Fall 2008

Occam Audio MTM4 (Kit B option)
A fantastic small speaker!

Difficulty Level
  
  

  I met Jeff Keyes of Occam Audio at VSAC 2008 in the Creative Sound/Occam Audio room, as well as seeing an old friend: I met the WR125ST at CES 2002. When I first saw its little copper phase plug and had a listen, I recognized the impressive aspects of the XBL^2 motor as applied to a (nearly) full-range driver. Some time later I'd gotten a set of these wonderful little drivers in my hands and have been using either these or their 'big' brother the FR125 in an application ever since. What makes this motor structure unique is a shaped pole-piece that is 'split', creating a motor which Adire Audio (now defunct, there are currently licensees using XBL^2) claim to linearize the BL curve, allowing greater output with lower distortion. I had to request a review sample of this kit, to do a little comparison. I've built a couple designs around this driver, generally opting for the smooth low-end roll-off and bumped response of a higher-Q sealed box. The competition for the MTM4 lay in wait already, the best implementation of this product family I'd yet come up with. The Galacticons (pictured here). These use the FR125 vs. the WR125, allowing the omission of the tweeter.

The Basics
The Occam Audio MTM4 uses a pair of the WR125ST and a large-diameter high-output dome from Wavecor in a small 'variovent' or 'aperiodic' enclosure. The crossover is 2^nd order, and has some tweeter padding. As you might have inferred from the name, this is a midrange/tweeter/midrange (MTM) style speaker, with a pair of mid-woofers above and below a centrally-mounted tweeter. This is also known as a D'Appolito array, since this configuration creates some directionality in the vertical plane and some other benefits yet creates some off-axis response anomalies which may be less desirable.

The kit came in as seen here. Packing was excellent, and presentation very good. The fit of the components was excellent, and I was tempted to assemble it raw and have a go. I could have done this (those customers purchasing pre-finished enclosures will not have to wait) but I took the longer road, put a black/copper combo finish on them, and 'did it right'. In process, I also had to build some stands, my existing stands being either too short or too tall for these very modestly sized speakers. Off to the store to make some very basic stands. Cinder blocks, to be precise. Mock me not, they are heavy, hugely strong stands that are well-suited to the job, and can look half-respectable with a thick coat of paint or five. I chose to seal them with white shellac-based primer and paint with boat paint (durable, thick stuff suitable for keeping the dust sealed in the concrete). The design was very similar to this but adapted for height, thanks "Santi" for the very functional design (

The crossover was easy to zip up, but could have been somewhat better documented, despite the inclusion of a DVD. For me, as an experienced DIYer, it was incredibly easy, and took no time at all. The finishing took a number of hours, but the crossover, wiring, and assembly flew. With a pre-finished cabinet, a pair of these could be easily done in an hour or two.

This is a very nice theater speaker, in which the drivers are magnetically shielded, and the size small but power handling and output relatively high. They absolutely require a sub-woofer, for which I used a "Castle Acoustics Classic". Finished properly by you (or Occam), they're very attractive. The large-format quad of screws which bolt the whole assembly together are also used as magnetic anchors for the grille.

We've almost all heard bad MTM implementations, with poor transient and off-axis response. The smaller driver sizes made possible in the MTM4 by the high excursion mid-woofers and the small flange of the neodymium tweeter allow not only for greater treble extension from the mid-woofers (easing the job on the tweeter), but much closer spacing of the driver centers. This advantage comes through in a more coherent sound than larger-scale MTMs, and many of the advantages of the very small format, such as very good imaging and low cabinet coloration were present here. It didn't hurt that the cabinet was of high quality, though if I had to pick nits, I'd have the inside cut so as to have an irregular interior surface.

So How Does It Sound?
The imaging for the most part as said was excellent, which lent itself well to video applications. I enjoyed a number of films through these, and the dialog was always clean and clear. The background scores were easily discerned, not buried as some MTMs can do- obscuring of the fine, low level cues that help tell a trombone from a baritone behind speech and sound effects. Again, they do need a sub-woofer, having a bass roll-off at around 100 Hz. The better the sub you can pair with these, the better the overall sound will be. Better still, use a stereo pair and get higher output, lower distortion, and usually flatter in-room response. The higher output level achievable with the MTM4 than many of their contemporaries was invaluable in preventing these very small speakers from sounding very small in any but the positive ways. They sound more like a nice medium sized speaker than a small speaker, in terms of clean output, which is a big advantage in a domestic balance situation. In other words, you can probably put them in the living room without too much fuss. They're very attractive and have a handy mounting bracket option for near-wall installs.

With music, the MTM4 played very nicely on rock, clean and clear and punchy. I listened to a variety of rock fare, from ACDC to JethroTull and back again. The intelligibility was very good, and there was a lot of life and vitality from this little design. The near-absence of cabinet coloration was a plus, as was the low diffraction minimalist baffle design. They disappeared fairly effectively.

