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  Mirage OM-7 Omnipolar Loudspeaker

      Date posted: September 24, 2000

Mirage OM-7

Size: 43 3/4″ H x 11 5/8″ W x 14 3/8″ D
Sugg. Retail: $2800pr Cherry, $3000pr Black Gloss (CAN)
Manufacturer: Audio Products International Corp.,
3461 McNicholl Ave, Scarborough, Ont. M1X 1G5
(416) 321-1800, FAX 321-1500
www.miragespeakers.com

(Reprinted from the Summer/Fall 2000 Audio Ideas Guide)

     The second-largest speaker in the OM series, the OM-7 doesn’t quite dominate a room like the larger OM-5, which has an active subwoofer built in. But both need to be given a little space, being Omnipolar.

     What is Omnipolar you ask? Here’s how the Mirage design team (Ian Paisley, Andrew Welker, and Stefan Hlibowicki) put it: “Mirage research and development centers on how the ear/brain processes sound. To realistically portray a recorded sound, a loudspeaker must control both direct and reflected sound within the listening space. The reflected sound must be optimally shaped by the off axis dispersion qualities of the drivers and the form of the enclosure.”

     ”Mirage speakers use the space around them to create a coherent soundfield composed of direct and reflected sound. The result is extraordinarily real; the sound is simply there, in space, unrelated to the speakers.”

     All that makes me think of James T. Kirk, and “the final frontier” and “to boldly go…” (and split infinitives). I once created a colour ad for Advent’s SoundSpace Control (an early surround box) using an U.S.S. Enterprise model. Here the soundspace control is through the speakers, the OM-7 having back-to-back identical midrange drivers and tweeters in a space so shallow that the only way to mount them is to invert the rear pair so the midrange metal-coned driver is behind the less-deep front tweeter and the rear tweeter below behind the front midrange. The reason for this is to keep the front and rear drivers close together but facing in opposite directions so their wide dispersion will result in full omnidirectional radiation rather than the bipolar patterns of the previous wide-baffled M series speakers.

     The tweeters are 1″ Pure Titanium Hybrid (PTH) domes, while the midranges are polypropylene cones with butyl surrounds. The rear-ported woofer section employs a single 8″ polypropylene driver. Crossover of the former to the latter is a quite low 200 Hz, while mid-to-tweeter transition is at 2 kHz.

     Let’s explore the spatial aspects of Omnipolar a little more before getting to the measurements. Here’s what Mirage has to say about it: “Some might argue that bouncing much of the speakers’ output off surrounding surfaces would blur the acoustic detail or ambience of the original recording. This is a common misunderstanding that can be clarified by `the Haas Effect’. It postulates that the brain cannot differentiate direct from reflected sound (in terms of its arrival at the ear), if the time difference between them is within the so-called `time of fusion’ - 20 milliseconds (20 thousandths of a second). It would take the equivalent of more than 20 feet of difference in travel distance, reflected to direct sound, to exceed this time. Add to that the substantial time differences that will typically exist in large recording spaces (or as simulated in studio recordings), and it is clear that the listening room acoustics are quite insignificant when compared to those incorporated in the recording itself. And, depending on room size, little, if any, of the original ambience need be lost.”

     That said, the OM-5s, as the biggest Omnipolar speaker yet, are meant for large rooms, perhaps those large enough to exceed the Haas time boundaries in their reflections, while the Om-7s are suited to more commonly sized spaces. Our room is quite large in length at 32 feet, but only 12′ wide, with a good deal of damping behind the speakers with Tube Traps and Room Tunes, plus various other absorptive panels to control both corner boom and first sidewall reflections, as well as other extra energy. However, there remain enough reflections to allow the OM-7s to do their thing; it is by no means a dead room.

     Of course, we don’t measure speakers at the listening position, accurate measurements only possible at the point in the room where it is farthest away from all reflective surfaces. In measuring, it is not the Haas Effect we worry about, but the successful gating of the LMS system, that is, making sure that the measuring microphone hears all directly radiated energy before reflections, these latter being gated (or filtered out) by the sophisticated computer system.

