Wilson Audio Sasha W/P loudspeaker Measurements

Sidebar 3: Measurements

Other than the in-room and nearfield measurements, for which I used a Joe D'Appolito–designed Mitey Mike II and an Earthworks QTC-40 microphone, respectively, the quasi-anechoic measurements of the Wilson Audio Specialties Sasha W/P were performed using DRA Labs' MLSSA system and a calibrated DPA 4006 microphone. Wilson specifies the Sasha's voltage sensitivity as 91dB/2.83V/m, which was confirmed by my own measurements. This is usefully higher than average. On the other hand, the Sasha's plot of impedance magnitude and electrical phase (fig.1) reveals the speaker to be a demanding load for the partnering amplifier to drive. Not only does the impedance drop below 4 ohms for most of the lower midrange and upper bass, with a minimum value of 2 ohms at 86Hz, but there is also an amplifier-unfriendly combination of 3 ohms and –43° phase angle at 61Hz. I'm surprised that Art's Shindo Corton-Charlemagne monoblocks, even when wired for 4 ohm operation, worked as well as they did, though I'm not surprised that his Shindo Haut-Brion, with its 16-ohm output secondaries, weren't happy driving the Wilsons.

Other than the effect of the tweeter's primary diaphragm resonance at 20kHz, the impedance traces in fig.1 are free from the small wrinkles that would imply the existence of cabinet resonances of various kinds. And indeed, investigating the vibrational behavior of the enclosure panels with a plastic-tape accelerometer, I found very little of note. The side panels of both W and P of the W/P were extremely inert; I show only a cumulative spectral-decay plot calculated from the accelerometer's output while it was fastened to the top panel of what I still like to call the "WATT" (fig.2) to demonstrate just how low in level were the only two modes I could find. The heroic construction of this speaker's two enclosures is very effective at suppressing panel resonances.

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Fig.1 Wilson Sasha W/P, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

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Fig.2 Wilson Sasha W/P, cumulative spectral-decay plot calculated from output of accelerometer fastened to top panel of upper enclosure (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The low-frequency saddle centered on 26Hz in the impedance-magnitude trace in fig.1 suggests that this is the tuning frequency of the woofer enclosure's large, rear-firing port. This was confirmed by the woofers' nearfield response (fig.3, black trace), which has the expected minimum-motion notch at that frequency. (This is the frequency at which the pressure exerted by the port resonance on the back of the woofer cones holds them still, all the acoustic output therefore coming from the port.) The output of the port itself (fig.3, red trace) peaks between 20 and 40Hz, with a smooth, well-controlled rolloff above that region. Commendably, no midrange resonances are visible in its response. The woofers start to roll off above 80Hz with initially, a 12dB/octave slope that increases above 300Hz. Again, the midrange output of the woofers is free from resonant spikes.

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Fig.3 Wilson Sasha W/P, acoustic crossover on listening axis at 50", corrected for microphone response, with nearfield responses of midrange unit (blue), woofer (black), and port (red), all plotted below 350Hz.

Below 350Hz, the blue trace in fig.3 shows the midrange driver's response, plotted in the nearfield (and including the contribution of its own 1"-diameter, rear-facing port). The initial rolloff from 400 to 100Hz is very gentle. Though this does steepen to 12dB/octave below 80Hz, this is not the usual 24dB/octave slope you get from a reflex design; I suspect the port is used to increase the midrange unit's power handling rather than to extend its low-frequency response.

Higher in frequency in fig.3, the midrange (blue) and tweeter (green) responses were taken at 50" in the farfield, on an axis I calculated to be equivalent to the position of Art's ears in his listening chair. Art sits with his ears 39" from the floor, and the in-room measurements I had performed in his room indicated that each of his ears was 87.5" from a Sasha tweeter. Following the excellent instructions engraved on a metal plate on the Puppy's top panel, Peter McGrath had set the speakers up in Art's room with the shortest set of spikes between each Sasha's two enclosures. In the room I use for measurements in inclement weather, I measure a speaker at a distance of 50" rather than 87.5" because, with the speaker raised off the floor so that its tweeter lies midway between floor and ceiling, all reflections from the room boundaries and the mike stand arrive at the mike more than 4 milliseconds after the direct sound from the speaker itself. Simple trigonometry indicated that, with the microphone at 50", it would have to be 2.8" below the tweeter in order to be on a straight line from the tweeter to Art's corresponding ear. I used this mike position for all of my farfield measurements.

The output of the midrange drive-unit rises gently by a couple of dB between 400Hz and 1kHz, and rolls off above 2kHz, with several peaks evident in its out-of-band response. I assume that these are cone breakup modes—the lowest-frequency mode, at 4.1kHz, is not very well suppressed by the crossover. The tweeter covers the region above 3kHz rather than the specified 1.8kHz, and has a basically flat response within its passband, with a couple of small peaks balanced by adjacent dips. Its output does rise above 15kHz due to the tweeter resonance at 20kHz, but the peak is not that high. The output drops rapidly above 20kHz.

Fig.4 shows how all these individual drive-unit responses sum in the farfield, on the reference axis. The slight rise in output in the upper bass will be due entirely to the nearfield technique used to measure the response below 300Hz. Under anechoic conditions the Sasha will extend down to 40Hz, with then a slight shelf apparent before the rolloff reaches its eventual 24dB/octave slope below 28Hz. A little too much energy is apparent at the top of the midrange; whether this will be perceived as a forwardness to the Sasha's balance, or whether the relative lack of energy in the presence region will be heard as a slightly laid-back quality, will very much depend on the music played. This graph was taken with the Sasha's grille removed; with the grille in place, which is how AD auditioned the Wilsons, reflections from the grille frame filled in some of the depressions between 2 and 8kHz, at the expense of reducing output above 10kHz by 1dB or so. Although I don't usually comment on how closely a pair of speakers matches, the two Sashas were extraordinarily close in response, matching to within 0.2dB or less over most of the audioband, with differences of up to 1dB evident in only two very narrow regions: between 1100 and 1500Hz, and between 4 and 5kHz (not shown).

