Product Review

ACI Force Subwoofer

April, 2004

Arvind Kohli



● Driver: 10� Long Throw
● MFR: 20 Hz � 250 Hz
3 dB
● Power Amplifier: 250 rms, with

    < 0.5% THD
● Dimensions: 13.5" H x 16.75" W x

● Weight: 48 Pounds
● Available Finishes: Architectural,

    Oak, Black Oak, Mahogany, Walnut,

    Maple, Rose walnut, Cherry
● MSRP: $799 USA

Audio Concepts, Inc.


�You can have it all�, is the promise of modern life. Great job, great family, great house, great health, great vacations, great food, great sex, etc., ad nauseum. Well, I never did subscribe to that notion. I have always believed that if you try really hard and with some luck, you can only have some of �it' at any given time. If you don't believe me ask Messrs. Jagger and Richards, as they seem to have said something very clever about this a long time ago.

The same seems to hold true for speaker design. Of the several thousands of models available, there isn't a single one that will give you all you can get in terms of frequency response, dispersion, detail, dynamics, imaging, low distortion, power handling, and cost. Perhaps, there are a few models that achieve great success in most of these areas, but cost more than a car, and in some cases several cars.

We each have to decide what is important to us, and how best to attain it. My priorities and preferences have steered me down the mini-monitor route, and for the most part, I am quite content to live with the compromises that are adjunct to that choice. Of course, every once in a while, I want to believe that even if I cannot have it all, maybe I can have just a little more.

What I am talking about here is bass extension.

Most mini-monitors provide adequate bass response, at least for music listening. My current reference the Dynaudio Contour 1.3 MkII has a rated extension of �3 dB at 42 Hz, and has useful output well into the 30's. So, for most of my listening, a subwoofer would not be doing any work at all. But on many tracks with synthesizer and organ contributions, I am missing out on that lowest octave of human hearing, that is so very tactile and emotion rousing.

As much as I desire that last octave or so, I would rather not have a subwoofer than have one that has any of the following three shortcomings common to sub/sat setups:

1) The crossover point does not go down low enough. This results in a substantial overlap with the monitors and an unacceptable bass hump for a small range of frequencies. Most subs on the market today offer a low-pass filter that is variably selectable from 40 Hz to 120 Hz. Mating those subs with speakers that have a �3 dB point lower than the mid 40s (which is somewhat common) will invariably result in a bass hump. That may only occasionally be audible, but will also be persistently annoying.

2) Lack of a phase control. If the frequencies of the satellite and subwoofer are completely out of phase, then the region where the sub and sat overlap frequencies will experience cancellation, and the lower frequencies could sound out of synch with the rest of the music. You can also adjust for phase by altering the placement of the sub, but the ideal spot for phase may not be practical for your living considerations.

3) Inertia of the woofer. If the driver in the sub cannot start and stop with the same quickness as the woofer in the sat, then you have not extended the response of the satellite, but simply added another device that plays low frequencies, but with a different tonality. To achieve woofer quickness, many approaches have been taken, some of the more common ones being a very light weight woofer cone, a servo-mechanism that corrects the woofer's movements to the signal, and several smaller diameter drivers instead of a single large one.

Let us see how well the ACI Force can overcome these major obstacles and maybe even give me some more of �it'.

The Manufacturer

ACI was founded in 1977 by Mike Dzurko in La Crosse, Wisconsin. Initially, they focused on custom building and experimentation, and this evolved into offering select designs as speaker kits for the DIY market. Gradually, ACI began custom assembling the speaker kits for clients. Both the kits and assembled models were sold consumer-direct, and ACI is still consumer-direct in North America. For a given manufacturing cost of any audio product, I would guess that a factory-direct model of marketing/distribution should be priced 30% to 50% less than the same product sold through traditional retail channels.

During the early years, ACI also contracted to develop and build systems for other audio companies. Eventually, the number of DIY customers dwindled, and ACI turned its focus totally to the design and manufacture of finished speaker systems.

Since 1987, ACI's headquarters have been in a converted church (circa 1867). This building houses the corporate offices and final test and assembly facilities. ACI outsources its enclosures from local cabinetry shops. The drivers for ACI subwoofers are custom made in the US. Crossover design and manufacture are done in-house, and so is final assembly. ACI's headquarters also houses a professional recording studio. This studio serves an additional purpose of having the ACI staff constantly exposed to live music, and they can get feedback on their products used within the studio. Brilliant.

