Sidebar: Measurements, from April 2015 (Vol.38 No.4)
Sam Tellig wrote about Spendor's D7 loudspeaker in his column, "Sam's Space," in the September 2014 issue, concluding that this English floorstanding speaker was "a splendid Spendor," and that "one might seriously question the need to spend any more than $6495 for a pair of speakers" (footnote 1). What he found especially rare, for a relatively small speaker, was the D7's "authority, weight, and speed. You could spend far more for a speaker and not get such a fast, highly resolving sound, and such a sweet midrange."
We don't usually publish measurements of the products covered by Stereophile's columnists, unless the writer feels that there is something about the sound that merits further investigation. But in the case of the D7, some readers complained about the lack of measured results in comments posted to the website reprint of Sam's report. In response, Jay Rein, of Bluebird Music, the brand's North American distributor, posted that "Bluebird Music has repeatedly said the D7s are the best speakers Spendor has ever made. We are more than willing to back up this claim by providing a pair of D7s to be measured and tested as you do in your regular reviews."
Accordingly, I asked Rein to ship a pair of D7s to me for measurement. (The serial numbers were 324000289 and '290.) I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Spendor D7's frequency response in the farfield, and an Earthworks QTC-40 for the nearfield responses.
Footnote 1: The D7's price was reduced to $5995/pair in January 2015 (add $1000 for a premium finish).
Fig.1 Spendor D7, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
The D7's voltage sensitivity is specified as 90dB/W/m; my estimate was close, at 89.3dB(B)/2.83V/m, which is above average. The D7's nominal impedance is specified as 8 ohms, with a minimum magnitude of 4.5 ohms. Fig.1 shows the measured impedance magnitude (solid trace) and electrical phase angle (dotted). The minimum impedance was indeed 4.5 ohms, at 183Hz, and the magnitude remains above 6 ohms in the upper bass and from the upper midrange upward. Although the phase angle exceeds 40° at 53 and 76Hz, the magnitude is high at those frequencies, ameliorating the extra current demand imposed by the impedance phase.
Fig.2 Spendor D7, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of side panel level with mid/woofer (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
Though there is a slight discontinuity at 125Hz, the impedance traces are free from the small wrinkles in the midrange that would suggest the presence of cabinet resonances. Nevertheless, when I investigated the vibrational behavior of the enclosure panels with a plastic-tape accelerometer, I found a strong mode at 391Hz that was most pronounced on the side panels level with the midrange/woofer (fig.2). This mode disappeared lower on the panel, replaced by one much lower in magnitude, at 348Hz. It's possible, therefore, that despite this mode's high amplitude, the areas affected are small enough that it won't lead to coloration or midrange congestion.
Fig.3 Spendor D7, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with nearfield responses of: mid/woofer (green), woofer (blue), port (red), with complex sum of nearfield responses, respectively plotted below 500Hz, 500Hz, 950Hz, 300Hz.
The black trace above 300Hz in fig.3 shows the Spendor's farfield anechoic response, averaged across a 30° horizontal window centered on the tweeter axis. Below 300Hz, the black trace shows the complex sum of the nearfield outputs of the mid/woofer, the woofer, and the vent in the base of the enclosure. The green trace is the mid/woofer's output, the blue trace the woofer's, and the red trace the vent's, each scaled in the ratio of the square root of the radiating area. The port has a peak in its upper-midrange output, but this will be subjectively inconsequential given that the port fires away from the listener. The port's output covers a much wider bandpass than in a traditional reflex alignment; perhaps that slight discontinuity in the impedance traces at 125Hz is due to the vent's actually behaving with some aspect of transmission-line tuning, which extends the port's output higher in frequency than is usually the case. The port's output is also low in level, which suggests that the alignment has been optimized for the fact that the port is so close to the floor.
The minimum-motion notch in the woofer's response (blue trace) occurs at 27Hz and the mid/woofer's notch at 22Hz, but the two drive-units' outputs in the mid- and upper bass are otherwise similar. However, the woofer's output starts to gently roll off above 100Hz, and is 3dB below the mid/woofer's output at 500Hz. The D7 is thus a "2½-way" rather than a three-way design, with the low frequencies shared by the LF drivers. This seems to be an effective technique, judging from ST's favorable comments about the D7's bass reproduction: "I take back everything bad I've written about floorstanding speakers: troublesome bass, poor integration of drivers in the nearfield, etc. . . . Few loudspeakers can convey the power without the aid of a pesky subwoofer or two. Or without pooping out. The D7s could, given enough power."
Fig.4 Spendor D7, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.
Fig.5 Spendor D7, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15–5° above axis, reference response, differences in response 5–10° below axis.
Higher in frequency in fig.3, the soft-dome tweeter's output drops off rapidly above 18kHz, but the D7's farfield response is impressively even, with a couple of small peaks balanced by equally small dips. Whether this will translate into a flat perceived balanced will depend also on the speaker's dispersion, of course, and the D7's horizontal radiation pattern is shown in fig.4. This graph reveals that the mid/woofer's output drops off rapidly to the speaker's sides above 2kHz, which results in an off-axis flare at the bottom of the tweeter's passband. Whether the flare results in a slightly bright balance, or the off-axis lack of energy half an octave lower in frequency results in a slightly polite balance, will depend both on the music played and on the listening-room acoustics. Fig.4 also indicates that the tweeter maintains its top-octave output over a wider angle than is usual with a 1" dome. In the vertical plane (fig.5), the D7 should be listened to within 5° of the tweeter axis, which is 35" above the floor.
Fig.6 Spendor D7, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Turning to the time domain, the Spendor's step response on the tweeter axis (fig.6) is reminiscent of BBC-legacy designs, with the tweeter connected in inverted acoustic polarity and its output leading that of the positive-polarity lower-frequency units by a quarter of a millisecond or so. The decay of the tweeter's step blends with the start of the woofers' step on this axis, however, implying optimal crossover design. The decay of the D7's output is clean in the tweeter's passband (fig.7), though a ridge of delayed energy is apparent at 4.2kHz, the frequency of a small peak in the D7's HF-axis response.
Fig.7 Spendor D7, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
Overall, the Spendor D7 offers excellent measured performance. I must agree not only with Sam Tellig but also with Martin Colloms, who wrote of the D7, in the July–September 2013 issue of The HiFi Critic: "here is a modern interpretation of the classically neutral, accurate and well integrated design."—John Atkinson
Footnote 1: The D7's price was reduced to $5995/pair in January 2015 (add $1000 for a premium finish).
