Mirage OM-7 loudspeaker Measurements

Sidebar 2: Measurements

The Mirage OM-7's quasi-anechoic sensitivity was fractionally below average, at an estimated 86dB(B)/2.83V/m. However, it will appear to be more sensitive in-room due to the presence of its rear-firing drive-units, which is presumably why Mirage quotes a "room efficiency" specification of 90dB. Its impedance (fig.1) drops below 4 ohms slightly in the upper bass and midrange, the former coinciding with quite a severe phase angle. Optimistically specified receivers would best be avoided with the OM-7. The "saddle" at 35Hz in this graph's magnitude trace indicates the tuning frequency of the twin rear-facing ports.

Fig.1 Mirage OM-7, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

No obvious wrinkles or discontinuities can be seen in fig.1 that would imply the presence of cabinet panel resonances. Nevertheless, a high-level mode at 348Hz on the front baffle (fig.2) might obscure lower-midrange clarity. The bass bin was relatively well-braced, with this mode well-suppressed in comparison, and another, at 250Hz, only slightly higher in level.

Fig.2 Mirage OM-7, cumulative spectral-decay plot of accelerometer output fastened to center of front baffle. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

To the right side of fig.3 is shown the output of the midrange/tweeter section on the tweeter axis at 50", assessed with DRA Labs' MLSSA system and a calibrated B&K microphone. The speaker is quite flat through the midrange and mid-treble regions, disturbed by a small suckout at 2kHz. However, the region covered by the tweeters gently rises for three octaves, reaching +8dB at 20kHz compared with the reference level at 1kHz. As El Bee noted, the balance will have a "slight whitening."

Fig.3 Mirage OM-7, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield midrange, woofer, and port responses plotted below 500Hz, 1kHz, and 800Hz, respectively.

On the right-hand side of fig.3 are shown the individual responses of the front midrange unit and woofer and one of the ports, measured in the nearfield using the small Mitey Mike II from Old Colony Sound Labs. The two lower-frequency units appear to cross over at around 150Hz, with symmetrical 12dB/octave slopes. The woofer actually covers quite a restricted frequency range—about an octave and a half, centered on 80Hz, with the ports handling most of the speaker's midbass output. The woofer's minimum-motion point, indicated by the sharp notch at 40Hz, is a little higher in frequency than I expected from the impedance graph. The ports roll off steeply above their passband, with only a well-suppressed peak apparent at 600Hz.

Fig.4 shows how all these individual responses sum on the tweeter axis, with the result averaged across a 30 degrees horizontal window. The midrange and low treble are basically flat, with the rising top octaves balanced by a somewhat underdamped woofer alignment, and with no true deep bass present. However, with the upper-bass bloom and a -6dB point of 30Hz (referred to the 1kHz reference level), the OM-7 will sound bigger than it is—as LB found in his auditioning.

Fig.4 Mirage OM-7, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the nearfield midrange, woofer, and port responses plotted below 300Hz.

Not only the on-axis response affects a speaker's perceived tonal balance, of course. Its off-axis behavior also plays a role in all but the largest, most heavily damped rooms. Figs.5 and 6 reveal the OM-7's lateral radiation pattern: fig.5 shows the actual responses plotted every 5 degrees out to the speaker's sides, fig.6 just the differences between the off-axis responses and that on the tweeter axis. The first thing to note is that the on-axis suckout at 2kHz fills in to the sides, suggesting that it would probably be a good idea not to toe-in the Mirages all the way to the listening position. These two graphs also show that this would also usefully suppress the forward top octaves a little.

Fig.5 Mirage OM-7, lateral response family at 50", from back to front: responses 90 degrees-5 degrees off-axis, tweeter-axis response, responses 5 degrees-90 degrees off-axis.

Fig.6 Mirage OM-7, lateral response family at 50", from back to front: differences in response 90 degrees-5 degrees off-axis, reference response, differences in response 5 degrees-90 degrees off-axis.

The off-axis directionality around 1kHz is probably due to interference between the two midrange units. But whether it is due to the OM-7's relatively wide baffle or its use of rear- and front-firing drivers, the contour lines in these graphs are not as smooth or as uniformly spaced as we see with speakers that throw accurate, well-defined soundstages. I do note that while El Bee liked the Mirages' imaging, it was not as well-defined as possible.

In the vertical plane (fig.7), a suckout at the upper crossover frequency develops 10 degrees or more below the tweeter axis, with a corresponding energy excess in the same region 10 degrees or more above that axis. The Mirage is otherwise not that fussy about listening axis, except that the best integration between the tweeter and midrange occurs just below the tweeter axis.

Fig.7 Mirage OM-7, vertical response family at 50", from back to front: differences in response 15 degrees-5 degrees above tweeter axis, reference response, differences in response 5 degrees-10 degrees below tweeter axis.

In the time domain, the OM-7's step response (fig.8) indicates that the front tweeter and midrange unit are connected with positive acoustic polarity (the sharp up/down spike at 3.6ms and the lazier triangle 0.4ms later), while the woofer is connected with negative polarity (the broad, negative-going hump centered between 5 and 6ms). The actual drive-unit polarity doesn't matter except when it comes to waveform preservation—and the jury is out on that subject.

Fig.8 Mirage OM-7, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

What does matter, at least when it comes to the frequency response, is how the individual drive-unit step responses integrate: there should be no discontinuities. Fig.8 shows that while the midrange step does smoothly hand over to the woofer, that step starts very slightly late, compared with the decay of the tweeter step. The solution, as implied by fig.7, would be to lower the measuring axis slightly below the tweeter axis.

Finally, the Mirage's cumulative spectral-decay or waterfall plot (fig.9) indicates some delayed energy coincident with the on-axis crossover notch. But aside from that and some mid-treble hash, this plot is quite clean.—John Atkinson

Fig.9 Mirage OM-7, cumulative spectral-decay plot at 50" (0.15ms risetime).

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