With music with more space, and more quiet sections, they didn't fare as well. There was an upper treble haze, a slight splashiness that obscured fine detail and ambiance cues. I attribute this to the large dome tweeter, which breaks up within the "audio band" (the breakup peak is at about 17 kHz). Most metal domes don't breakup until about 25 kHz, and some tweeters can go much higher before breaking up. Typically, smaller tweeters have breakup modes higher in frequency, meaning they 'keep it together' through the audio band before losing it. This one is larger and thus, breaks up earlier. While less noticeable on a film score or rock album, it was discernible on classical tracks and less compressed or processed sounds.

So... You Didn't Like The Treble?
I didn't say that.
This was a fairly minor effect, but one which would only be bothersome to some listeners. The advantages of the larger sized dome are that it can be crossed over lower or with shallower slopes (good for coherence), and at lower frequencies, which helps keep the mid-woofers out of their directional and breakup frequency ranges. And indeed this area was well handled, the crossover intruding itself only mildly (every crossover is audible, in my opinion), without the harshness that comes from low-end distortion on overdriven tweeters. Even at high levels where the mid-woofers were pumping pretty well, there was nothing offensive or grating about their performance (at least not at any volume level I'd subject my ears to). The top-octave coloration was not nearly as objectionable as (poorly controlled) metal dome breakup, or overdriving of a smaller tweeter, both of which are common in small speakers playing loudly. The drivers chosen for this speaker support big, high output sound, which no other speaker this size I've heard can replicate.

So What About The Ones That Look Like Eyeballs?
How do these compare?
The spherical enclosures of the "Galacticons" are cast aluminum, and the shape optimal for a nearly complete absence of diffraction. Further, the cones have been modified slightly, and the FR125 is a more expensive driver (though less than the combined cost of the MTM4 drivers). My understanding is that the FR125 has shorting rings that are absent in the WR (though the copper phase plug acts similarly to one in some respects).

The result is a different speaker. There’s a clarity and ambient ability that is not present with the WR, as well as the vaunted ‘single driver coherence’. There also was more impactful and present bass. The smaller, enclosures with their absolute lack of diffraction- make this a speaker that totally disappears. Why, then do I describe this as a different speaker rather than a ‘better’ one?

The MTM4 had the edge in clean output, big time, for starters. There are limitations to a single-driver speaker, and while the FR125 design can play reasonably loud, it doesn’t even approach the level of clean output the MTM4 can. At some point, the treble begins to suffer (from being reproduced on ‘top’ of bass signals that are making the cone excurt significantly). The MTM4 does not have this problem, maintaining its clean output to a very high volume level, without the treble falling apart.

Another advantage of the MTM4 was off-axis treble dispersion. The smaller spherical units using a full-range driver require careful focusing of the beam-width (read- placement sensitive), as they get fairly directional above a few kHz. The MTM4 is fairly wide in terms of dispersion, with the off-axis lobing issues of an MTM design pushed both higher in frequency and further off-axis than with most speakers of this type.

Summary
This is a fantastic small speaker. They do most things well, and are overachievers in some respects. Output level, coherence, and clean presentation are all strengths of this speaker. I can't stress enough how much the higher output level contributes to the value of a small speaker like this. Most small speakers completely crumble at even moderate output levels well below real-world SPLs. The MTM4 can actually portray an orchestra with a sense of scale, a rock band at rock-able levels, and keep up with your sub-woofer in terms of clean output. On the other side is the upper treble breakup and a somewhat veiled presentation at times. Naturally, this is a real-world speaker, and to me, the design trade-offs are well-chosen. There's very limited bass extension, and they're also very insensitive. They take some juice to drive at approximately 85dB/W/m nominal sensitivity. The driver selection that allows this is one that also is very small for the job, and that leads these speakers to a much greater sense of coherence than some other similar designs.

I look forward to trying these as home-theater surround channels, where the limited bass extension would be a non-issue, but we all have our deadlines. Single sentence summary: Occam Audio is aptly named and these kits embody the approach that the simplest solution is the best (AKA Occam's Razor).

The crossovers are simple, the drivers well-behaved, and the solution a very good one.

Specifications
Type: Two-way, three driver floorstanding loudspeaker 
Nominal Impedance: 6 Ohms
Frequency Response: 100Hz to 15kHz (+/- 3dB, on axis)
                                 200Hz-15kHz (+/- 1.5dB, on axis)
Sensitivity: 85dB/W/m
Enclosure Type: Aperiodic
Price (each) : $199 per Kit B (as tested, unfinished)
$149 Kit A (doesn't include enclosures, just baffle + parts)
$279 Finished (Satin in either Walnut or Cherry)
$49 upcharge for piano lacquer finish

Manufacturer
Questions about kit, enclosures and finished speakers
Contact Jeff Keyes
Lucid Acoustics

Voice: (435) 674-9990
E-mail: jeff@lucidacoustics.com
Website: www.occamaudio.com

Questions and details about CSS woofers
Contact Bob Reimer
Creative Sound Solutions
44-31255 Upper Maclure Road 
Abbotsford, BC 
Canada V2T 5N4

Voice: (604) 504-3954
Website: www.creativesound.ca