Thiel CS1.6 loudspeaker Measurements

Sidebar 3: Measurements

Thiel's literature goes into the efforts made by Jim Thiel to increase the CS1.6's sensitivity, and the speaker did indeed prove very sensitive, my estimate coming in 7dB above average, and 4dB above Thiel's own figure, at 94dB(B)/2.83V/m. This seemed unlikely, so I repeated the measurement, only to get the same result. However, the '1.6 does suck more than twice as much current as an 8 ohm speaker from the amplifier to achieve that high sensitivity, as revealed by its plot of impedance magnitude and electrical phase against frequency (fig.1). The phase angle is generally low—a good thing, in that it doesn't amplify the speaker's current demands—but the combination of 5.1 ohms magnitude and 48 degrees capacitive phase at 129Hz might stress an optimistically specified receiver or amplifier.

The saddle in the magnitude trace at 49Hz indicates the tuning frequency of the reflex slot, which in turn implies only moderate low-frequency extension. Unusually, there is no evidence of the tweeter's aluminum dome above 20kHz, but there is a peculiar step at 1.2kHz in both magnitude and phase traces, which indicates that there is some sort of resonant problem present at that frequency.

The fig.1 traces are also free from glitches in the midrange that would indicate the presence of cabinet resonant modes. However, listening to the CS1.6's cabinet with a stethoscope did reveal some high-level cabinet "talk" in the upper midrange. Fig.2 is a waterfall plot calculated from the output of a plastic-tape accelerometer fastened to the center of a sidewall. Some flexing at the reflex tuning frequency is apparent, but more bothersome are some high-level modes between 450Hz and 600Hz. These are even higher in frequency on the rear panel (not shown). In general, my experience has been that resonant modes this high in frequency tend not to be as subjectively bothersome as ones lower in frequency, where they will be excited more often and ring longer. However, I did wonder if this cabinet behavior was related to the midrange hardness at high playback levels that I noted in my auditioning.

Fig.1 Thiel CS1.6, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Fig.2 Thiel CS1.6, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet's side panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

To the right of fig.3 is shown the Thiel's farfield frequency response on its tweeter axis, averaged across a 30 degrees horizontal window, while on the left are shown the nearfield responses of the woofer and port and the complex sum of these two responses, taking acoustic phase into account. The usual notch in the woofer's output at the port tuning frequency can be seen at 49Hz, with the port's bandpass response peaking at the same frequency. On the face of things, these curves suggest reasonably good bass performance. However, the nearfield measurement technique does boost the apparent level of bass frequencies by 3dB, so it is probable that the CS1.6's low frequencies are actually shelved down a little, which is what I heard in my auditioning.

Fig.3 Thiel CS1.6, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the nearfield woofer and port responses and their complex sum plotted below 300Hz.

Higher in frequency, the Thiel's balance is basically flat, though some small peaks can be seen in the upper midrange, where I had noted some peakiness in my listening. A slight rising trend is also apparent in the mid-treble. The response falls off a little above 11kHz, though not enough in itself to make the top octave sound lacking in air, as I had found.

Fig.4 shows the CS1.6's lateral dispersion; ie, how the speaker's response changes as the measuring microphone moves to the side of the tweeter axis. The on-axis notch at 13.6kHz fills in to the sides, but while there is overall a little more off-axis energy above 11kHz than there is on-axis, this is not enough to compensate for the top-octave rolloff.

Fig.4 Thiel CS1.6, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90 degrees-5 degrees off-axis, reference response, differences in response 5 degrees-90 degrees off-axis.

Lower in frequency, the contour lines in this graph are evenly spaced, which correlates with the stable, well-defined stereo imaging I noted in my auditioning. However, note the depression in the off-axis traces between 1kHz and 3kHz. This is due to the woofer becoming directional at the top of its passband, by contrast with the tweeter, which radiates at full strength to the sides at the bottom of its passband. In all but very dead rooms, this off-axis flare in the presence region can make a speaker sound bright unless it is balanced by an on-axis response that is slightly dished in the same region.

A similar graph, showing the radiation pattern in the vertical plane (fig.5), reveals that a suckout appears at the crossover frequency if you sit higher than the tweeter, just 30" from the floor. It might be thought that this suckout could counteract the tweeter's lateral off-axis flare. However, it is a little low in frequency to do so optimally, and the top-octave energy rises somewhat above the tweeter axis.

Fig.5 Thiel CS1.6, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15 degrees-5 degrees above axis, reference response, differences in response 5 degrees-15 degrees below axis.

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