Each of the core members of the ACI design team have their own laboratory and listening room setups. ACI uses several electro-acoustic software and hardware tools to aid in the design and testing, including LEAP, MLSSA, CLIO, and TrueAudio. ACI maintains seven rooms/systems where they listen to and judge prototypes, finished designs, and modifications of existing products. Room treatments are also used to allow an accurate impression of the speaker being evaluated. These rooms are crucial to ACI's design process, and the Force was tested in all but one of these setups before being launched.

Like many other captains of the audio industry, Mike Dzurko's interest in audio gear grew from his love of music. He started playing the piano in Grade 1, later on he switched to the guitar and then the drums. In his high school years he was part of a rock 'n roll band that played several nights a week and was very much in demand in the local circuit. His college years took away from playing music, but his love for music now manifested into listening. This led to an unrest and dissatisfaction with the state of equipment at the time, and he started to build his own speakers. This was not his first attempt at it either; he had built his first set at the age of 12. In college, others quickly purchased the models he built for himself. The orders kept coming in, and that was the start of his speaker building days.

Mike has a Masters Degree in Educational Psychology, he feels his academic training has not only grounded him in psychoacoustics but also helped develop a methodology for listening that is used in the development of his products.

Mike does not focus on comparing his prototypes to the competition; instead the focus is to compare their designs to live music - a sensible approach indeed. In earlier days he enjoyed speakers from Beveridge, Quad, and Dahlquist. His philosophy states that the best audio products do not call attention to themselves, but simply involve you in the music. Indeed, my highest praise has often gone to products I find not easy to review, because listening sessions yield few notes and more involvement.

My sense of Mike's goal for the Force subwoofer can be summed in the following comment he made to me regarding his thoughts on a subwoofer' purpose, �Enhance, do not detract. Integrate seamlessly, and disappear unless you are called on.�


For all my setups, I used the speaker level leads feeding into the stereo inputs on the back of the sub. I found the best location to be 6� from the wall behind the main speakers. Starting recently, ACI subs will ship with a CD of test tones and setup instructions. Unfortunately, these were not available in time for the review's completion.

I will not go into the details of setup. It is fairly laborious and time-consuming, and the final settings were quite different for each speaker in each room.

here are three aspects of integrating a subwoofer with main speakers, the crossover, level and phase.

To set the crossover and level, I start with a middle-level volume setting and a crossover setting a few Hz below the �3 dB point of the main speakers. From there a RadioShack SPL meter and test tones takeover, and I start to plot the response levels for small adjustments to the crossover and level settings. Hopefully, you will also have established a few potential spots to place the subwoofer. Repeat the tests in all these spots; the combination that yields the flattest response is where you should leave it.

Setting the phase is a bit trickier, and should only be done after location, level, and crossover are finalized. Pick a recording of an instrument whose frequency response will overlap the speaker and subwoofer, and listen with the phase control in various settings. When there is no hint of lag in part of the signal, or the sound seems like it is coming from a single point in space, you have hit the jackpot.

The track I used to set the phase control is the �Bass Resonance Test' mentioned below, and I have found it excellent for this specific purpose. For example, this was my experience trying to mate the Force with the Titus: With the phase dial at the 0 degree mark, lower frequencies (the overlapping ones reproduced by the sub and the satellites) seemed dissipated across the entire speaker plane, while the higher frequencies seemed locked in a narrow and stable image in the center. Turning the phase dial to 180 degrees, there were now two distinct sources of sound in the overlap region, and I was much happier at the 0 degree setting. I concluded that nirvana would be closer to 0 than 180.

I found the perfect match at somewhere between 60 and 80 degrees (it is very hard to read these darn puny knobs). The image across all frequencies was narrow and stable in the center. The pluck of each string was delivered in its entire heft at once. The sound was glorious in its correctness. I suppose with a lesser sub that only has two phase settings, I would either have had to settle for the sound I heard at the 0 degree setting, or move the sub to a location that I would be forever explaining to my wife. I can just hear myself now, �I know it is in the middle of the room, but I finally got the phase locked in."

The Outside

The Force is fairly small as subwoofers go, even with the odd looking wood �cheeks� on the sides. The front panel sports the logo on the bottom, and the woofer is down-firing, with the enclosure raised on four legs. The rear of the sub hosts a series of control knobs that set this sub apart from most of its competition. The topmost dial is to control the overall level control, and is placed at the input of the preamp section in the signal path.

