Thiel CS1.5: Measurements

Sidebar 2: Measurements

Other than impedance, all acoustic measurements were made with the DRA Labs MLSSA system and a calibrated B&K 4006 microphone. To minimize reflections from the test setup, the measuring microphone is flush-mounted inside the end of a long tube. Reflections of the speaker's sound from the mike-stand hardware will be sufficiently delayed not to affect the measurement.

Fig.1 Thiel CS1.5, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

The Thiel is moderately sensitive at a calculated 86.5dB/W/m (B-weighted). Its impedance, however, averages 4 ohms, as can be seen from fig.1, which also reveals a drop to 3.3 ohms in the lower midrange. Note the moderate electrical phase angle, though. Low-powered tube amplifiers will probably be able to cope with the Thiel from their 4 ohm output taps. The tuning of the passive radiator is indicated by the minimum in the amplitude trace centered at 39Hz. This suggests quite good low-frequency extension for a small speaker.

Fig.2 Thiel CS1.5, anechoic response on tweeter axis at 54", averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield woofer and passive radiator responses plotted below 300Hz (top trace below 10Hz), as well as individual nearfield responses of woofer and passive radiator, also plotted below 300Hz.

This is confirmed by the nearfield measurements shown to the left of fig.2. The bandpass trace peaking just below 40Hz is the passive radiator's output, its level adjusted with respect to the woofer's in the ratio of their radiating diameters. The corresponding notch in the woofer's output is offset a little, at 31Hz. Adding the outputs of the woofer and radiator, taking phase into account, gives the top trace in fig.2. The speaker is basically flat down to 50Hz, with then a typical 24dB/octave reflex rolloff, reaching –6dB at 35Hz.

To the right of fig.2 is the CS1.5's quasi-anechoic response averaged across a 30° horizontal window on the tweeter axis. Superbly flat overall, it's broken only by a few small peaks and dips. The top octave seems to roll off a little prematurely, however, which is at odds with MK's auditioning comments.

Fig.3 Thiel CS1.5, horizontal response family at 54", 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.4 Thiel CS1.5, vertical response family at 54", normalized to response on tweeter axis, from back to front: differences in response 45°–5° above tweeter axis; reference response; differences in response 5°–45° below tweeter axis.

The Thiel's horizontal dispersion (fig.3) provides a possible answer to this anomaly, because the speaker doesn't narrow its high-frequency dispersion until way off-axis. The room's reverberant field will therefore be more energized in the top two octaves than is the case with, for example, the Swans Baton, which has a similar on-axis HF balance. Despite the wide horizontal radiation pattern, vertically (fig.4), suckouts develop in the Thiel's output if you sit much above or below the tweeter axis.

Fig.5 Thiel CS1.5, impulse response on tweeter axis at 54" (5ms time window, 30kHz bandwidth).

Fig.6 Thiel CS1.5, step response on tweeter axis at 54" (5ms time window, 30kHz bandwidth).

Fig.7 Thiel CS1.5, excess phase on tweeter axis at 54" (45°/vertical div.).

MK mentioned how much attention Thiel pays to time coherence: the CS1.5's impulse response (fig.5) has a classically time-coherent shape, broken up only by some ultrasonic ringing from the metal-dome tweeter. The step response (fig.6) also has a classically time-coherent shape, meaning that on the tweeter axis, all the frequencies arrive at the measuring microphone at the same time. This is also shown by the excess phase graph (fig.7). Virtually a straight line, it reveals the Thiel to be a true minimum-phase system.

Fig.8 Thiel CS1.5, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).

Finally, as well as the clean, uniform initial decay confirming the time-coherent behavior, the CS1.5's cumulative spectral-decay, or waterfall, plot (fig.8) shows a couple of low-level resonant modes in the mid-treble, exactly as MK predicted. I'm surprised he was bothered by this behavior, but I suspect that it is not generally masked by the music.

To sum up its measurements, the CS1.5 is another well-engineered, time-coherent, flat-responsed, neutral-balanced speaker design from Jim Thiel. As always with a Jim Thiel speaker, it demands a lot from the partnering amplifier. But in my opinion, having listened to all three speakers in this review, both at Muse Kastanovich's and in the Stereophile listening room, the CS1.5 is a lot of loudspeaker at an affordable price.—John Atkinson

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