Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Thiel TT1's frequency response in the farfield; and, for the nearfield frequency responses, an Earthworks QTC-40, which has a ¼" capsule and thus doesn't present a significant obstacle to the sound.
My estimate of the Thiel's voltage sensitivity was 88dB(B)/2.83V/m, confirming the specified figure. The speaker's electrical impedance magnitude and phase are shown in fig.1. Like earlier Thiel models, the TT1 is a fairly demanding load for the partnering amplifier to drive, with minimum impedance magnitudes of 3.2 ohms at 140Hz and 3.1 ohms at 1.4kHz (solid trace). These values are a little lower than the specified minimum of 3.7 ohms, though the electrical phase angle (dashed trace) remains low over much of the audioband. The wrinkle in both traces at 25kHz is presumably due to the first breakup mode of the tweeter's titanium dome, but the traces are otherwise free from the small discontinuities that would suggest the presence of enclosure panel resonances. Nevertheless, when I investigated the cabinet's vibrational behavior with a plastic-tape accelerometer, I did find resonances at 309 and 344Hz that were most pronounced on the curved sidewall level with the uppermost woofer (fig.2). Neither is very high in level, however, and I doubt they will have audible effects. Thomas J. Norton did find the speaker dead to the traditional knuckle-rap test.
In fig.3, the green trace above 350Hz shows the TT1's midrange and tweeter output in the farfield, and the blue trace above 350Hz the output of the woofers. Below 350Hz, the blue and green traces show the responses measured in the nearfield, as does the red trace, which shows the summed response of the ports on the rear panel. The acoustic crossover appears to be just below the specified 400Hz, and the woofers roll off smoothly above that frequency with that appears to be close to an 18dB/octave slope, suppressing any breakup modes in the aluminum cones. What would otherwise be a smooth high-pass rolloff for the midrange unit (green trace) is disturbed by a small peak at about 150Hz. The ports and the woofers behaved identically, and the well-defined notch at 34Hz in the woofers' output (blue trace) suggests that this is the ports' tuning frequency. However, the ports' output (red trace) covers a wider bandpass than usual, not rolling off until above 65Hz. Though both ports had a peak in their output at 600Hz, this is well down in level and will not affect the TT1's sound quality.
Fig.1 Thiel TT1, electrical impedance (solid) and phase (dashed) (5 ohms/vertical div.).
Fig.2 Thiel TT1, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of side panel level with upper woofer (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
Fig.3 Thiel TT1, acoustic crossover on tweeter axis, corrected for microphone response, with summed nearfield responses of ports (red), woofers (blue), and midrange unit (green), all plotted in the ratios of their radiating diameters.
Fig.4 shows the complex sum of the TT1's port, woofer, and midrange outputs, spliced at 300Hz to the farfield response and averaged across a 30° horizontal window centered on the tweeter axis. The gentle rise in the low-frequency response is almost entirely due to the nearfield measurement technique, though the unusual tuning of the ports means that the bass rolloff doesn't extend quite as low as implied by the 34Hz port-tuning frequency. TJN did comment on the TT1's lack of low bass. Higher in frequency, the farfield response is extraordinarily flat, meeting ±1dB limits from 400Hz to 11kHz. Wow!
Fig.4 Thiel TT1, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield responses plotted below 300Hz.
The Thiel's lateral dispersion (fig.5) is also textbook in nature: The contour lines are evenly spaced; the rolloff with increasing frequency is apparently gentle and smooth; and there are no off-axis flares or suckouts up to the cursor position at 12.25kHz. As TJN found, this behavior correlates with stable, well-defined stereo imaging. In the vertical plane (fig.6), the TT1's tonal balance is maintained over quite a wide window above and below the tweeter axis, which is 36" from the floor.
Fig.5 Thiel TT1, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.
Fig.6 Thiel TT1, 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.
In the time domain, the Thiel's step response on the tweeter axis (fig.7) reveals that the tweeter and midrange unit are connected in inverted acoustic polarity, the woofers in positive polarity. While the TT1's output is not time-coincident, which differentiates it from the late Jim Thiel's designs, it is time-coherent, in that the decay of each drive-unit's step smoothly blends into the start of the step of the unit next lower in frequency. This confirms the optimally realized, high-order crossover design. Finally, the TT1's cumulative spectral-decay plot on the tweeter axis (fig.8) reveals a superbly clean initial decay.
Fig.7 Thiel TT1, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.8 Thiel TT1, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
I was impressed by the Thiel TT1's measured performance. In most ways it resembles what I would expect from a speaker designed by Mark Mason, an engineer who worked with Paul Barton at PSB. It is a shame that Mason is no longer with Thiel, but his legacy remains in the sound and the measured performance of this excellent speaker.—John Atkinson
