	4.0		111.7	5.5		109.5	9.5
40	115.2	4.4		113.1	4.3		111.4	4.4
50	115.0	4.4		113.7	6.2		112.5	6.0
63	115.1	5.4		114.1	5.8		113.5	6.8
80	115.1	5.3		114.3	6.9		114.0	7.4
100	115.0	4.2		114.2	8.1		114.0	9.2
125	114.6	5.1		113.6	9.7		113.5	14.6

 

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 publically available CEA-2010 measurements are shown at 2 meter RMS, so we followed that convention.

The results produced by the Ultra-X13 are all very good, and we do see a couple of noteworthy features in this data set. One thing we see right off is that the maximum distortion levels are generally very low. For the most part, these burst tests are being amp-limited, and the driver is not being pushed into significant distortion except in very low frequencies that are generally outside that modes’ range of operation. Looking at the THX certified configuration of 2 ports open with an EQ 2 setting, we get a sense of what THX is looking for in terms of performance targets. As we see in the baseline response for the THX Ultra configuration, the response from 30 Hz and above is extremely flat, even when pushed to the highest limits in burst testing. It is still almost within the +/- 0.5 dB window that it has at nominal levels. There is very little compression that is impacting the response. What is more, distortion is extremely well controlled in this mode, hovering right around 5%. This is extraordinarily linear behavior, and ‘clean’ is the operative word for these test results, much as it is a description of the sub’s appearance.

The other configurations of the Ultra-X13 do not quite have the linearity of the THX Ultra mode, but they are still quite good. EQ 1 with one port open grants us deeper bass but at the cost of output above 20 Hz. The sealed operating mode can gain deeper bass than that, scoring a rare passing 10 Hz burst measurement, albeit at a very low SPL. 

One interesting characteristic is that the limiting harmonics are of relatively high orders. This may be due to the amplifier’s limiter clipping the shape of the signal. If it were the driver, one would expect to see the subwoofer limited by 3^rd-order harmonics, but it may be that the design optimizations of the driver has also reduced the lower harmonic distortions in proportion to the higher orders.

It should be stressed here that there is a very significant difference between just barely passing a CEA-2010 burst test and passing it with flying colors like as we see here with 5% distortion percentages. There is an audible difference in the character of a burst tone as distortion begins to creep up as a percentage of the measured sound. A passing CEA-2010 measurement does not necessarily mean that sub sounded ‘clean’ for that frequency and amplitude. While distortion is generally more difficult to discern in low frequencies, CEA-2010 testing can allow almost as much as 40% harmonic distortion, and that is very easily heard. We should also keep in mind that this distortion comes at the expense of the fundamental, the ‘intended sound,’ so it does not just tag along with the intended sound, it displaces it. Subwoofers with the same passing CEA-2010 measurements are not necessarily equal in sound quality, and one should always look distortion quantities in the test if they are interested in fidelity as well as output.

Frequency Breakdown of CEA-2010 Burst Measurements for the Outlaw Audio Ultra-X13 

The above graphs show the measured frequency spectrum of the increasing CEA-2010 burst tests. Essentially, it depicts 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 Ultra-X13 in its THX Ultra certified configuration of 2 ports open on the EQ 2 setting. We do see significant amounts of distortion in the frequencies below this mode’s range of operation of 20 Hz, and that is expected, since the driver and port are at odds with each other in those frequencies instead of working together. Above that point, distortion drops dramatically, and at 25 Hz and above, the most severe harmonics stay well over 20 dB lower than the fundamental at the worst. That means they will not even be close to 10% total harmonic distortion, and this is what we saw in the CEA-2010 table, where total harmonic distortion hovered around 5% for this range. This level of distortion would essentially be inaudible. The story here is that at 25 Hz and above, the Ultra-X13, in its THX Ultra mode, can not be pushed into making an audible sound that does not belong. This subwoofer is a highly-controlled system.

Those who want to know how distortion can affect low-frequency sound can read this Audioholics article: The Audibility of Distortion in Bass Frequencies.

      

Long term output sweeps of the Outlaw Audio Ultra-X13 in different operating modes

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 m from the microphone. We then conduct further 20 second sweeps by raising the gain by 5 dB until no more output could be rung out of the subwoofer. In these tests, we can see that unlike the burst tests, the shape of the response of the THX Ultra mode does compress a bit on the very highest sweep. The compression is relatively light in magnitude and would not make much difference in any practical usage. As one would expect in a variable tuned subwoofer, the most headroom can be had when both ports are open and in the EQ setting most suited for that operating mode. Relative to its response at nominal levels, some response is lost above 60 Hz, but it mostly compresses above 90 Hz, so it is unlikely to have a meaningful effect in real world content. In contrast, when run with 1 port open on the EQ 1 setting, the upper end actually flattens out at its maximum drive level, although response below 30 Hz does get compressed. The same effect is seen in the sealed mode.

