The impedance magnitude and phase angle for the Signet SL280 are shown in fig.1. The double peaks in the magnitude plot below 100Hz are typical of a reflex tuned cabinet. Note that the minimum impedance of the system is just below 7 ohms (at about 150Hz), indicating that no competent, appropriately powered amplifier (into 4–8 ohm loads) should have any difficulty in driving the Signets.
Fig.2 shows the impulse response. The ringing in the tail of the response is due to the ultrasonic peak in the tweeter's response—visible in the other plots and typical of a metal dome. Both drivers are connected in phase, and the response of the woofer lags that of the tweeter by about 0.2 milliseconds. This is clearer from the impulse responses of the individual drivers (not shown).
Fig.3 actually plots four separate measurements. The responses of the individual drivers (driven from their respective bi-wire terminals) are shown above 250Hz. Note the smooth, controlled rolloffs of the individual drivers, with no significant aberrations in the slopes which could color the response. The only significant anomaly is the narrow, 2–3dB blip around 900Hz. The 25kHz peak in the metal-domed tweeter is also evident—but all of its impact is well above the audible range. The measurements above 250Hz (as well as the impulse response and the data in the remaining curves) were MLSSA-derived. The curves below 250Hz (as well as fig.1) were measured with the Audio Precision System One test set. Below 250Hz are the nearfield low-frequency response measurements of the woofer and the port, with the curve to the far left showing the port output. It is evident that the port is tuned to around 30Hz. The output level for the port, as shown, is at an arbitrary level. There are various proposed methods of melding measured near-field responses of a low-frequency driver and port to form a coherent whole, but no consensus.
In fig.4, taken at tweeter level, the SL280's smooth response across the 30° lateral window is evident, implying a broad dispersion. With the exception of the minor dip and bump between 400Hz and 1500Hz, the midrange response is exceptionally smooth and level, the tweeter free of significant aberrations. Fig.5 provides a more detailed picture of the Signet's dispersion. In the curve closest to the front, taken 30° off the lateral axis of the loudspeaker, the dip at 4kHz clearly indicates that the Signets should be toed-in toward the listening position for best results. The next curve back was taken below woofer level. The middle curve, on the woofer axis, is clearly the optimum height—which happens to be very near to the axis used in my listening tests. On the dedicated stands, the tweeter is about 40.5" above the floor, the woofer 34.5". The height of the stands appears to have been well chosen to encompass the spread of average listening heights. The next to the rear curve, taken on the tweeter axis, is actually a bit smoother through the midrange but with what appears to be a narrow interference dip at 4kHz. And the rear curve, at 7.5° above the woofer axis, shows why these loudspeakers (along with most others, I might add) are best not listened to while the listener is standing.
These measurements agree closely with the results of the listening tests and would be impressive in a loudspeaker in any price range, let alone one costing $900/pair. The waterfall plot—more correctly termed the cumulative spectral decay—shows how the frequency response dies off over time after being hit by an impulse. In theory it should drop off immediately; no loudspeaker does, and the response of the Signets is among the best we have measured—regardless of price. There are no significant audible resonances in the high frequencies (the dark line around 15kHz is the computer monitor's scanning frequency; the resonance at the far right is the 25kHz peak in the metal dome's response). The minor resonance corresponding to the small rise around 900Hz may correlate with the slightly forward quality noted in the listening tests, but this is by no means certain, as the rise covers only a very narrow band.—Thomas J. Norton
