Signet SL-100 Speaker System Review

One of the most fundamental problems in speaker design is obtaining a uniform dispersion of the sound across the operating frequency range of the system. As the wavelength of the sound becomes comparable to the dimensions of the radiating diaphragm or dome, the output becomes increasingly directional, “beaming” like a searchlight along the forward axis of the speaker and becoming weaker along other axes. In some cases, obtaining the desired stereo effect requires that the listener be on the forward axes of both speakers, which must be angled slightly inward.

Another solution is to use two or more high-frequency drivers (tweeters) angled outward from the speaker’s forward axis. This can be effective, but such systems are prone to comb-filter effects, in which phase interference between the drivers’ outputs produces audible changes in the high-frequency sound as the listener’s head is moved or turned slightly.

The new SL-100 “Definitive Image” speaker system from Signet features a novel acoustic-lens tweeter design that is said to provide wide horizontal coverage throughout the tweeter’s operating range without comb-filter effects. Signet’s Ferrallipse acoustic lens takes advantage of a special property of the ellipse, which is an oval containing two focal points. The sum of the distances from the foci to the ellipse boundary is a constant. As a result, a sound originating at one focus of an elliptical reflector, regardless of its directive properties, will appear at the other focus as a single phase-coherent point source.

The SL-100 has two rear-facing 1-inch soft-dome tweeters located 7 inches apart and facing individual elliptical reflectors. Each driver’s dome is located at the outer focus of its reflector, nearest to the edge of the speaker enclosure, and the two reflectors have a common inner focus point located on a line midway between the drivers. In effect, all the high frequencies (from 5,000 Hz upward) radiate from a single point on the center axis of the speaker system and spread evenly across a horizontal angle of approximately 120 degrees.

The Signet SL-100 is a three-way floor-standing system measuring 36 inches high, 12-3/4 inches wide, and 16 inches deep. The tweeters and the Ferrallipse lens are located at the top of the front board, which slants slightly backward so that all the drivers radiate in phase. The midrange, from 500 to 5,000 Hz, is handled by a 3-inch soft-dome driver, and a 10-inch long-throw woofer operates in a vented enclosure whose two ports are in the rear of the cabinet. The frequency response of the SL-100 is rated as 40 to 20,000 Hz ± 2 dB, and its nominal impedance is 8 ohms. Suggested amplifier power ratings are from 25 to 250 watts per channel, and the rated sensitivity is 90 dB sound-pressure level (SPL) at 1 meter with an input of 1 watt.

The SL-100 is furnished with rubber feet and metal spikes, for use on carpeted or bare floors, respectively. Gold-plated five-way binding posts that accept banana plugs, spade lugs, or stripped wire ends are recessed into the rear panel. The walnut-grain cabinet has a removable black cloth grille, and each speaker weighs 66 pounds. Price: $1,450 per pair.

Lab Tests

The room response of the Signet SL-100 speakers, though quite flat overall, showed slight dips (3 to 5 dB) at 600 and 5,000 Hz. The high-frequency dispersion was excellent, with no significant difference between the response curves of the left and right speakers—respectively at angles of 0 and 30 degrees to the measuring microphone—all the way to our 20,000-Hz upper measurement limit.

When we combined the close-miked woofer response with the output from the port openings, the result was flat within 2.5 dB overall from 39 to 300 Hz, falling off to about — 6 dB at 500 Hz, the nominal crossover frequency, and at 26 Hz, the low end of the woofer’s range. We also measured the midrange response with a close microphone placement; it was uniform within ± 1 dB from 800 to 5,000 Hz except for a peak of several decibels at 3,500 Hz. The effective crossover from the woofer was at 600 Hz, where the output of each driver was down 6 dB from the average level in the octaves above and below that frequency.

The composite response, combining the woofer and room measurements, was within ±4 dB from 26 to 20,000 Hz. The middle and upper bass, from 60 to 200 Hz, was slightly elevated (by about 2 to 3 dB) compared with the higher frequencies, and a strong, clean bass output was maintained for more than an octave below that range.

The system’s sensitivity measured 91 dB SPL, and its impedance averaged about 5 ohms. The minimum, 2.7 ohms, was measured at 6,100 Hz, and the impedance reached 16 ohms at the upper-bass resonance of 53 Hz. The lower-bass resonance was below our 20-Hz limit, where the impedance was 30 ohms. The bass distortion was measured at a drive level of 2.5 volts, equivalent to a 90-dB SPL at 1 meter with a pink-noise input. Distortion was unusually low: less than 0.4 percent down to 65 Hz, under 2.4 percent to 24 Hz, and only 4.4 percent at 20 Hz.

Our quasi-anechoic response measurements, using the IQS FFT analyzer, revealed several interesting properties of the Signet SL-100. Most striking were periodic output variations through the middle and high-frequency range that were suggestive of comb-filtering resulting from interference between drivers. This is precisely the effect that the Ferrallipse lens was designed to eliminate, but it showed up strongly in our dispersion measurements made at angles of 0 and 45 degrees.

Aside from the cyclic response dips, however, these measurements confirmed the excellent dispersion that had been so evident in our room-response tests. The group-delay response (an indicator of phase linearity) showed some of the same variations, although it also indicated that the average output timing of all the drivers was matched very closely over the full measurement range from 180 to 22,000 Hz.

The SL-100 handled high-power tone bursts with exceptional ease. At 10,000 and 1,000 Hz, our amplifier clipped (at 1,510 and 1,290 watts, respectively) before any speaker limitations were apparent. At 100 Hz, where most 10-inch drivers would hit their stops at perhaps 200 or 300 watts, the SL-100 took a 610-watt single-cycle burst before emitting a slight rattle.

Comments

Compared with many other speakers, on first hearing the Signet SL-100 had a warm character, with an impressively deep bass output. At higher frequencies the sound was seamlessly blended and well dispersed. We listened for any signs of beaming or comb-filtering effects as we walked past the front of the speakers, using both music and pink-noise signals, but we could find none. This suggests that our FFT measurements, which were made at a single microphone “listening” position, were showing effects that would not be apparent to a human listener using both ears.

An excellent subjective test for speaker phase alignment is to listen through a spatial modifier such as the Carver Sonic Hologram, whose full effect requires that the speakers have close phase alignment in the horizontal plane. The SL-100 passed this test with flying colors, which tends to confirm that its two tweeters actually work like a single, centered, wide-dispersion tweeter.

The SL- 100’s were equally satisfactory with a variety of program material played though amplifiers of all power classes, from medium to very high. Since they are somewhat more sensitive than most speakers of similar size that we have tested, they can deliver a very high sound level when driven by a modestly rated receiver, yet they thrived on the full output of a super-power amplifier. They proved to have a full, flat response out to the highest audible frequencies in listening comparisons with other speakers on hand, most of which, though comparable in size to the Signet SL-100, did not have its extended bass performance.

Although the speaker’s interchangeable rubber and spiked feet offer the user a choice in installation, the instructions warn that the spikes can damage carpets and advise that the final position of the speaker be determined before any feet are attached. We found that the speakers sounded just fine when the cabinets were placed directly on the floor without any feet.

Overall, the Signet SL-100 impressed us with its superb bass, especially for its size, and its ability to provide clean, well-dispersed sound at high volume levels. Altogether, a very nice job.

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