The good news from these graphs is the responses in any of the modes do not lose shape until driven to their limit. The character of sound in any program material would not substantially change throughout the majority of the Ultra-X13’s dynamic range. Furthermore, there was never a moment when the subwoofer ‘complained,’ or made any noises of mechanical distress. The driver never bottomed out nor did I hear any obvious distortion. This is significant because these tests are the toughest that are conducted for our reviews. If the Ultra-X13 could have bottomed out, it would have during this round of testing, but it didn’t. This subwoofer is very well protected against damage from high excursions.

     

Outlaw Audio Ultra-X13 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. As with the burst output distortion measurements, the Ultra-X13 proves to be a very linear and well-controlled subwoofer for continuous output as with short-term output. While the 1 port open, EQ 1 mode would seem to have the least distortion, at a glance, the ‘THX Ultra,’ 2 ports open, EQ 2 configuration has the least distortion in the frequency range that are much more commonly used among these operating modes, wavering from 5% to 6% THD from the low 20 Hz range to around 90 Hz at the very highest drive level. At the nominal drivel level of the 95 dB sweep, total distortion floats around 1% of the sub’s output. That is ridiculously low distortion for a subwoofer. Keep in mind that this range is the supremely flat response range seen in the baseline response. The Ultra-X13 is keeping its promise of extremely accurate sound reproduction all around, in both realms of linear and nonlinear distortions.

For those who are wondering about the spike of distortion in the sealed graph, what is happening is the driver is being pushed very hard as it goes down in frequency. To reproduce an octave lower for any frequency, the driver has to quadruple its excursion to maintain the same amount of output, and we can see here where the driver is running out of linear throw. A limiter kicks in and restrains the driver’s motion as we go a bit lower in frequency, so distortion falls back down as the driver is reined back into a more comfortable operating range.

    

     

    

Component harmonics of the Outlaw Audio Ultra-X13 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 up to the fifth harmonic. The individual harmonics can give us a clue as to what might be the cause of some quirk or non-linearity. The Ultra-X13 bears some noteworthy features. One feature to note is the lack of fourth or fifth harmonic distortion products above 25 Hz in the ‘2 ports open’ mode. This absence of distortion carries over to higher harmonics as well. There are minute quantities present, and they do add up, but they don’t add up to much. That is all the better for the Ultra-X13, since higher-order harmonics are much more audible and objectionable than lower-order harmonics.

In the sealed operating mode, we see that the third and fifth harmonics are the chief contributors of distortion, and that the second harmonic stays at relatively low levels. This indicates that the driver has seen some significant optimization, since even-order harmonics signifies that the woofer is being inhibited by something on only one side of its travel. The small but even spread of the second harmonic out to upper frequencies look to be caused by inductance. It is kept in check relatively well considering the how much movement the coil is undergoing and how much current is pulling through it. The shorting rings are seen to be paying off here.

 

Outlaw Audio Ultra-X13 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. The Ultra-X13 gives a very good showing here, as almost all delay for every mode is kept under 1 cycle except below 20 Hz where a slight excess of delay would be very difficult or impossible to discern. For deep bass, the sealed configuration fares the best, which is expected since the port output is normally delayed by a cycle. We do see the effects of a high-pass filter in the sealed configuration, but it is very mild. No group delay from the Ultra-X13 would even be close to being audible. The bass from the Ultra-X13 is tight and does not suffer from overhang, and that much is demonstrated in this chart. Again, it shows itself to be a superbly linear sub with very high sound quality.

 

Outlaw Audio Ultra-X13 low-pass filter effects on response

The above graph depicts the effects of the Ultra-X13’s low-pass filter, and the curves ending from left to right are filter settings of 40 Hz, 60 Hz, 80 Hz, 120 Hz, 160 Hz, and bypassed. The low-pass slope is a 4rth order, 24 dB/octave rolloff. This slope should make it easy to blend with ported speakers if the Ultra-X13 is used in setups that lack modern bass

Outlaw Audio Ultra-X13 Subwoofer Conclusion

Those readers who have made it this far may have noticed I have not said much critical about the Ultra-X13 and that the tone of this review has been generally positive. This is because the Ultra-X13 has given me virtually nothing to complain about. This is a subwoofer that has lots of strengths and no real weaknesses. If there is any drawback at all to this sub, it is that the Ultra-X13 is somewhat large and a bit heavy. That is difficult to hold against it, because this is something buyers will know going in, and besides, this kind of low-frequency performance requires an enclosure of this size. Reducing the size will also reduce the deep bass performance by the laws of physics, and I will gladly accept the larger size for its terrific deep bass performance. As for heaviness, it could be worse. The Ultra-X13 should be picked up and carried by two people, but it is movable by a single healthy adult by being ‘walked’ on its spiked feet. As was said before, it should not be dragged, but moving it by lifting one end up and pivoting it on its feet is doable.

