Thiel CS2.3 Loudspeaker Measurements

Sidebar 3: Measurements

The Thiel featured above-average sensitivity, at an estimated 90.5dB(B)/2.83V/m. However, like all Jim Thiel designs, it is quite a demanding load for an amplifier to drive. Its plot of impedance magnitude and phase (fig.1) reveals it to remain below 5 ohms throughout the midrange and treble, with a dip to 2 ohms at 450Hz. Note also the combination of low impedance and high capacitive phase angle in the upper bass—wimpy amplifiers stay home. The twin peaks in the bass reveal the ABR to be tuned to a low 34Hz, which implies good bass extension.

Fig.1 Thiel CS2.3, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

Fig.1 was free from resonance-induced ripples other than above 20kHz, where the tweeter's innocuous dome resonance makes its presence known. Using a simple plastic-tape accelerometer, I found the CS2.3's cabinet to be well braced, with only low-level vibrational modes present at 250Hz and 685Hz. Fig.2, for example, shows a waterfall plot calculated from the accelerometer's output when it was fastened to the center of the side panel. The 685Hz mode is highest in level, but this is at a high enough frequency that its subjective effect will probably be zero.

Fig.2 Thiel CS2.3, cumulative spectral-decay plot of accelerometer output fastened to center of cabinet sidewall. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

When it came to assessing the Thiel's frequency response, I must admit to some difficulty. To the left of fig.3 are shown the individual responses of the woofer, the ABR, and their complex sum (top curve). The woofer's minimum-motion point is slightly lower than the peak of the ABR's output, but the overall picture painted by this graph is of a speaker that offers good bass extension, as BD found. Higher in frequency, however, things look problematic. There is a lack of presence-region energy and a slight positive shelving of the range covered by the tweeter section of the coaxial drive-unit. Given BD's positive comments on the CS2.3's balance, I wondered what was going on.

Fig.3 Thiel CS2.3, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with nearfield woofer and ABR responses and their complex sum plotted below 300Hz.

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