Thiel CS.5 loudspeaker Measurements
Unusually, I found the CS.5 to be a little more sensitive than its specification suggests, at about 89dB/2.83V/m (B-weighted). Its impedance, however, is, as with all Thiels, quite demanding, as can be seen by taking a gander at fig.1. Remaining below 6 ohms pretty much throughout the bass and midrange, it drops to a minimum value of 3.3 ohms at 200Hz. It does stay higher in the treble, however, and the electrical phase angle (dotted trace in fig.1) is generally lowish. But "8 ohm-rated" A/V receivers should still be shunned by Thiel owners if "cheap-amplifier meltdown syndrome" is to be avoided.
Fig.1 Thiel CS.5, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
The small wrinkle in the magnitude trace at 26kHz reveals the presence of the tweeter's ultrasonic "oil-can" resonance. Other than that, there are no discontinuities or wrinkles in the trace, implying a well-constructed, relatively inert cabinet, as LB found in his auditioning.
The "saddle" centered on 40Hz in fig.1's magnitude trace indicates the port tuning frequency, something confirmed by the individual port and woofer nearfield curves to the left of fig.2. (Their levels are weighted in the ratio of their radiating diameters.) The woofer's minimum-motion point can be seen to lie at 39Hz, though the maximum port output actually occurs nearly 20Hz higher in frequency. The result is an elevated upper-bass region, seen as the top trace to the left of fig.2, which is the complex sum (amplitude and phase) of the woofer and port outputs. If mild, this kind of bass response can lead to a feeling of powerful low frequencies. If extreme, however, it endows a speaker with "one-note bass." I note that LB found the CS.5's bass to err somewhat on the excessive side, lacking a little pitch differentiation. However, the upside of this is that you get bass that extends down to 33Hz—a low -6dB point for the size of the cabinet.
Fig.2 Thiel CS.5, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with nearfield woofer and port responses and their complex sum plotted below 300Hz, 650Hz, and 300Hz, respectively.
To the right of fig.2 is shown the Thiel's quasi-anechoic response on the tweeter axis, averaged across a 30 degrees lateral window. Relatively smooth, it features a broad depression in the low treble, very similar to the balance of the classic LS3/5a. The slight excess of mid-treble energy is anchored by the exaggerated lower mids and upper bass to give a subjectively pleasing balance. Treble detail will be well-presented, if not quite to the point of excessive brightness. The sharp spike at 26kHz is the tweeter resonance mentioned earlier. It will not be audible except to infants and dogs, neither of whom have sufficient disposable income to afford the Thiels. If you listen with the grille removed, the CS.5's top two octaves are boosted, which will make the sound too bright. Leave the grille in place, as it optimizes the speaker's dispersion.
I was concerned that the HF axis would be too low for comfortable listening. Fig.3, which shows how the Thiel's response changes with height—just the changes with respect to the tweeter-axis response are shown—reveals that the sound doesn't change appreciably as long as you sit with your ears somewhere between the tweeter and just above the top of the cabinet. This is still quite a low listening height, however, at a maximum of about 33" from the floor. Fig.3 shows that at listening heights much above that, a deep suckout appears in the crossover region, which will make the CS.5 sound very hollow. Sit below the tweeter and the response actually is more flat, but that is unrealistically low.
Fig.3 Thiel CS.5, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 20 degrees-5 degrees above tweeter axis; reference response; differences in response 5 degrees-10 degrees below tweeter axis.