Thiel CS6 Loudspeaker Measurements part 3

Another way of looking at a speaker's time coherence is to examine not its phase response as such, but the phase deviation left over when that due to the speaker's departure from a flat amplitude response is removed. In a minimum-phase system—one that has just the right amount of phase deviation for its frequency response—the phase and amplitude responses are related mathematically by the Hilbert Transform. Subtracting the Hilbert-transformed amplitude response from the measured phase response will leave what is called the "excess phase"; ie, the speaker's departure from a true minimum-phase system. The CS6's excess phase on the tweeter axis is shown as the top curve in fig.8—other than the slight negative error above 20kHz, which is due, I believe, to the measuring microphone's own phase error, the CS6's excess phase is better than ±10 degrees from 200Hz to 20kHz, which is excellent. To put this into context, the bottom curve in fig.8 is the excess phase response of the B&W Silver Signature, which is typical for a speaker with nonaligned drive-units and a high-order crossover.

Fig.8 Thiel CS6, excess phase response on tweeter axis at 50" (top); B&W Silver Signature, excess phase response on tweeter axis at 50" (bottom). (90 degrees/vertical div.)

The CS6's cumulative spectral-decay plot on the tweeter axis (fig.9) confirms the existence of the interference-related suckout on this axis, but is otherwise very clean through the low and mid-treble. There appears to be some hash present in the high treble, but this might actually be due to early reflections of the tweeter's output from the edges of the midrange cone. I noticed no HF graininess or grittiness during my auditioning.

Fig.9 Thiel CS6, cumulative spectral-decay plot on HF axis at 50" (0.15ms risetime).

Finally, I commented earlier that the CS6's cabinet seemed rigidly constructed. This was confirmed by looking at the output of a simple plastic-tape accelerometer attached to the various surfaces. The only modes present are at high frequencies —535Hz and 750Hz—considering the size of the panels involved, and although I could hear them with a stethoscope, their subjective effect was minimal. Interestingly, the concrete baffle produced quite a large impulse response with this test, but its frequency content was balanced, with no resonant modes apparent except at very low levels.—John Atkinson