Extended Test Bench
|Frequency response (at 2 meters)
T6 tower 42 Hz to 20 kHz ±4.5 dB
C5 center 59 Hz to 20 kHz ±3.6 dB
S5 surround 61 Hz to 20 kHz ±4.3 dB
Sensitivity (SPL at 1 meter with 2.83 volts pink noise)
Bass limits (lowest frequency and maximum SPL with limit of 10% distortion at 2 meters)
All of the curves in the frequency-response graph are weighted to reflect how sound arrives at a listener's ears with normal speaker placement. Measurements were made at 2 meters to ensure that full effects of cabinet diffraction and front panel reflections were included. The T6 was measured on the ground to take floor reflection into account; the Image C5 was measured on a 6-foot stand; and the S5 was measured on a 6-foot stand with a large piece of plywood behind it to simulate a wall. Response of woofers and ports was close-miked, summed, and spliced to the quasi-anechoic midrange/treble response.
All of these speakers show an admirably flat response-except that they all share a sharp upper treble peak centered at about 17.5 kHz. For most listeners, that peak will be barely if at all audible. The C5 center speaker measures best; below 13 kHz, its response is ±1.4 dB. It also shows no significant lobing off-axis, and the response is fairly consistent at angles of 0° to 30°. The T6 isn't quite so smooth, with its most audible dips occurring at 1.2 kHz and 6 kHz, and it also has a floor dip - a common anomaly with tower speakers - centered at about 240 Hz (the dip is not shown in this curve because the measurement splice occurred near that frequency). However, the T6's off-axis performance is about as good as it gets: below 7 kHz, the response curves are almost identical whether you're on axis or 45º off. The S5 surround speakers measures fairly flat on the axes of its tweeters, and its averaged response sums pretty flat for a bipolar surround speaker.
All of these measurements were taken without grilles. The grilles reduce treble response by -2 to -4 dB between about 6 kHz and 12 kHz.
Because the T6 tower speaker has substantial bass output, I measured it as a subwoofer. At our bass output measurement threshold of 10% THD at 2 meters, bass output maxes at 63 Hz, where it hits 111.4 dB, and remains solid down to 40 Hz at 100 dB. The C5 center and S5 surround both deliver plenty enough bass to work well at the industry-standard 80 Hz subwoofer crossover frequency.
The impedance curves for the Image series run a little low for a line that's intended for use with relatively inexpensive electronics. The T6 has a nominal impedance of 5 ohms, drops to a low of 2.7 ohms at 410 Hz, and runs below 4 ohms between 90 Hz and 840 Hz. The C5 and S5 are a little gentler, with a nominal 6 ohms impedance and each dropping below 4 ohms only in a narrow range centered around 220 Hz. Sensitivity with a 1-watt signal at 1 meter is good for all models, though: 89 dB for the T6, 88 dB for the C5, and 87 dB for the S5.
-- Brent Butterworth
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