Mirage Omni 250 Surround Speaker System
The radiation pattern of dipoles also got the attention of the folks at THX, who embraced it as a useful way of designing surround speakers that produced an enveloping sound but were difficult to locate—both desirable characteristics for film surround channels. However, dipole surrounds are typically designed around conventional enclosures with multiple drive units configured to simulate the radiation pattern of a planar dipole.
But boxless, flat-panel dipole speakers can have limited low-end response due to cancellation of the long-wavelength bass frequencies radiating out of phase toward the front and back. Clever design helps get around this shortcoming, but either limited bass extension, limited power handling in the bass, or both, still constrain many dipole designs.
Mirage took a different route beginning with the now-classic M-1. In the decade or so since that time, most upscale, multidirectional Mirage speakers have emulated the M-1's so-called bipolar design. With conventional (and often identical) drivers radiating in phase from the front and back of a conventional enclosure, these speakers produced spatial characteristics similar to those of a dipole, but without as pronounced a null at the sides or the bass cancellation.
Sound All Around
Mirage has now gone a step further down the multidirectional path with their new Omni series. Instead of radiating fore and aft with multiple drivers, the design feature in the new line is something Mirage calls the Omniguide. This consists of an upward-facing midrange driver (or, in the smaller models, a combined midrange/woofer) with a device mounted directly above it that looks suspiciously like a souvenir from a Roswell gift shop, but is actually an upward-facing tweeter. The structure enclosing the back of the tweeter is rounded to provide a diffuser for the midrange/woofer mounted just below it, directing most of that driver's sound to the sides in all directions. A smaller diffuser above the tweeter serves the same function for the high frequencies. The baffle on which this apparatus sits is angled slightly toward the front or, in the case of the Omni FX surround speaker, downward.
There are six models in the Omni line; we used four of them in various applications. The Omni 250 is a ported, floorstanding design with a single Omniguide on top and an additional 5.5-inch woofer firing directly toward the front. The Omni CC center-channel speaker uses a smaller midrange and two woofers. The Omni FX aims its Omniguide downward when you mount it high on a wall, spreading its radiation pattern out to the sides for good diffusion. And the Omni 60 is the largest of the line's two bookshelf models, usable for any channel. My original intent was to use the 60s as surrounds, alternating with the Omni FXes. Instead, I ended up pressing one Omni 60 into service as an alternative to the dedicated center-channel Omni CC. More on that a bit further on.
All of the Omni speakers use similar technology: woofers and midranges with Polypropylene Titanium Deposit Hybrid (PTDH) cones, Pure Titanium Hybrid (PTH) dome tweeters, magnetic shielding all around, and high-quality binding posts. Apart from the high-quality drivers and Omniguide feature, the Omnis look similar in build quality to other good speakers in their price range, with solid cabinets that are not obsessively heavy or big-boned, finished in vinyl veneer. Even the Omni 250 tower, at 45 pounds, can be easily carried by a person of average strength. But the 250 is a little narrow and top-heavy. To improve its stability, a set of wide feet are included, plus spikes.
Most of the Omni speakers come with two-piece grilles (one-piece for the FX). The cabinets are much more attractive with the grilles in place (that Omniguide does look peculiar), but the convoluted framework required to support the grille material is no sonic plus. It's a little trickier to remove and replace the grilles on the Omnis than on conventional speakers, but for serious listening, you should take the grilles off. A word to the wise: While the top of the grille is curved, that probably won't prevent the life of the party from setting a drink on it. And you don't want liquids spilled on these speakers.
Mirage's OM-200 subwoofer consists of two 8-inch drivers on opposing sides of its cabinet and operating in phase, an arrangement said to minimize vibrations. When operated at high levels, the OM-200 did seem "quieter" to the touch than most subs. Both low- and high-level inputs are provided. As with most subs, there are controls for level and crossover frequency; a switch lets the user bypass the onboard crossover when using the unit with a pre-pro's or A/V receiver's bass management. Unlike on most subs, these controls are conveniently located at the top front of the enclosure, along with a continuously variable (–180° to +180°) phase control.
I did experience a quality-control problem with one of the Omni 250s: a rattle somewhere in or around the Omniguide. This problem occurred only on the occasional low-frequency note, and 99% of the time it did not appear to affect the performance of the speaker in any way, so we didn't request another sample.
Mirage's instruction manual provides only the most rudimentary instructions for using the speakers, in several languages (including Russian!), but setup wasn't particularly complicated. I positioned the Omni 250s about 8 feet apart, to the left and right of my projection screen, with the center speaker located on a stand directly below it. The OM-200 sub was placed in the right front corner of the room where numerous other subs have performed well, and the Omni FX surrounds were located several feet behind the seating area. Because the surrounds were only about 4 feet off the floor, and on a shelf rather than on the wall, I positioned them upside down, with the Omniguides firing up toward the ceiling rather than down toward the floor.
I did most of my music listening to CDs in 2-channel mode, with the left- and right-channel Omni 250s operating down to 80Hz and the OM-200 subwoofer taking over below that point.