Mirage SM-.4 / Subwoofer Speaker System Review

In recent years “three-piece” speaker systems have begun to reappear (they enjoyed a brief popularity in the early days of stereo but soon disappeared from the market). The concept is attractive: two small, unobtrusive “satellite” speakers, placed for best imaging and handling most of the audible frequency range, together with a bass module (sometimes called a subwoofer) that can be placed almost anywhere in the room. We have tested several such systems and have found them to give generally very good results.

The Canadian-built Mirage SM-.4/Subwoofer combination is the latest three-piece system to come to us for evaluation. The SM-.4 is a small acoustic-suspension bookshelf speaker with a 6-inch woofer and a 3/4-inch dome tweeter. The crossover frequency is not specified, but our measurements suggest that it is between 2,500 and 3,000 Hz. The tweeter faces a small plastic cone whose purpose is apparently to improve the polar dispersion, and it is surrounded by a ring of felt to smooth its high-frequency response. The Mirage SM-.4 has no balance controls. A pair of binding posts 3/4 inch apart are set into its rear surface.

The Mirage Subwoofer is a fairly large box containing two acoustically and electrically isolated 10-inch acoustic-suspension woofers (in other words, a stereo pair) and a crossover network that matches it to any of several small Mirage speakers, including the SM-.4. It can also be used with other brands of satellite speakers. Electrical separation of the two drivers is somewhat unusual, the traditional approach being to sum the low frequencies of both channels for reproduction by a single driver. The woofers are on the ends of the box near its bottom edge. The enclosure is finished on all sides, and the manufacturer states that, if required for better bass balance, the box can be turned over to increase the distance from the woofer cones to the floor.

The internal crossover of the Mirage Subwoofer is at 300 Hz. An array of binding posts set into one of its larger surfaces provides for connecting the driving amplifier and routes the signals above 300 Hz to the satellites. Separate sets of connectors are provided for either the Mirage SM-.4 or SM-1 speakers and for the SM-.5. A toggle switch marked increase and decrease adjusts the level of the woofer output relative to that of the satellites. It is also possible to use biamplification to match the Subwoofer to other speakers whose characteristics may not match those of the bass module.

The Mirage SM-.4/Subwoofer system is rated to handle up to 200 watts per channel of program material, and a minimum amplifier rating of 20 watts per channel is recommended. System impedance is quoted in conjunction with which satellites are used; with the SM-.4’s the system is rated at 6 ohms with the bass switch set for increase and at 10 ohms with it set for decrease.

As with most similar three-piece systems, the Mirage components give best results with the satellites on stands that place them at ear level, a foot or two from the wall, and spaced slightly wider apart than the distance from them to the listeners. Mirage offers optional wooden stands for this purpose. The SM-.4 speaker is 16 inches high, 10 inches wide, and 6-3/4 inches deep, and it weighs 11-3/4 pounds. The Subwoofer is 25-1/2 inches long, 15-1/2 inches wide, and 19 inches high. It weighs about 80 pounds. The Model T-4 stands are 28 inches high and are finished in black to match the cloth grille of the SM-.4 speaker. The Subwoofer’s grilles are dark brown and beveled on the end next to the top of the cabinet; they can be turned over if the Subwoofer is operated in an inverted position. Prices: SM-.4, $199 per pair in walnut-grain vinyl finish, $259 per pair in genuine oak veneer; the Subwoofer, $399 finished in walnut-grain vinyl, $529 in oak veneer; T-4 stands, $50 a pair.

Laboratory Measurements

Most of our measurements were made separately for the Mirage SM-.4 and the Subwoofer, although the room response was measured with them operating as a system. The 6-inch woofer of the SM-.4 developed its maximum output at about 100 Hz, falling off at 12 dB per octave below 70 Hz. With increasing frequency, the output fell off very smoothly to the vicinity of 1,000 Hz, above which it leveled off and was very flat (with minor irregularities) up to 10,000 Hz, rising to its midrange level at 16,000 Hz and dropping off slightly at 20,000 Hz. From 700 to 20,000 Hz, the output of the Mirage SM-.4 varied only ± 2 dB, which is excellent performance for any speaker. Its output variation, including the close-miked woofer response, was ±4 dB from 50 to 20,000 Hz.

