Stand Loudspeaker Reviews

Though SOTA specifies the Panorama as being an 8-ohm load, the speaker can be seen from fig.1 actually to drop somewhat lower than that, reaching a minimum value of 5.1 ohms at 135Hz and averaging 4.4 ohms in the top octave. This should not present amplifiers with problems unless, like the B&K ST-140, they have problems driving loads much below 8 ohms. That the damped slot on the speaker's base does not act as a reflex port can be shown by the fact that the impedance in the bass features the single peak typical of a sealed-box alignment.

As noted above, choosing the correct listening axis with the Panorama is critical. This is shown by fig.2, which shows the speaker's anechoic response at a 48" distance (calculated by DRA Labs' MLSSA system) on what might seem to be the intuitive listening axis, between the two drive-units (green), with the mike looking down at the tweeter (red), and with the microphone level with the bottom of the cabinet (blue). (The speaker was sitting on a conventional stand for these measurements, meaning that the only tiltback was that due to the sloped baffle.) The only response not afflicted with severe treble suckouts is the latter, confirming that the speaker needs to be drastically tilted back if the listener is to get anything like a neutral treble balance.

The top octave is still peaky on this axis, however, which is a function of the tweeter used. This has a strong resonance around 16kHz or so, significantly lower in frequency than the analogous resonance featured by a typical metal-dome type such as the MB unit used in the Hales Signature loudspeaker. Though many listeners like the sound of this Focal tweeter, feeling that its exaggerated top octave adds a sense of "air" and subjective focus to the sound, I prefer a more neutral treble balance.

This audible top-octave peak can also be seen in fig.3, which shows the anechoic responses of the individual drive-units measured at 48" on this optimal axis. The tweeter can be seen to roll out steeply below 3kHz, though there is a strong diffraction-related notch noticeable at 2.5kHz. The woofer response is plotted above 500Hz, where the data are reliable. Though it starts to roll out above 2kHz, the use of a 6dB/octave low-pass filter means that this rollout is very shallow. In fact, the presence of strong cone-breakup modes between 3kHz and 5kHz—interestingly, the impedance of the woofer on its own also indicates either a resonance or the presence of some kind of notch filter at 3250Hz—means that the woofer's level is not significantly lower than that of the tweeter throughout the presence region. In my humble opinion, the designer's choice of a first-order low-pass filter was not optimal, given this Focal woofer's relatively severe problems in the treble. I find it significant that although Thiel uses a similar Scanspeak drive-unit in their Thiel CS5, which also features first-order crossover slopes, Jim Thiel rolls it off above 400Hz.

The lefthand curve in fig.3 shows the Panorama's bass response measured in the near field; ie, with the microphone almost touching the center of the driver's dustcap. Though the rolloff is shallow, starting off at 9dB/octave or so, the speaker's –6dB point is a high 50Hz, correlating with Tom Norton's feeling that the sound was too lightweight too much of the time. With this kind of overdamped alignment, it would be possible to reinforce the midbass with a near-wall placement without the sound becoming too boomy in the upper bass. However, this would undoubtedly compromise the Panorama's excellent imaging and soundstaging by adding integrity-destroying early reflections of the sound from the wall.

With the speakers out in the room, sitting on 24" stands and tilted back, carrying out a spatially averaged response measurement—the 1/3-octave response of each speaker is individually measured at 10 positions in a rectangular window some 72" by 21", centered on the listening position—reveals a balance nicely correlating with Tom's description of the sound (fig.4). Though the midrange is quite smooth, the bass is lightweight and prematurely rolled-off, while the extreme treble is peaked up. A touch of liveliness can also be seen between 3 and 4kHz, the region where the woofer has problems.

Turning to the time domain, fig.5 shows the step response on an axis midway between the drive-units, while fig.6 is that on the optimal low axis. Both relatively time-coherent, the former shows the output of the tweeter slightly preceding that of the woofer on this axis. On te optimal axis, the speaker offers a time-coincident output. Looking at the individual step responses of each driver (not shown here) reveals that the drivers are connected with the same polarity and that both appear to be inverting.

I said "relatively time-coherent." The Panorama's acoustic phase response between 200Hz and 20kHz on the optimal axis at the 48" microphone distance (correcting, of course, for the time taken for the sound to reach the mike) is shown in fig.7. It can be seen that the phase error is still large in the treble, though looking just at the phase error that remains after that due to a minimum-phase system with the Panorama's amplitude response has been subtracted (fig.8) indicates that below 7kHz or so, the speaker is quite close (+5°, –50°) to a minimum-phase system.

The MLSSA system's postprocessing power allows you to examine how a speaker's frequency response changes as the impulse sound dies away. Fig.9 shows the Panorama's resultant cumulative spectral-decay, or "waterfall," plot, calculated from the impulse response data taken on the optimal axis. The tweeter's strongly resonant behavior in the top octave is readily apparent, as is a resonant ridge at 3730Hz, this due to the strong woofer mode noted above. As this is the region where the ear has its maximal sensitivity, I would expect this to be audible as an added brightness to the Panorama's sound and it will make the speaker very sensitive to the matching electronics. It could also contribute to the Panorama's vividness of imaging by adding "outlines" similar to the Mackie lines you get when you underexpose a low-contrast scene and then process the resultant black-and-white negative with weak developer. Dark areas in the subsequent print are surrounded with contrast-enhancing white lines where the developing solution had become exhausted.

The recent debate (instigated by Peter Aczel of The Audio Critic, footnote 1) concerning the effect of altitude on loudspeaker performance prompted me to carry out measurements on the SOTA Panorama both in TJN's Los Angeles condo and here in Santa Fe. Fig.10 shows the quasi-anechoic response of the Panorama at a distance of 48" on the tweeter axis in Santa Fe (red), and in Los Angeles (blue). As can be seen, there are no significant differences apart from those due to the impossibility of positioning the B&K measuring microphone at exactly the same position in the two locations.—John Atkinson