     In general, the larger a speaker system and the more drivers it has, the trickier it is to measure. However, given that the midrange and high frequency drivers are closely concentrated in space, this difficulty is mitigated. Looking at the top curve, we see a Pink Noise Sweep (PNS) of both the rear radiation of the OM-7, and they are very similar; the Summed Axial Response (SAR: the sum of the axial measurements below) can be distinguished from these by the mild rolloff at the highest frequencies.

     Looking at upper bass and into the midrange, the OM-7 is very smooth, but response tapers off above 1 kHz, down 5 dB at 2 kHz. It may be that the midrange is contoured this way on axis to compensate for the extra energy in the room from the rear drivers, but the very nature of the PNS measurement in its assessment of overall radiated energy suggests that the octaves above 1 kHz could ideally be a little flatter. But listening did not suggest a reticence in the midrange, so maybe the Mirage engineers are right in voicing the OM-7 this way.

     We can also see this dip in the quasi-anechoic and axial measurements below, the latter closely clustered to 30o off axis, with that definite dipole-like null in the highest frequencies at 60o; in other words, this speaker is not quite omnipolar, though its 60o midrange is definitely so.

     A look at the lowest octaves shows an unusually gradual rolloff, the response down only 4 dB at 20 Hz. This is due to the quite large and deep woofer enclosure volume combined with the tuning of the rear port, which uses the room boundaries to enhance the lowest frequencies. Note that the PNS done of the rear drivers shows a steeper rolloff for the simple reason that the OM-7 was turned around, its port therefore facing into the room rather than at the back wall. That tells us, by the way, that deep bass is not omnidirectional so much as room directional.

     The impedance of the system is well controlled, reaching a high of 11 ohms at crossover, and a low of just under 4 ohms at 90 Hz, with a broad dip again to 4 ohms centred at 500 Hz. The phase angle through the crossover region is fairly mild, +/-30o or less, meaning that the drivers are very well matched, and ensuring that this system, while it will want some amp current, is otherwise quite easy to drive, and should sound exceptionally coherent and image well.

     In listening I found the OM-7 as predicted by the measurements, with a big, wide soundstage that was slightly forward, with good detail, and a very fast, live presentation. But it has to be said that a speaker design like this gives the room the big decisions about tonal balance and soundstage: the more reflective and live the room is, the bigger the soundstage and the brighter the sound will be. Positioning will be very critical for optimum tonal balance, moreso than for a dipole, which is less sensitive to sidewalls because of its inherent cancellations to the sides. Positioning for good deep bass is less critical, though you obviously can’t stick them in corners.

     Looking back at the review of the first speaker in this series, the OM-6 (Spring 97), we can see similarity in the measurements, and in the perceived tonal balance: I called it “one of the most innovative and interesting speakers to come through our testing and listening process, one with remarkable realism, lifelike dynamics, and a degree of spatial re-creation that will appeal to many high end audiophiles.” In the newer OM series, the OM-7 replaces the OM-8, while the larger OM-5 replaces the previously reviewed OM-6.

     Considerable time was also spent listening to the big guy, the OM-5, and though the measured performance is similar to that seen here, the powered bass section provided deep bass that overwhelmed our room, which admittedly is something of a tuned pipe below 30 Hz; in a bigger room, the OM-5 could provide better deep bass than even most pairs of separate subwoofers. And the ability to adjust deep bass balance and level will allow tuning it to almost any size of room, as long as it’s big enough, that is, bigger than ours.

     I spent quite a bit of time comparing the 7 to our resident Veritas 1.8s, too, and found their tonal differences less notable than their spatial: The 1.8 provides a very natural and deep soundstage, but the 7 goes beyond this to energize the room with its rear drivers. Whether you like this effect will, as always, be a key to buyer acceptance. But what may impress many listeners is the beautifully balanced and extended bass performance of the Mirage when set up a metre or so from side and rear surfaces. Except for high level home theatre use, you won’t feel the need for a subwoofer.

     And, speaking of beautiful, I can’t conclude without a comment about the subtly gorgeous cherry finish of our review Mirage OM-7 pair; these speakers make a statement in elegance as the warm cherry glow complements the black grilles and woofer cabinet. If you buy into the Omnipolar concept and agree with me about the looks, a pair of OM-7s may turn out to be just right for your room and your ears.

Andrew Marshall

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