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.