 

The overall performance is very good. There are some subwoofers in its same price range that can get louder and dig a bit deeper, but the advantage of the Ultra-X13 is its consummate fidelity, especially in its THX Ultra certified configuration. As was said before, in this mode, it cannot be pushed to make an unwanted noise, and it can not be driven into error. It maintains its extremely linear response at all drive levels in burst tones and almost all drive levels in long term tones until it gets to the last few dB, where a minor amount of compression occurs, but burst tones are more representative of the transient nature of real world content than the continuous sound of a long-term sweep. Every aspect of its performance is solid, and, in my opinion, it has no shortcomings. I have dealt with subwoofers that were higher performing in one metric or another, but the Ultra-X13 put up a very good showing in every single measurement. Nowhere is it lacking or below average.

The Ultra-X13’s splendid acoustic measurements translate into a great sound, as I found out in my time listening to music and movies with it. It sounded great, period. I could happily live with this sub. For my sometimes extreme tastes, I think I would want two, but I am certain most people would be very satisfied with one. This is a subwoofer that, at 2 meters in an outdoor ground plane setting, can belt out a tight average of 115 dB from 30 Hz to 125 Hz and do so with only 5% distortion, and that is more than enough headroom for most people. I think the average user would rarely crank it that hard, but, if they ever wanted to, the Ultra-X13 will do that without any danger to itself. This is a subwoofer that can be pushed hard without fear of being over-driven. If an owner wanted to impress their friends, it is fine to turn to maximum volume for a short while, just to “see what it can do.” We all know the type of guy who does that as soon as they get any kind of new toy (and yes, it’s almost always a guy who does that), but the Ultra-X13 can take it. Of course, it shouldn’t be subjected to that sort of abuse indefinitely, because if it was, it would break down eventually, but for short periods, go ahead and let it rip; the Ultra-X13 can handle it.

The Ultra-X13 earns our Bassaholics “Extreme” Room Rating, which means it should be able to achieve 115 dB peaks at the typical listening position in a 5,000 cubic foot. The Ultra-X13 just barely clears the hurdle to achieve that with respect to maximum output at 31.5 Hz and above, but it clears it nonetheless, and it doesn’t struggle at all in doing so, since its distortion is so low.

Those wishing to know more about our room rating system can read this article: Bassaholic Subwoofer Room Size Rating Protocol.

It is worth mentioning here that clean, distortion-free bass will sound significantly quieter than bass with audible quantities of distortion. The reason is that harmonic distortion is sound that is higher in frequency than the sound that it is a product of, and, in the subwoofer frequency band, human hearing rapidly rises in sensitivity to sound as frequencies move upward. To see how severe this is, look at the frequency range of 10 Hz to 100 Hz in this chart of the equal loudness contours. What that means is that normal human hearing can hear the upper frequencies of the subwoofer band much more easily than the lower frequencies. For example, a 90 dB tone at 30 Hz only has to be 70 dB at 70 Hz to be heard as the same loudness level, even though it is a 20 dB difference in sound pressure level. To put that another way, a 30 Hz tone (which is a deep bass sound) must be ten times more powerful in amplitude than a 70 Hz tone (which is considered a mid bass sound) to be perceived by human hearing as the same loudness! Since upper bass is so much more easily heard than lower bass, when harmonic distortion does become audible in bass frequencies, it can be substantially louder to human hearing, even against a non-distorted bass sound of a much greater amplitude.

Bottom Line

A big, black, ported subwoofer stamped with the THX logo is sure to get dismissed by audiophiles as merely a ‘home theater’ sub that is primarily meant for making explosions louder, but in the case of the Ultra-X13, that would be a mistake. In fact, there are less expensive options that are more suited for that task. For those who want accuracy first and foremost, the Ultra-X13 is a great choice. For those who want their music reproduced without any additions or subtractions, the Ultra-X13 is an ideal candidate for that job; the output is faithful to the input. Acoustic summations and cancellations of room modes will inevitably take their toll on the bass response, so it might be worth getting multiple subwoofers to even out the response for those who want a truly neutral response in-room. Outlaw Audio does give a hefty discount for the purchase of duals, at $2,500 a pair (not including shipping), but the $1,500 cost of a single is still a good deal, in my view.

Outlaw Audio offers a 30-day full refund for those who wish to return the Ultra-X13 for any reason. They will also pay for return shipping, although not the initial shipment cost. The buyer receives a 3 year warranty on all parts along with Outlaw Audio’s good reputation for customer support. The Outlaw Audio Ultra-X13 is a fine product and a good value, in my opinion. I wish that every product that is sent to me for review gave me as much cause to praise and so little cause to complain. 

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
Fit and Finish
Ergonomics & Usability
Features
Dynamic Range
Performance
Value