The wide response was confirmed by the IQS FFT quasi-anechoic response measurement, which showed the Mirage SM-.4 to be flat within ±4 dB up to 19,000 Hz on the speaker axis. In fact, the tweeter output remained strong well beyond 20,000 Hz. The SM-.4 had excellent dispersion; over a ± 45-degree angle in the horizontal plane the output varied by no more than ±6 dB over the full audio frequency range.

The impedance of the Mirage SM-.4 reached its minimum of 4 ohms at 20 and 200 Hz, with a peak of 20 ohms at its 75-Hz woofer resonance. The speaker sensitivity was 87.5 dB at 1 meter with a 2.83-volt driving signal. The SM-.4 had excellent phase characteristics, with a group delay variation of less than ±0.3 millisecond up to 20,000 Hz.

The Mirage Subwoofer had its maximum output at 60 to 70 Hz. The output was 5 dB below that maximum at the 300-Hz nominal crossover frequency, and it fell off rapidly above 300 Hz to about 30 dB below average bass level at 800 Hz. We made no separate sensitivity measurement for the Subwoofer, but its distortion was measured with a constant input of 3.76 volts (the value required to develop a 90-dB sound-pressure level at 1 meter from the SM-.4 satellite). The Subwoofer’s distortion was very low, rising gently from 0.3 per cent at 100 Hz to 0.93 per cent at 50 Hz and 1.63 per cent at 40 Hz. These are exceptionally low distortion readings for any speaker, but the Subwoofer’s lower frequency limit was apparently just below 40 Hz, since its distortion increased to 10 per cent at 35 Hz and 18 per cent at 30 Hz.

The system impedance (Mirage SM-.4 and Subwoofer combined) reached a minimum of 4 ohms at 700 Hz (and at 85 Hz in the increase position of the Subwoofer switch). Above 1,000 Hz, the impedance was generally in the 6- to 10-ohm range. The major effect of the bass-level switch occurred between 50 and 300 Hz; when the switch was in the decrease position, system impedance was 8 ohms or higher from 20 to 250 Hz, and it dipped significantly below 8 ohms only between 400 and 1,500 Hz.

Forming a composite frequency-response curve for the combination of the Mirage SM-.4 and the Subwoofer was difficult because of the physical separation between the two units and the many uncontrolled placement variables that could affect their combined performance in different rooms. The bass level switch shifted the woofer output by about 5 dB. In our room, a preference for one setting or the other was often a function of the program material, although the decrease setting generally gave a better balance between bass and midrange levels. We would estimate the typical overall system response to be about ± 5 dB from 30 to 20,000 Hz; using the increase setting would not extend the lower frequency limit significantly, but it would give additional emphasis to the midbass.

Comment

Heard by themselves, the Mirage SM-.4 speakers are open, clean speakers with a trace of brightness, especially in comparison with our reference speakers. We soon became accustomed to this brightness and accepted it as the norm, however, so that it made the reference speakers sound a trifle dull! (Actually, most of us soon become accustomed to the sound characteristics of our systems and—assuming they are reasonably good—tend to judge others by comparison to what is a familiar sound.) Counterbalancing the slight brightness at the top end is the relatively boom-free and uncolored lower midrange of the SM-.4, which makes it a very pleasant sounding speaker by itself (especially so in view of its low price).

Adding the Subwoofer makes the system much more muscular and solid-sounding. A clean, strong bass output down to 40 Hz is all most of us ever need to give a sensation of “all there” sound, and the Mirage three-piece system certainly provides that. It seems to us, however, that the “Subwoofer” designation is here applied to what could more accurately be called a “bass module.” In our view, any speaker that operates much above 60 Hz can hardly be called a "subwoofer,” and the Mirage Subwoofer goes up to 300 Hz. This is indeed a small point, but the lower frequency limit of this entire system is within the range achieved by many fairly compact speakers on their own. The Subwoofer does not at all extend below a “normal” woofer’s range, though its distortion level at lower frequencies is admirably low.

Terminological nit-picking aside, the Mirage SM-.4/Subwoofer combination is an attractive and versatile speaker system at a reasonable price. As with any speaker, it is difficult to make any accurate predictions of how it will perform in an arbitrary room environment, but its three-piece configuration should give the user much more flexibility in speaker placement and in many cases better overall results than might be achieved with two conventional speakers.

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