Vandersteen Audio Treo loudspeaker Measurements
Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Vandersteen Treo's frequency response in the farfield, and an Earthworks QTC-40 for the nearfield and spatially averaged room responses. As I suspected from my auditioning, the Treo is relatively insensitive. My estimate of the Treo's voltage sensitivity was a low 82.5dB(B)/2.83V/m, the same as the BBC LS3/5a. However, the impedance remains between 6 and 10 ohms for almost the entire audioband (fig.1), with a benign electrical phase angle. The minimum magnitude is 5.5 ohms, at 26 and 75Hz; the Treo will be relatively easy for the partnering amplifier to drive.
The slight discontinuity just below 200Hz in the impedance traces implies that there is some kind of resonant mode in this region. Investigating the panels' vibrational behavior with a simple piezoelectric plastic accelerometer, I did find a low-level mode at 195Hz on the side panel level with the subwoofer (fig.2), as well as some slight flexing in the midbass. There was also a mild resonant mode present just below 600Hz on the rear panel (not shown). Overall, the Treo's enclosure seems well damped and relatively resonance free.
The saddle centered on 26Hz in the impedance-magnitude trace suggests that this is the tuning frequency of the downward-facing port. Its output, measured in the nearfield and scaled in respect to the outputs of the other radiators in the ratio of its radiating diameter, is shown as the red trace in fig.3 and peaks between 20 and 50Hz, with a smooth rolloff above that range broken by a slight plateau at 150Hz. While the port is tuned to 26Hz, the two low-frequency drive-units have slightly different minimum-motion notches in the nearfield responses: the subwoofer's (green trace) lies at 22Hz, the woofer's (blue trace) at 25Hz. These figures imply excellent low-frequency response for the Treo, which was indeed what I heard. The subwoofer's output effectively fills in the gap between the woofer and port outputs.
Fig.3 reveals that the Treo's woofer covers a wide range, not handing off to the midrange unit until 1kHz, which means that it is actually responsible for most of the Treo's midrange output. Both the woofer and the midrange unit (black trace below 5kHz) roll off very gently outside their specified passbands. Both, however, are well behaved outside those bands. I have shown the Treo's upper-frequency response measured on the midrange axis, as that is what I had understood from Richard Vandersteen's own measurements to be the optimal listening axis. The grille was left on for all the farfield measurements, as it provides a smooth acoustic environment for the drive-units. The treble output on this axis is generally flat, though with a slight lack of energy in the region of handoff to the tweeter. That same lack of energy can be seen in the Treo's farfield response averaged across a 30° horizontal window centered on the midrange axis (fig.4). Other than slight excesses of between 400 and 600Hz and between 9 and 14kHz, the response is very flat, falling within a ±2dB limit across the entire audioband. The 3dB rise in output in the midbass is an artifact of the nearfield technique used to assess the response below 300Hz in this graph. The Treo's woofer alignment is actually maximally flat, and reaches its 6dB point at the port tuning frequency of 26Hz.
The Treo's lateral dispersion, referenced to the midrange-axis response, is shown in fig.5. The apparent off-axis peak at the cursor position just below 5kHz is actually due to the suckout in the upper crossover region filling in to the speaker's sides. The speaker's lateral radiation pattern is otherwise well behaved. In the vertical plane, the dispersion, again referenced to the Treo's output on the midrange axis, which is 36" from the floor with the speaker supported by its cones, is shown in fig.6. (Work by Home Theater's Tom Norton when he was with Stereophile showed that the average seated listener's ears are the same 36" from the floor.) The response is maintained 5° below the midrange axis, but as you move above the tweeter axis, the region covered by the tweeter is boosted and the suckout in the crossover region deepens.
Fig.7 shows how this quasi-anechoic behavior sums in my listening room. To generate this trace, I average 20 1/6-octavesmoothed spectra, taken for the left and right speakers individually using SMUGSoftware's FuzzMeasure 3.0 program and a 96kHz sample rate, in a vertical rectangular grid 36" wide by 18" high and centered on the positions of my ears. This eliminates the room acoustic's effects, and integrates the direct sound of the speakers with the in-room energy to give a curve that I have found correlates reasonably well with a speaker's perceived tonal balance. With the boundary reinforcement offered to the port's output by the floor, the Vandersteen's low frequencies extend at full level to 25Hz. The Treo's spatially averaged response is impressively flat in the decade of 400Hz4kHz, above which it smoothly rolls off as a result of the increasing absorptivity of the room's furnishings. However, there is a lack of energy in the lower midrange that I couldn't eliminate by experimenting with the speaker positions. I suspect that it results from the woofer and midrange being similar distances from the floor and sidewalls in my room.
In the time domain, the use of a sloped-back baffle and first-order crossover filters gives a step response on the midrange axis that generally resembles the right-triangle shape of a true time-coincident design (fig.8). The sharp leading-edge spike suggests that the tweeter is a little too hot, while the double arrival after the spike implies that the optimal axis is actually just below the midrange unit (fig.9). The cumulative spectral-decay plot (fig.10) is clean in the midrange and top octaves, but is dominated by a hashy-looking region in the upper crossover region. I am not sure where this behavior stems from; it's fair to note that it is absent from the cumulative spectral-decay plot taken on the same axis and sent me by Richard Vandersteen.
With a time-coincident first-arrival sound and a basically flat response, the Vandersteen Treo offers respectable measured performance.John Atkinson