Just below that are two knobs for the low-pass filters, each rated at 12dB/octave. This makes a lot more sense than one control set at 24 dB/octave or any other slope. Having two independent filters allows you to not only pick the crossover point but also the slope at which to integrate with the main speakers, very thoughtful. Each filter control allows the choice of frequencies to be continuous from 35 Hz to 250 Hz. I must applaud ACI on two accounts here: first on keeping the crossover selector continuous and not at fixed frequencies, and secondly on having the range extend down to 35 Hz, instead of the more common 40 Hz. That extra 5 Hz certainly made a difference in preventing a bass hump in the 40 Hz region, with my Dynaudio 1.3MkII for mains. I would be even happier if the lower range was extended even further below.

Why even bother having a subwoofer then you ask? Well, some speakers (like the Dynaudios I have) are down 3 dB in the low 40's range, but have a very shallow rolloff below that. To properly integrate with such speakers, you probably want to start somewhere around 30 Hz and use a slope somewhat less than 24dB/octave. Such a subwoofer would then also be able to be mated with what are generally considered to be �full-range' speakers, with the �3 dB point in the 30's. It may seem extravagant to some, but then even these systems could be extended to 20 Hz.

The last control knob allows the phase to be set continuously from 0 to 180 degrees. Again, I must applaud ACI on not only having a phase control but also making the control continuous, instead of the more commonly provided two settings of 0 and 180 degrees. The benefit of having a continuous control was discussed in the Setup section above.

If I have a complaint, it is with the size of the controls. I wish the knobs and markings on the control dials were larger and better graduated, so I could note exactly where I set them. Mike took this comment to heart and had also received the same feedback from others. The knobs currently supplied, he explained, are part of the Alps pots they use. They have since been able to source a larger knob that will start to roll out into production shortly.

Below the control knobs are the RCA inputs and outputs, starting with a left and right input, and then a �'theater'' input intended to be fed with a �sub-out� line from a processor or receiver. And finally, there are left and right line-level high-pass outputs, with a fixed filter point of 100 Hz. These outputs are offered so you can input the full bandwidth line-level input from your preamp, and pass the frequencies below 100 Hz to the subwoofer and only the frequencies above 100 Hz to the amps and speakers for the main channel. The benefit is reducing the demands on the main channels, and letting the sub do the hard work. While, I think this has its merits, I would not endorse this approach. Regardless of how meticulously implemented, you will still be introducing an extra set of connections and circuitry in the path of the main channels. My stated goal is to reduce the distance, connections, and circuits the signal has to go through before it reaches my ears. Moreover, 100 Hz is too high a frequency to allow a subwoofer to play; human ears can localize sound at that frequency.

The bottom of the rear panel holds a rocker style power switch and a fixed power cord. The left half of the rear panel has the heat sinks from the amplifier running virtually the full length. The main body of the sub is finished in textured satin black. Optional side cheeks are available in a variety of finishes for an extra $100 to $175. Frankly, these cheeks do not do much for me; I would rather not have them.


Mike Dzurko furnished most of the technical information below.

The woofer in the Force is a 10� custom design unit, referred to as the SV10. It is a cone made of pressed pulp. By Mike's admission this is a �relatively heavy' design, but he eschews the conventional notion that a lighter cone is necessary for the driver to be �fast', i.e., be able to stop and start quickly in relation to the signal. Ironically, this conventional view is the foundation of electrostatic speaker design, and although ACI approaches the issue from a contrary perspective, their subs are regarded by many to be a fine match for electrostatics. Go figure.

According to Mike the issue of paramount importance is cone stiffness. The nature of the beast is such that a subwoofer's operation will create a tremendous amount of air pressure inside the cabinet, and the cone is subjected to fairly long excursions from its resting point. These events create a great amount of stress on the surface of the driver, and if it were to buckle under and lose its shape while in motion, this is often referred to as �breakup', resulting in audible distortion. Perhaps this is the kind of distortion that shakes loose our dental fillings, courtesy of the teenage in the hotrod next to us while stopped at a traffic light.

