Snell Type C/IV loudspeaker Measurements
Sidebar 3: Measurements
The LEDR test on the Chesky Test CD produced a fairly good "up" impression, but with the image turning inward toward the top of its extension. The "over" and "lateral" tests were reproduced quite well, with solid images moving between the loudspeakers. Driving the Type C/IV with a sinewave oscillator revealed cabinet resonances at 65Hz, 200Hz, 500Hz, and 660Hz, the last being the strongest.
Even in the worst case, with the HF control at maximum and the rear tweeter on, the Type C/IV presents a fairly easy load to a power amplifier, evinced by the impedance magnitude and phase plot of fig.1. Although the impedance dips narrowly to just over 4 ohms at about 100Hz, and more broadly between 4kHz and 10kHz, the impedance is much higher through most of the band. Most amplifiers should have no trouble driving the C/IV. Turning the HF control to its minimum setting gave the impedance plot shown in fig.2a benign load through the treble. The woofer and port tunings, evinced by the impedance peak at 35Hz and the minimum at 22Hz, are very low in frequency, accounting for the C/IV's excellent feeling of extension.
Fig.1 Snell C/IV, HF control maximum, rear tweeter on, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
Fig.2 Snell C/IV, HF control minimum, rear tweeter on, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
Fig.3 shows the C/IV's MLSSA-derived impulse response on the tweeter axis at 48", taken out of doors on a high stand to eliminate the floor reflection. The ultra-high-frequency ringing is typical of metal-dome tweeters, the general shape consistent with that of other loudspeakers using high-order crossover slopes. Fig.4 shows the step response.
Fig.3 Snell C/IV, impulse response on tweeter axis at 48" (5ms time window, 30kHz bandwidth).
Fig.4 Snell C/IV, step response on tweeter axis at 48" (5ms time window, 30kHz bandwidth).
Looking next at fig.5, the Type C/IV's frequency responseFFT-derived from the anechoic portion of the impulse response and averaged over a 30° lateral windowwe can see that the loudspeaker is very flat. There is, however, a very slight depression between 1kHz and 8kHz. In addition, a crossover-related dip around 3kHz is apparent, and the treble has a slightly rising response above 6kHz. These measurements were made with the grille on, the tweeter-level control at "Optimal." For comparison, fig.6 shows Snell's own measurement of the C/IV, averaged spatially across 30° lateral and vertical windows at the Canadian NRC facility and frequency-wise with a 1/5-octave window. Again, there is a suggestion of a broad but shallow depression between 1kHz and 8kHz or so.
Fig.5 Snell C/IV, anechoic response on tweeter axis at 48", averaged across a 30° lateral window, with nearfield woofer and port responses plotted below 300Hz and 500Hz, respectively.
To the left of fig.5 are shown the C/IV's nearfield woofer and port responses, measured with an Audio Precision System One. As can be seen, the port, its 23Hz tuning revealed by the LF minimum in the woofer's response, really extends the system's low-frequency output below the woofer's rolloff. Note the extreme flatness between 60Hz and 300Hz, but with a very slight peak between 70Hz and 100Hz. These measurements confirm the impressions of good LF extension and smoothness noted during the auditioning.
For comparison, Snell's measured frequency response of the C/IV in fig.6 also shows respectable LF extension, though this anechoic measurement reveals that the rear-mounted port's output doesn't quite match the woofer overall. (The matching of the separate woofer and port traces in fig.6 is only approximate.)
Fig.6 Snell C/IV, anechoic response on tweeter axis at 48", averaged across 30° lateral and verical windows, 1/5-octave smoothed (NRC measurement supplied by Snell).
Finally, the Type C/IV's "waterfall" plot is shown in fig.7. The decay is quite clean initially, but with a bit of "hash" through the midrange and lower treble, especially around 4kHz. The dark ridge at 16kHz is an artifact of the computer monitor's display.
Fig.7 Snell C/IV, cumulative spectral-decay plot on tweeter axis at 48" (0.15ms risetime).
Overall, the Type C/IV measured very well, with a strong correlation between measurements and listening impressions.Robert Harley