B.E.S. SM 80 Speaker System Review

The SM 80 is the latest addition to the line of hi-fi loudspeakers made by Bertagni Electroacoustic Systems, Inc. (B.E.S.). The SM 80, the least-expensive B.E.S. speaker, employs the same unique construction as the company’s other models. Its principal radiating surface appears to be an almost flat plastic diaphragm, roughly oval in shape and measuring about 8-1/2 x 14 inches, mounted on a rigid cast-metal frame. The driving voice coil and magnet assembly are suspended over the center of the diaphragm on a metal bridge. A similar magnet/voice-coil structure for the tweeter is mounted at the upper left of the frame. Since the front grille is opaque and not removable, we could not see the tweeter diaphragm (or the front of the woofer diaphragm), but the magnet structures were visible through the rear grille.

Because it uses the radiation from both sides of the diaphragms to create a “wrap-around sound” quality, the B.E.S. SM 80 should be installed a few feet in front of a wall. The manufacturer suggests experimenting, however, with unorthodox positioning such as having one or both speakers at right angles to the listener. The SM 80’s planar diaphragms radiate equally from both front and back. In its normal freestanding position, it has a nearly omnidirectional horizontal polar pattern (actually, a figure-8) with output nulls at each side. The system’s frequency response is rated as 60 to 19,000 Hz ±3 dB, and its nominal impedance is 8 ohms. The crossover between the woofer and tweeter takes place at 1,500 Hz.

The SM 80 stands upright on an oak-veneered wooden base that is 14 inches wide and 9 inches deep. The speaker proper is 15 inches wide, 24 inches high, and 4 inches deep. Its front and rear are covered with dark-brown cloth, and the sides are finished in simulated-oak vinyl veneer. Each speaker unit weighs about 18-1/4 pounds. Price: $330 per pair.

Lab Tests

We placed the B.E.S. SM 80 speakers about 3 feet from the wall behind them and 6 to 8 feet apart, though their locations did not seem to be at all critical. The room-response curve was somewhat irregular in the lower midrange, and the bass output dropped off rapidly below 76 Hz. Although the response curves from the left and right speakers (measured on axis and 30 degrees off axis, respectively) differed by 7 to 10 dB over much of the audio range above 1,000 Hz, they had similar shapes. For our close-miked woofer-response measurement we placed the microphone at the center of the lower grille.

The bass response sharply peaked at 76 Hz and dropped to a plateau about 10 dB lower in the 130- to 400-Hz range. Above 400 Hz it became very ragged, possibly because of interference between soundwave components radiating from different parts of the large bass diaphragm. The bass output fell precipitously—about 30 dB per octave—below 76 Hz. The bass and room-response curves spliced easily, with some guidance from our quasi-anechoic FFT response measurements. The speaker also had a distinct peak between 200 and 300 Hz that showed up in both the FFT and room-response measurements. B.E.S. supplied us with response curves run on our test samples, and in spite of the very different test conditions, they matched ours closely. From our composite curve, the response was ±4 dB from 270 to 18,000 Hz, with peaks of 10 or 11 dB at 250 and 76 Hz. The speaker’s effective bass-response limit was about 50 Hz.

The FFT measurements not only confirmed the validity of the room-response measurements but also showed the same polar-response changes, with relatively little difference in frequency response between the on-axis and 45-degree off-axis measurements but a substantial Change, typically about 10 dB, in overall output level. The phase characteristics of the speaker were quite good, with a group-delay variation of ±0.4 millisecond over most of the range from 1,500 to 23.000 Hz. The response irregularities that we found at several frequencies showed up, however, in the group-delay readings as sharp peaks of at least 2 milliseconds at 14.000 to 15,000 Hz, 9,000 Hz, and below 1,000 Hz.

The SM 80 was quite sensitive — our 91-dB measurement surpassed its rated value by 3 dB. When we drove it with 2.5 volts (which corresponds to a 90-dB sound-pressure level at 1,000 Hz), the bass distortion was very low from 100 down to 50 Hz, increasing from 0.28 to 2 percent over that range. The output fell off so rapidly below 50 Hz, however, that we could not make measurements at lower frequencies. The speaker’s impedance curve was among the flattest we have measured, between 5 and 6 ohms from 20 to 1,000 Hz except for a rise to 10 ohms at 76 Hz and to 6 ohms at 250 Hz—the latter coinciding with the sizable resonant output rise at that frequency. Impedance dipped to a minimum of 3.1 ohms at 5,000 Hz, returning to 5 ohms at 20,000 Hz.

Pulse tests of the speaker’s power-handling ability revealed that it distorted with inputs of 223 watts (into 5.5 ohms) at 100 Hz and 600 watts (into 6 ohms) at 1,000 Hz. Our amplifier clipped at 640 watts (into 3.9 ohms) before the speaker’s output showed any significant distortion at 10,000 Hz. Clearly, the SM 80’s rated maximum/minimum-power recommendations (75 and 5 watts, respectively) are very conservative, especially since it is fuse protected.

Comments

Any speaker that radiates over a wide angle is likely to produce irregular frequency-response curves in our FFT quasi-anechoic measurements because of interference from late-arriving boundary reflections (analogous to the multipath reception of FM signals). Nevertheless, such speakers usually sound much better than one might expect from looking at the response curves. The B.E.S. SM 80 was no exception. For the most part it sounded much like the contours of its room-response curve would suggest, with an additional sense of spaciousness derived from its dipolar radiation pattern.

There was a noticeable “chestiness” in the sound of this speaker corresponding to its accentuated lower-midrange output. Its high end was strong, with a tendency toward brightness consistent with its somewhat ragged high-end response measurements. The bass was surprising: it seemed much stronger than one would expect from a speaker with a 50- or 60-Hz cutoff, but this was probably an effect of the strong peak at 76 Hz.

Overall, the B.E.S. SM 80 sounded larger than it is, probably because of its wide dispersion characteristics. After a short period of listening to it, it was easy to accept its colorations as normal since they were never overbearing or unpleasant. Undoubtedly, some people will like the sound of the SM 80 and others will not, but that can be said about almost any speaker, regardless of price. My advice is the same as usual: listen before you buy. Among speakers selling for $330 a pair, the B.E.S. SM 80 holds its own very well.

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