The issue of the driver being able to start and stop quickly is key from a musicality point of view. The ideal driver will perfectly replicate the onset, decay, and stop of a note. A sluggish driver will display some lag in starting to move after the onset of the transient, and will continue to move of its own volition after the signal has ceased to exist. The persistence of the driver's own inertia will make the reproduced signal sound different than what the recorded instrument would sound like in a live setting.

Mike's view is that the motor driving the cone needs to have enough energy to get a nimble response. In this case, the voice coil of the SV10 has a winding length of 41mm and four layers of wire. The magnet assembly is comprised of two 42 oz units for a total of 84 oz. I can follow the logic of a powerful motor being able to move the cone quickly from a resting state. But my understanding of physics would make a heavy cone and relatively heavy driver slower to come to a stop. Mike countered that concern by conceptually explaining that the motor also serves as a brake.

Other subwoofer manufacturers incorporate a servo that compares the motion of the cone to the signal and makes corrections accordingly; such is not the case either with the Force. Nonetheless, the true test of this subwoofer's ability will come in the listening; the theoretical pro and cons can then all be laid to rest.

The driver is attached to the body of the unit via a Santoprene surround, thread-locked screws, and adhesives used in the Aerospace industry. An extra long pole-piece along with the compliant surrounds was developed in order to allow for a long-throw design. Throw is a term used to refer to the distance the driver can travel, �Xmax' is more specific in that it refers to the peak linear travel in one direction. The larger the Xmax value the greater the sound level can be attained with minimal distortion. Xmax for the Force is rated at 16mm, with a margin of safety to prevent mechanical damage.

The Force uses the very same amplifier as in the Titan model; this unit is rated at 250w RMS at less than 0.5% distortion. There is a different personality card for each model that varies specific characteristics of the amp in order to optimally function with the rest of the model's configuration.

The warranty is fully transferable, and is 5 years. For the first year they will also pay for the freight both ways within the US. There is also a 30-day money back guarantee, which is almost mandatory for anyone selling the factory direct route.

The Sound

To test the �quality' of bass I had to borrow my trusty Triangle Titus 202 from the setup in the bedroom. The mid/bass driver in this speaker has no crossover components in its path, and the driver is made of a light doped-paper, mounted on a very stiff spider. The result of all this is a very clean and lifelike reproduction of the lower notes, at least as far down as it goes (-3 dB at 65 Hz). If a subwoofer can keep up with this unit, it can probably keep up with anything.

A track that seems ideally suited for this test is "Bass Resonance Test" (Chesky Jazz and more Audiophile tests Vol 2, Chesky, JD68). It is a nearfield recording of a double-bass in the middle of a cavernous room, so as to record only the true tonality of the instrument and not the reinforcement from the walls. Without the subwoofer, the Titus delivered a very clean and fast rendition of each pluck and the resulting resonance and decay from the body of the instrument. This is a very realistic depiction in terms of tonality and speed, but lacks in frequency extension (a double bass can extend down to 40 Hz). Switching on the Force added �legs' to the presentation, and the picture was now complete. However, there was no compromise in the coherence of the sound, the instrument was firmly placed a few feet in front of me. If I closed my eyes, I was convinced that the sound was emanating from an instrument at that specific spot in front of me, and not from the two bass drivers of the Titus and the woofer of the Force spread over a 3-D triangle in the space of my listening room (quite an achievement if you think about it).

The woofer of the sub was in perfect lock-step with the nimble mid/bass drivers of the Titus. Now I must give myself some credit for the effort expended in properly dialing in the sub, but that credit would only be the tip of the iceberg. The majority of the credit would be due to Mike Dzurko and team at ACI, who designed a product that has a responsive driver and enough controls to be able to be perfectly mated with this speaker.

For a more realistic application I wanted to try a track where the notes requiring a subwoofer were infrequent. I wanted to see if it was worthwhile having a subwoofer for nominal duties. So I threw in a newfound treasure "Tal Posta in 5 beats' (Essence of Rhythm, Ustad Zakir Hussain, Emarcy, 536-943-2). This album records the tabla in a close-miked setup. While this does result in the bass piece sounding a bit overdone, it serves the purpose nicely for this test. Well, the Force proved to be quite transparent, I had to check to make sure the sub was actually turned on. It was only once I turned it off that I noticed that the belch of the bass piece of the tabla was not quite as vociferous, and something indeed was amiss. In practice this is exactly what you should expect of an excellent subwoofer that is well mated into your system. Its presence should only be missed when absent, but not obviously noticeable when present . . . like the right amount of ice in your whisky, I suppose.

