Thiel CS2.3 Loudspeaker Measurements part 2
Accordingly, I repeated the measurement at an 80" mike distance, about as far away as I can get in Stereophile's new measurement/listening room. The results are shown in fig.4. The top trace is the 50" measurement; the bottom trace is the one taken at 80". Yes, the lack of energy starts to fill in. But note the much larger effect of microphone distance on the absolute level of the tweeter. At the farther distance, the treble is better matched to the speaker's midrange. I have not experienced this kind of inverse proximity effect before—usually, the bass starts to tilt up as the microphone gets closer to the speaker. Perhaps this is a function of the CS2.3's unique coaxial, mechanically crossed-over midrange/HF unit. Whatever, fig.4 suggests that, at a normal listening distance in a room, the CS2.3's overall balance will be much flatter than is implied by fig.3. (When I visited the Thiel facility a couple of years back, I noted that Jim Thiel does measure his speakers much farther away than I can.)
Fig.4 Thiel CS2.3, 1/3-octave-smoothed anechoic response on tweeter axis at 50" (top) and 80" (bottom).
In a room, a speaker's perceived balanced will also be affected by its off-axis behavior. Fig.5 shows the changes in the Thiel's response as the microphone is moved at 5° intervals to the side. The high-frequency dispersion is excellent, correlating with the accurate, stable imaging. And in the presence region there is more energy to the speaker's sides, which will ameliorate the effect of the on-axis suckout. But note that an off-axis depression develops in the lower crossover region.
Fig.5 Thiel CS2.3, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90 degrees-5 degrees off-axis; reference response; differences in response 5 degrees-90 degrees off-axis.
I note that BD actually listened with his ears several inches above the tweeter axis, which is a low 32" from the floor. Fig.6 shows the actual responses of the CS2.3, plotted from 10° below the HF axis (front of graph) to 20° above it. The flattest response overall is actually the lowest, at just 20" from the floor. Moving above the tweeter axis at this 50" microphone distance fills in the presence-region suckout, but introduces another suckout at the lower crossover frequency. The effect of this in a room will be hard to predict; lobing in this frequency region is hard to perceive in-room, and the speaker's total power output is still smooth through this region.
Fig.6 Thiel CS2.3, vertical response family at 50", from back to front: responses 20 degrees-5 degrees above HF axis; reference response; responses 5 degrees-10 degrees below HF axis.