Below you will find free-air impedance plots of woofers that are taken with various voltage levels. The measurements are taken with Praxis loudspeaker measurement tool. A logarithmic chirp signal (sine sweep) of length 1sec is used. Sampling frequency was 48Khz. Praxis uses a voltage divider method for impedance measurements.
The list of the measurement curves and their corresponding voltage levels at the speaker terminals is*:
Yellow Curve: 0.120 Vrms
Orange Curve: 0.235 Vrms (+6db)
Cyan Curve: 0.470 Vrms (+6db)
Blue Curve: 0.930 Vrms (+6db)
Red Curve: 1.300 Vrms (+3db)
*These voltage levels are representative, the output of the amplifier
was hold constant for each measurement between drivers. But because voltage divider
method is used, the voltage at the driver terminals varied from driver to driver. Exact values
are listed below for each driver.
Besides the changes seen in Fs and Qts parameters, two things took my attention while taking these measurements.
One of them was the change of Le (voice coil inductance) of a driver could be
seen in the upper frequency ranges of these plots.
The other thing was that, with higher voltages applied, the shape of the impedance curve around resonant frequency neighborhood
was having anomolies. On some of drivers this was showing up as non-symetrical resonance peak curve, and on some
others there appeared to become new oscillations right after the resonance peak returns to normal level. This may be the
result of drivers not being secured well-enough while taking the measurements, but it was interesting to see that the results
were different for different drivers. I plan to replicate these measurements with securing the drivers in a different way and see what effects that'll have.
I also added partial T/S parameters extracted from each curve. Vas and Mms are not given for each case because I haven't done these
measurements with added mass for each voltage level.
Eton 8-800/37 Hex 8" Woofer:
Full Range
2kHz-20kHz Range focused:
Vrms
Qts
Qes
Qms
Fs
Vas
Mms
Fs/Qts Change wrt 0.25Vrms
0.125V
0.435
0.527
2.495
38.321
-
-
5.52%
0.25V
0.398
0.478
2.392
37.111
61.03lt
23.28g
0.0%
0.5V
0.366
0.445
2.055
35.252
-
-
-3.30%
0.99V
0.344
0.420
1.897
33.176
-
-
-3.43%
1.35V
0.359
0.436
2.046
32.458
-
-
3.04%
Peerless CSX-176 7" Woofer:
Full Range
2kHz-20kHz Range focused:
Vrms
Qts
Qes
Qms
Fs
Vas
Mms
Fs/Qts Change wrt 0.25Vrms
0.125V
0.467
0.557
2.921
55.535
-
-
1.18%
0.25V
0.452
0.542
2.711
54.395
-
-
0.0%
0.5V
0.432
0.522
2.485
51.825
-
-
0.31%
0.99V
0.406
0.493
2.293
49.028
-
-
-0.35%
1.35V
0.391
0.484
2.234
47.793
-
-
-1.57%
Scan-Speak 18W-8545 7" Woofer:
Full Range
2kHz-20kHz Range focused:
Vrms
Qts
Qes
Qms
Fs
Vas
Mms
Fs/Qts Change wrt 0.24Vrms
0.12V
0.345
0.396
2.659
31.500
-
-
-1.53%
0.24V
0.340
0.390
2.650
30.575
47.88lt
16.71g
0.0%
0.47V
0.353
0.403
2.836
30.083
-
-
5.23%
0.93V
0.383
0.430
3.454
29.993
-
-
12.92%
1.25V
0.409
0.453
4.177
30.006
-
-
18.42%
Scan-Speak 21W/8554 8" Kevlar Woofer: (Added on 9/25/2004)
T/S params of this driver are very stable with different voltage levels.
