Mirage OM-6 loudspeaker
The current popularity of subwoofers, however, may have inadvertently sown the seeds that will change all that. It has given rise to an interesting and sensible hybrid: a one-piece loudspeaker whose woofer is separately powered from the rest of the system. The user provides the amplifier to drive the midrange/tweeter portion of the loudspeaker.
Such a system has a number of advantages. First, the woofer amplifier and driver can be tailored to each other. The power output of the amp can be designed to complement the power handling and sensitivity of the woofer, and low-frequency equalization can be used if needed. There is also the possibility (though not a guarantee) that such a direct connection to the driving amplifier will offer better control of the woofer's performance. Second, the woofer crossover frequency can be lower. A passive crossover at or below 100Hz—particularly the low-pass leg—has a number of serious technical liabilities. Third, the bass characteristics and power output of the amplifier the user must furnish become less significant, allowing the user a potentially wider selection of candidates (but see more on this below). And, finally, the powered woofer allows for user adjustments—woofer level and perhaps contour—that are not feasible in a passive design.
Mirage is not the first company to come out with such a powered, self-contained subwoofer design, but this feature, significant as it is, is not the only attraction of their new OM-6 loudspeaker. This model also incorporates a number of concepts that Mirage has used successfully in earlier bipolar designs (some of which remain in production), and at least one new refinement on the bipolar principle.
Recall that both bipolar and dipolar loudspeakers radiate energy front and back. Bipolar loudspeakers (invariably designed around conventional dynamic loudspeaker drive-units) differ from dipolar designs (often, but not always, flat panel radiators) in that the front and back radiation are out of phase in a dipole, in phase in a bipole (which means that the cones on both sides move away from the cabinet when presented with a positive voltage step). In a dipole, there is a deep null in the sound output at the sides of the enclosure due to cancellation of the opposing phase wavefronts. In a bipole, the response dips at the sides due to the natural limits of driver dispersion at higher frequencies, but the front and back radiation do not cancel each other out.
The Mirage engineers reasoned that if you could make a bipole cabinet as narrow and shallow as possible, the response dip at the sides would become less pronounced. Carried as far as is physically possible, the response would begin to resemble an omnidirectional design. (It can't be a true omni, of course, because the front and rear drivers, particularly the tweeters, do not have full 180° dispersion at all frequencies.) Mirage dubbed this design "Omnipolar," and the OM-6 is the first commercial realization of the concept (footnote 1).
The narrow front/shallow depth aspect of the Omnipolar idea is immediately evident in the OM-6: Its upper portion is very thin. To accommodate the cabinet volume needed by the woofers, however, the bottom of the enclosure is much deeper, with a sloped top surface where the woofer controls are located (more on this below). The bi-wirable connections for the midranges and tweeters are on the lower rear panel, along with the connector for a detachable power cord for the subwoofer amplifier. The top and back of the woofer section are the only surfaces not covered in black grillecloth. A thin strip of black lacquered wood dividing the woofer and upper modules (which cannot be separated) and a removable gloss-black top complete the traditional Mirage look. But because of the OM-6's unusual shape, any physical resemblance to Mirage's bipole designs ends here.
Three separate grille elements cover the front and sides of the subwoofer enclosure. All are removable, though the loudspeaker looks decidedly industrial without them. The top mid-tweeter module is covered in a grillecloth "sock" that can be slipped down to uncover the drive-units. But the system is not designed to be used with the grilles out of position, and the IDQ (Interior Design Quotient) with the sock down and the bottom grilles removed is probably less than zero. I did all of my listening with all of the grilles in place. In this configuration, the OM-6 is a very attractive package—though definitely unconventional.
All of the drivers in the OM-6 use Mirage's latest technology, including the same titanium hybrid tweeter used in the company's flagship M-1si loudspeaker. The upper module contains two midrange/tweeter arrays, one radiating toward the front, the other toward the back. To keep the cabinet as shallow as possible, the front and rear tweeters and midranges are mounted back to back; that is, while the front tweeter is mounted on top, the rear tweeter is mounted on the bottom, and vice versa for the midranges. The overall depth of the mid-tweeter module is just over 4.5".
The two 8" woofers are mounted on opposite sides of the lower, sealed cabinet, in the same bipolar arrangement used by Mirage in a number of their separate subwoofers. This bipolar arrangement is said to minimize cabinet vibrations by cancellation: the movement of the woofer on one side is counterbalanced by the equal and opposite motion of the woofer on the other side. This will definitely reduce the tendency of the woofers to "rock" the cabinet, but I suspect the general complexity of the vibration modes makes cancellation of cabinet vibrations themselves considerably less than perfect. (There's no dispersion advantage in mounting the subwoofer drivers in this fashion, as each driver is largely omnidirectional below its fixed crossover frequency of 80Hz.)
At the top of the subwoofer cabinet is a line-level input to the subwoofer amplifier, a two-position phase control (zero or 180°), and controls for both subwoofer level and contour. The latter provides for a moderate degree of LF cut or boost, specified to peak at +3dB at 45Hz. There are two ways the subwoofer may be driven: directly at line-level via an additional interconnect from the preamp, perhaps using a Y-connector; or by tapping the drive signal for the subwoofer from the system's speaker-level input. (The latter mode is engaged via a third position on the upper-panel phase switch.)
Footnote 1: While Mirage didn't originate the bipole concept—the original Bose 901 was actually a bipole of sorts—they were the first, to my knowledge, to coin the term. And Mirage does have a trademark on the word "Omnipolar."