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Fig.4 Wilson Sasha W/P, anechoic response on listening axis at 50", without grille, averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield responses plotted below 300Hz.

The Sasha's lateral dispersion, referenced to its on-axis response, is shown in fig.5. While at first the speaker's off-axis behavior seems a bit peaky between 4 and 8kHz, this is actually due to the on-axis suckouts in this region filling in to the speaker's sides; in a typical room, the Wilson's mid-treble balance will be smooth and neutral. A slight reduction in off-axis energy between 700Hz and 1.3kHz might mitigate the effect of the slight on-axis boost in the same region. As is normal with a 1" tweeter, the dispersion narrows in the top octave, but not to the usual degree. The tweeter also puts out a little more energy to the sides above 20kHz than it does on axis, which is why there appear to be off-axis peaks in this region. In the vertical plane (fig.6), a small suckout develops in the upper crossover region for listening axes below that Art used for his auditioning, while the same region gets a little peaky if you sit on or above the tweeter axis. Again, the wide range of inter-cabinet spikes supplied by Wilson, in combination with the detailed setup information, will allow the speaker's in-room balance to be optimized for a wide range of listener ear heights and listening distances.

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Fig.5 Wilson Sasha W/P, lateral response family at 50", normalized to response on listening axis without grille, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.

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Fig.6 Wilson Sasha W/P, vertical response family at 50", normalized to response on listening axis without grille, from back to front: differences in response 15–5° above axis, reference response, differences in response 5–15° below axis.

On my way home from the 2010 Salon Son et Image in Montreal, I visited Art Dudley in upstate New York to pick up the Sashas for measurement chez Atkinson. Before packing up the Wilsons, I took advantage of the opportunity to perform my usual spatially averaged in-room response with the speakers driven by Art's preferred Shindo tube monoblocks. The result is shown as the red trace in fig.7. The highs are well extended, though with an excess of energy at the top of the midrange. As I said earlier, whether this will be heard as an excess, or whether the listener's ears will instead latch on to the relative lack of energy in the mid-treble, will very much depend on the recording played. The low frequencies are also well extended, with useful output evident to below 25Hz. However, there is also a lack of energy apparent in the lower midrange. I suspect this is due to the Allison Effect, in which reflections from the walls and floor cancel the direct sound. With the Sashas set up in AD's room, the distances of each midrange unit from the adjacent wall and the floor were very similar, meaning that the effect of each reflection will be coincident in the frequency domain. Against this measured effect must be set the fact that the presence of such reflections is something the ear learns and can compensate for, at least to some extent.

The blue trace in fig.7 shows the in-room response of Art's reference Audio Note speakers, which he places in the room's corners with an extreme amount of toe-in. The corner placement gives rise to a more lumpy bass response than the Wilsons', which were 5' or so in front of the wall behind them. The Audio Notes evince far more midrange energy, which I heard in my own auditioning of them as coloration. However, the toe-in gives the English speaker a very well-controlled if somewhat mellow-balanced treble region.

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Fig.7 Wilson Sasha W/P, spatially averaged, 1/6-octave response in AD's listening room (red); spatially averaged response of Audio Note AN-E/SPe HE (blue).

Finally, looking at the Wilson's time-domain performance, its step response on the listening axis (fig.8) indicates that its midrange drive-unit is connected in inverted acoustic polarity, the woofers in positive polarity, and that the decay of the midrange's step smoothly blends with the rise of the woofer step. This indicates optimal crossover design. It is difficult to see from this graph what polarity the tweeter is connected in; however, looking at the Sasha's step response with the midrange driver disconnected indicates that it, like the woofer, is connected with positive acoustic polarity. The tweeter's output thus interrupts the smooth negative-going step of the midrange unit. However, the decay of the tweeter's step blends in nicely with the rest of the rise of the midrange step on this axis, which, presumably, is why there was no lack of crossover-region energy apparent in fig.4. The cumulative spectral-decay plot on the Sasha's listening axis (fig.9) is relatively clean, though some low-level modes can be seen to be associated with the peaks in the mid-treble.

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Fig.8 Wilson Sasha W/P, step response on listening axis at 50" (5ms time window, 30kHz bandwidth).

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Fig.9 Wilson Sasha W/P, cumulative spectral-decay plot on listening axis at 50" (0.15ms risetime).

It took me longer than usual to comprehend what the Wilson Sasha's measurements were telling me. Some of the individual measurements raised my eyebrows a little, but taking in the entirety of the speaker's measurements, it becomes apparent that the Sasha is the result of its designer carefully balancing each parameter to achieve a good-sounding whole. And judging from not only Art Dudley's auditioning but also my own, the Sasha does achieve that goal. With the Sasha, David Wilson has finally eliminated the upper-bass "blump" endemic to earlier generations of the WATT/Puppy, and that I felt to be that otherwise excellent-sounding speaker's Achilles' heel. But its impedance is still going to mean careful auditioning with a would-be purchaser's amplifier, to see if the latter is up to the task of driving the Sashas.—John Atkinson

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Wilson Audio Specialties
2233 Mountain Vista Lane
Provo, UT 84606
(801) 377-2233
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