Thiel CS7.2 loudspeaker Measurements

Sidebar 3: Measurements

Like most Jim Thiel designs, the CS7.2 features quite a low impedance over most of the audio band (fig.1), with a magnitude dropping to below 2.7 ohms above 8kHz and ranging between 2.9 ohms and 4.2 ohms between 60Hz and 8kHz. The electrical phase angle is generally low, however, meaning that a good amplifier rated into 4 ohms will be able to drive the speaker. The minimum value at 24Hz in the magnitude trace in fig.1 reveals the tuning of the passive radiator, implying excellent low-frequency extension. My estimate of the Thiel's B-weighted voltage sensitivity was 85dB/2.83V/1m—slightly lower than specified, and ruling out the use of amplifiers of less than 100W, in my opinion.

Fig.1 Thiel CS7.2, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The impedance traces were free from resonance-caused wrinkles, and an investigation of the cabinet walls' vibrational behavior revealed just two resonant modes of any consequence (not shown). These should have only minimal subjective consequences; their relatively high frequencies, 350Hz and 535Hz, indicate a very well-braced cabinet, considering its size.

Assessing the amplitude response of a physically large loudspeaker featuring first-order crossover slopes is an enterprise fraught with difficulty due to the significant overlap between the drive-units, which gives rise to interference effects. Please bear this in mind when you examine fig.2, which shows the CS7.2's quasi-anechoic response averaged across a 30-degree horizontal window on the tweeter axis, spliced to the complex sum of the nearfield radiator, woofer, and midrange responses below 500Hz. The speaker's bass extends down to 25Hz, below which it rolls off quite steeply. There appears to be a lack of energy in the upper midrange, but this might be due to interference in the crossover region between the two midrange units at the relatively close (50") microphone distance I have to use.

Fig.2 Thiel CS7.2, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the nearfield midrange, woofer, and passive radiator responses plotted below 500Hz.

However, what are undoubtedly real aspects of this response are the narrow but quite intense peak just below 6kHz and the large peak at 21kHz. The latter is due to the tweeter dome's "oil-can" resonance and, despite its closeness to the audio band, should have no audible consequences. With program material possessing energy in the 6kHz region, however, the lower-frequency peak will be audible as a steeliness or a lack of mid-treble clarity. When I auditioned the speaker for myself, I was surprised at how unobtrusive this resonant problem was for much of the time—it is very narrow, meaning that it will be excited only if significant program energy exists between 5 and 6kHz. But Brian does talk about a touch of treble "hardness" that occasionally intruded; I imagine that this peak was the cause of this comment.

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