The only other question I had left was, how much would the sub add to a speaker like the Dynaudio Contour 1.3 MkII, which is very respectably extended in terms of bass response (-3 dB at 43 Hz, and a very gentle roll off below that)? For most of the music I listen to, there was not much added by the Force to make me justify adding a component to my system. However, the Force really made a noticeable difference on tracks with synthesizer or electric bass induce tremors, such as "The long ships" (Enya, Watermark, Reprise, 926774-2) and "Rimshot" (Eryka Badu, Baduism, Universal, UD53027).

Although, I had not originally intended to test this sub in a home theater setting, I just could not resist. The audio equipment in my home theater is relatively modest and is comprised of five NHT Superzero speakers, an Onkyo SR500 receiver, and a Velodyne F1500R subwoofer. Admittedly, it will not take much to load bass response in a smallish home theater like mine at 1700 cu ft, even though it has large openings into two equally large rooms. I was surprised to find with the Force that a setting well below midpoint on the level dial resulted in as flat a response down to 20 Hz as I could expect with any subwoofer. Now, with the Velodyne, the same output is realized with the level knob barely turned past the 1 mark. The point is either subwoofer is amply able to serve a layout like mine for home theater applications. Then the differentiating criteria is, which subwoofer is better able to blend in better with the rest of the system or is better acoustically.

On standard bass heavy scenes from movies like Jurrasic Park, Eyes Wide Shut, and The Matrix, I was hard pressed to tell the difference between the two subwoofers. In scenes where there is audio information well below 20 Hz or such test tones, the Velodyne seemed a hair better, but only when I was comparing directly and specifically looking for even the slightest difference. At those frequencies the notes are felt and not heard, and it is very difficult to be certain that there is a difference at all. I would have to say that for bass response and extension in my home theater setup, I would be indifferent between the two subwoofers. Even though in absolute terms the Velodyne is capable of lower notes and greater SPLs than the Force, at least in my specific setup that extra capability is wasted. In my small room, I barely got the Force to work up a sweat.

Directly comparing the two subs on a level matched reproduction of the Chesky track, I felt the ACI was just a hair better in terms of keeping up with the speed and tonality of the main speakers. This is not surprising, since the Velodyne is equipped with a servo that compares and corrects the woofer's motion to the signal 3,500 times a second.

The ACI was much easier to integrate, and had a wider range of placement options, whereas the Velodyne seemed to only have one spot where the phase is in sync with the main speakers. This method may yield a position that is not optimal for the flattest output, and then you would have to choose between the lesser of two evils.


Since it applies so well here, I cannot resist but to draw upon a carnal metaphor; it is not the size of the wand, but it is the magic you do with it. With its 10� driver and 250 watt amp, this sub is likely to get sneered at by those convinced that real subwoofers only come with kilowatt plus amps and at least a 15� driver.

Unfortunately, I was able to compare the Force to only one other sub. And in my two setups, the ACI did not yield an inch, except in the below-20 Hz region. It was definitely superior in its ability to integrate with the main speakers and allowing for a variety of placement options. Kids today call a decision like this a no-brainer. It could not get any easier to choose the Force over something like the Velodyne F1500R, in a small room. Of course, the Velodyne wins in a very large room, with its ability to deliver 16 Hz sound.

The most concise conclusion I can pass on is that I have put my money where my pen is, and I bought the ACI Force. Keep in mind this is in spite of the fact that it will be mated with the Dynaudio's for now. And considering the extension of those speakers, the subwoofer will hardly ever be called to action. But I am hooked on the bottom-most octave, and I am confident this sub will be able to add that last bit in perfect tandem with almost any speaker I choose for my mains. In the end I did indeed find a way to get some more of �it�, thanks to the folks at ACI.

 - Arvind Kohli -

Speakers: Dynaudio Contour 1.3MkII; Triangle Titus 202, NHT Superzero
Integrated Amplifiers: Cayin 265Ai; Creek 5350 SE
Receiver: Onkyo TX-SR500
Digital source: Sony DVP-NS755, Panasonic A320
Subwoofers: Velodyne F1500R
Power Conditioner: PS Audio P300
Video Display: Toshiba 51HX81
Cables: DIY

    Related to the article above, we recommend the following:

Primer - Speakers



� Copyright 2004 Secrets of Home Theater & High Fidelity

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