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Woofer Impedance (Cont.')

I would rate the sub a 5 Ohm unit even though factors, especially near the free air resonance frequency, of inductive and capacitive reactance are left out for simplicity. At frequencies nearing resonance, this value would easily exceed 6-8 Ohms and at frequencies nearing it's high frequency cutoff, the value would drop to under 4 Ohms. Overall, the sub was a fairly easy load to drive, unlike some units that dip under the 2-3 Ohm mark and draw gobs of current from already overtaxed transformers.

The Satellites

Like the subwoofer, the satellite specs are not specified by the manufacturer. As I couldn't open them, I couldn't verify any particular spec for comparison purposes.

Satellite Impedance
Frequency (Hz)
Voltage (mV)
Current (mA)
Impedance (Z)
315
369
43.0
8.58
1000
339
70.8
4.79
3150
330
64.4
5.12

Nearing their low frequency cutoff and port output range, the impedance soars well beyond the indicated 8.58 Ohms. The entirely too small midrange driver is so overtaxed, that a port centered at 170Hz had to be installed.

Frequency Response

The first piece I measured was the satellites. I was stunned that such a small driver had such poor high frequency extension. The HF rolloff wasn't something to severe, but it started far too early in the curve. At a mere 5kHz, the 2" drivers are being ineffectively utilized and will not exhibit anything over 10kHz, let alone come close to the 18kHz spec as claimed by PolkAudio.

Farther down the frequency scale, the same driver had a noticeable upper midrange peak in a very critical range. The 1-3dB boost will throw most voices abnormally forward of where the mixing engineer intended.

The bass was tested last and was the biggest disappointment. After consulting with PolkAudio directly, their response was to the effect of "most people want it that way." The sloppiness of the bass was the worst experienced at TargetPC. The "Q" factor, sometimes referred to as the system damping factor, was so high that it might as well have been off the scale. In order to reduce power requirements and cost, little damping is employed with the AMR90's. The frequency response shows a fairly flat, low extension though so PolkAudio's engineers have 100% utilized the underdamped characteristics.

Overall Frequency Response
Bass (rel. 100Hz)
Midrange (rel. 1kHz)
Highs
65-79Hz = +3-+6dB
1.8-3.1kHz = +1 to +3dB
5kHz = -3dB
58Hz = -3dB
 
Severe rolloff above 8kHz
56Hz = -6dB
   

Rated R.M.S. Power Output

Subwoofer measurements weren't possible in this case, but satellite measurements were. There is a rather large caveat though: 99% of AMR90 buyers won't be able to max out these speakers. The cause? Gross insensitivity. The manufacturer rates these as attaining full volume with 500mV: more like 1000mV! Since the vast majority of soundcards max out well before 500mV, you may only attain 25-35% of maximum output. In any case, my SB Live! maxes out at a relatively high 590mV and using those numbers satellite power was a minuscule 1.00 watts per channel (4 watts satellite total) at 1kHz. Just for the heck of it, I hooked the speakers to a frequency generator, boosted the input and achieved a much improved 2.41 watts per channel ouput (9.64 watts satellite total). Based on the more typical 500mV or less computer sound card output, I'd strongly suspect that the 5 driver output would be in the 8-12 watt range, which is far below the rated 30 watt level.

 





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