Sidebar 3: Loudspeaker Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Vandersteen Quatro Wood CT's frequency response in the farfield and an Earthworks QTC-40 mike for the nearfield responses. I left the loudspeaker's grille in place for the measurements, as it provides the necessary acoustic boundary conditions for the drive-units.
Vandersteen specifies the Quatro Wood CT's sensitivity as 87dB/2.83V/m. My estimate was 83dB(B)/2.83V/m, which is lower than average but within experimental error of the 83.5dB/2.83V/m I measured for the earlier version of the Wood. The nominal impedance is specified as 8 ohms ± 3ohms. The original Wood's impedance lay at or above 8 ohms for the entire audioband. By contrast, as shown by the solid trace in fig.1, the Wood CT's impedance lies below 6 ohms for the entire midrange and low treble as well as in the top audio octave and above. The minimum magnitude is 4.5 ohms between 155Hz and 200Hz, and while the electrical phase angle (dotted trace) reaches –47° at 85Hz, the magnitude at that frequency is 9.4 ohms. The new speaker is still not a difficult load to drive.
The red trace in fig.4 is the subwoofer's nearfield response repeated from fig.3. The Vandersteen's farfield response, averaged across a 30° horizontal window centered on the tweeter axis, is shown as the blue trace above 300Hz in fig.4. The midrange is slightly down in level, and there is a slight lack of energy between 4kHz and 7kHz. Other than that, the Quatro Wood CT's balance is superbly flat and even up to 19kHz.
Like its predecessors, Vandersteen's Quatro Wood CT offers a true time-coincident output, due to the stepped-back sub-baffles for the upper-frequency drivers, the first-order crossover slopes, and the fact that all its drive-units are connected in positive acoustic polarity. Fig.9 shows the speaker's step response on the tweeter axis. The initial arrival is an almost textbook right-triangle shape, though the tweeter's output arrives slightly before that of the midrange unit. This confirms that the optimal axis will be 5° below the tweeter axis. Finally, the Quatro Wood CT's cumulative spectral-decay plot (fig.10) is impressively clean.—John Atkinson
Footnote 1: This graph was not taken with MLSSA, by the way, so the Y-axis level is not calibrated. It cannot be compared with cabinet vibrational cumulative spectral-decay plots in other Stereophile loudspeaker reviews.
Fig.1 Vandersteen Quatro Wood CT, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
The traces in fig.1 are free from the small discontinuities that would imply the presence of cabinet resonances of various kinds, and the enclosure's panels seemed inert to the knuckle-rap test. However, I did find a resonant mode just above 500Hz on the sidewalls level with the woofer when I investigated the enclosure's vibrational behavior with a plastic-tape accelerometer (fig.2, footnote 1).
Fig.2 Vandersteen Quatro Wood CT, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of sidewall level with woofer (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 1kHz).
The impedance-magnitude plot has a peak centered on 65Hz, which suggests that this is the sealed-enclosure tuning frequency of the front-mounted woofer, though the use of an in-line high-pass filter in the amplifier will roll off the output above this frequency. The red trace in fig.3 indicates that with the speaker driven by the M5-HPA amplifier, the woofer's nearfield response rolls off cleanly in the bass, with a –3dB point at 100Hz as specified. The nearfield response of the Vandersteen Quatro Wood CT's subwoofer (green trace) peaks between 30Hz and 80Hz, though its upper-frequency rolloff is disturbed by a boost between 120Hz and 200Hz. This is due to the settings of the graphic equalizer chosen by Richard Vandersteen when he set up the speakers in my room. In the farfield, the woofer crosses over to the midrange unit (blue trace) at 725Hz—a little lower than the specified 900Hz— with symmetrical first-order slopes.
Fig.3 Vandersteen Quatro Wood CT, responses of midrange unit (blue), woofer (red), and subwoofer (green) with 11-band equalizer as set by Richard Vandersteen.
Fig.4 Vandersteen Quatro Wood CT, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response (blue), with the complex sum of the nearfield midrange and woofer responses plotted below 300Hz (blue) and the nearfield subwoofer response (red).
Fig.5 shows the Vandersteen Quatro Wood CT's horizontal radiation pattern, normalized to the tweeter-axis response and plotted up to 45° to the speaker's sides rather than the usual 90°. This was because the speaker's bulk and 110lb weight meant I had to perform the quasi-anechoic measurements in my listening room rather than in my backyard; the proximity of room boundaries meant that it wasn't possible to measure the speaker's output more than 45° to its sides. The contour lines in this graph are impressively even, implying stable stereo imaging. In the vertical plane (fig.6), with the traces again normalized to the tweeter-axis response, it appears that the mid-treble suckout seen in fig.4 fills in 5° below the tweeter axis but deepens above it. As the tweeter is 40" from the floor with the speaker sitting on its spikes, the Quatro Wood CT gives its flattest treble response 36" from the floor, which we have found to be the average height of a seated listener's ears.
Fig.5 Vandersteen Quatro Wood CT, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° off axis, reference response, differences in response 5–45° off axis.
Fig.6 Vandersteen Quatro Wood CT, 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.
The red trace in fig.7 shows the Quatro Wood CTs' spatially averaged response in my room. This is generated by averaging 20 1/6-octave–smoothed spectra, taken for the left and right speakers individually in a vertical rectangular grid 36" wide by 18" high and centered on the positions of my ears. For reference, the blue trace shows the spatially averaged response of the Vandersteen Model Seven Mk.IIs, which I reviewed in May 2016. The two speakers have similar measured responses above 400Hz, smoothly sloping down above 1kHz, though the Model Seven has more high-treble energy in-room, perhaps due to the fact that it has a rear-facing tweeter to increase the reverberant-field energy in this region. Both speakers' powered subwoofers extend their output to 20Hz, though the Quatro CT excites the lowest-frequency room mode to a greater extent than the Model Seven.
Fig.7 Vandersteen Quatro Wood CT, spatially averaged, 1/6-octave response in JA's listening room (red); and of Vandersteen Model 7 Mk.II (blue).
Fig.8 compares the Quatro Wood CT's in-room response (red trace) with that of the DALI Callisto 6 C that I reviewed in September 2019 (blue trace) and my long-term reference, the KEF LS50 minimonitor (green trace). The KEFs and Vandersteens have broadly similar responses from 1kHz to 20kHz, with the slightly sloped-down output due to the increased absorptivity of the room's furnishings as the frequency rises—what you don't want to see in this kind of measurement is a flat treble response, which corresponds to a rising on-axis output. By contrast, the DALIs have too much treble energy. All three speakers have a small peak in their outputs in the upper midrange but different patterns of peaks and dips in the lower midrange. The small KEFs roll off early in the bass, but the DALIs and Vandersteens have very similar low-frequency in-room balances.
Fig.8 Vandersteen Quatro Wood CT, spatially averaged, 1/6-octave response in JA's listening room (red); of DALI Callisto C 6 (blue), and of KEF LS50 (green).
Fig.9 Vandersteen Quatro Wood CT, step response on tweeter axis at 50" (5ms time window, 48kHz bandwidth).
Fig.10 Vandersteen Quatro Wood CT, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
Footnote 1: This graph was not taken with MLSSA, by the way, so the Y-axis level is not calibrated. It cannot be compared with cabinet vibrational cumulative spectral-decay plots in other Stereophile loudspeaker reviews.
