Nowadays the words "compression horn" aren't bandied about much when discussing home stereos. Conversely, the term "unique" has become quite common in gear descriptions; in fact, many folks seem to believe that "most unique" is proper English usage. It's not. Many of the components that get the "most unique" title aren't all that special, but what if I told you there was a new speaker that does qualify as unique (not most unique or very unique) because it combines old technology in a new way to create a more room-friendly loudspeaker that interacts less with a room's acoustics than most conventional loudspeaker designs?

This is what you get with the latest offering from Spatial Audio, the M3. It's a loudspeaker that uses an open-baffle design combined with a compression driver in a coaxial arrangement to achieve a sonic result that is unobtainable from any conventional box, line-array, electrostatic, or even dipole design.

Spatial Audio is the brainchild of Clayton Shaw, whose first company, Emerald Physics, was founded in 1978. In the beginning Emerald Physics was a "project company" where Shaw worked on prototypes for his other clients, which included a number pro audio manufacturers. In 2006 Shaw took his latest proof-of-concept project to the Rocky Mountain Audio Show, where he met the owner of Underwood Audio, Walter Liederman. Liederman was so impressed by the prototypes that he encouraged Emerald Physics to build what came to be the Emerald Physics CS2, which debuted at the 2007 Rocky Mountain Audio Fest. By 2009 Emerald Physics had over 25 dealers, but Underwood Audio has sold the most speakers by far. In 2010 Shaw sold Emerald Physics and all its intellectual property to Underwood Audio and agreed to continue designing Emerald Acoustics loudspeakers for the next three years under a non-compete clause. The models 2.3 and 2.7 were Shaw's last Emerald Physics designs.

In 2010 Shaw also created Spatial Audio, whose first products were computer audio installations using Mac Minis and a software suite that combined sophisticated EQ with the ability to handle multiple channels of crossovers. I reviewed the Spatial Audio computer system for The Absolute Sound in 2012. In 2014, Shaw released the first Spatial loudspeakers, the M1 and M2, followed in 2015 by his latest designs, the M3 and M4.

This review will focus on the M3 loudspeaker. Its base Turbo model is priced at $1,995 per pair. The Turbo S version sent for review adds the latest Spatial M25 compression driver and ups the price to $2,595 per pair. Besides its unique design (which I'll explain in detail in the next section), the M3 has a 20-year warranty on parts. That's how confident Clayton Shaw is that his design, using drivers that were sourced from pro audio and sound reinforcement equipment makers, will withstand all the rigors of home use.

The Spatial M3 is a two-way design with the 800-Hz crossover point between its coaxial compression driver and its twin 15-inch mid-woofers. With an efficiency of 94 dB at one watt/meter, the M3 is a highly efficient design with a very even four-ohm nominal impedance curve that can be driven successfully by even a small-wattage power amplifier. The M3 is an open-baffle design that has no enclosure or box. Instead the two 15-inch-diameter drivers are mounted on a three-inch-thick baffle that measures 17 inches wide by 42 high. The M3's dispersion pattern is tightly controlled by the design to be 80 degrees horizontal and vertical over its entire frequency range from 32 Hz to 20 kHz.

To accomplish its high efficiency and controlled dispersion, the M3 uses a compression driver for its tweeter/midrange. A compression driver employs a different technology than a conventional dynamic driver. It is called a compression driver because the area of the loudspeaker diaphragm is significantly larger than the throat aperture of the horn to which it is attached. Horn-loaded compression drivers can achieve very high efficiencies, around 10 times the efficiency of direct-radiating dynamic driver loudspeaker. A "lens" in the throat of the horn extends and smoothes the driver's upper-frequency response, while the shape of the horn controls the dispersion pattern. Unlike loudspeakers that use a dome or ribbon tweeter, the M3 has the same dispersion patterns for its upper frequencies as its lower ones. When you walk around the M3, you will notice that the loudspeaker does not generate much sound from the back or sides. This is due to its open-baffle design, which actually cancels side sound while reducing the sound coming from the back. This is similar to what happens with an electrostatic line array, but the M3 does not generate as much sound to the back because the compression driver does not generate a portion of its output to the rear the way most electrostatic line arrays do.

Bass response in the M3 design is also different than with a conventional sealed- or ported-cabinet loudspeaker. Most loudspeakers' bass response is omnidirectional below 100 cycles or so and forms higher low-frequency SPL pressure levels around and behind the speakers themselves. However, an open-baffle design is different. With no cabinet or port arrangement to reinforce the bass frequencies, an open-baffle design does not generate the same pressure zones around the loudspeaker. Instead the bass is directional--it comes out of the front of the loudspeaker in a directional dispersion pattern controlled by the driver's diaphragm shape. This dispersion pattern interacts with the room far less than any conventional cabinet design's omnidirectional bass dispersion pattern.

One of the principal drawbacks of most commercially available off-the-shelf compression drivers is that they do not have linear frequency, especially in the 1,000- to 4,000-Hz region that is so critical for proper upper midrange response. Clayton Shaw got around this issue by using a digital EQ to correct these issues on his earlier Emerald Physics designs. For the Spatial M3, Shaw was able to work directly with a driver manufacturer to develop a compression driver with a different lens configuration that no longer needed digital EQ to keep it more linear in frequency response.

The Hookup

Setting up the M3 Turbo S speakers in my dedicated listening room was relatively simple. The previous loudspeakers were a pair of Emerald Physics CS 4.3 loudspeakers, and not surprisingly the Spatial M3 Turbo S models ended up with a very similar placement and toe-in. I angled the M3s so that they faced directly at the center listening position. I used the spikes that came with the M3 to anchor them firmly to the concrete floor under my carpet.

For most of the review, I used six-foot runs of Audience AU24SX speaker cable to connect the M3s to a series of amplifiers--including the April Music Eximus S1, NuPrime ST-10, Pass Labs X-150.3, and Bel Canto M600s. I also used WireWorld Eclipse 7 speaker cable during the review.

My main listening room has, in residence, a pair of JL Audio Fathom F-112 subwoofers that I usually use to augment my system's low bass. At first I tried a standard THX-type crossover setup using 80 Hz as the crossover point between the M3s and the Fathoms. After a few weeks I noticed that, while there was a lot of bass in the room (especially in pressure zones), I was not getting as much bass as I wanted at my listening position, so I tried something different. Instead of using the 80-Hz crossover that is built into my Parasound P7 preamplifier, I removed it from the circuit and gave both the M3 and the JL Fathoms a full-range signal. Then I used the Fathoms' built-in crossover to set them at 50 Hz. This forced the M3 to cover more of the bass region. Because the M3's bass is more directional, I found less bass build-up in the room's pressure zones and more bass getting to my ears.

Another advantage to using this crossover scheme is that it reduced the 60-/120-Hz low-level hum from the JL Audio Fathom subwoofer from barely noticeable to nothing. The positive effects of a completely hum-free system (even at extremely low volume levels) are myriad.

With most full-range speakers, especially less-efficient ones, you gain quite a bit of headroom by rolling off the low bass into a subwoofer. However, because the M3s are such efficient speakers with exceptional power handling (they are really hard to destroy via high SPL levels), there is no practical advantage to rolling off their low frequencies, except perhaps if you are attempting to use a five-watt or less power amplifier to drive them. In most rooms, using any amplifier that puts out more than 50 watts RMS, you should run the M3s full range so that you get all the advantages of their more directional bass presentation.

Click over to Page Two for Performance, The Downside, Comparison & Competition, and Conclusion...

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