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Upcoming Seminar:
"Loudspeaker
Measurement"
Lone Star Audio Fest
May 2-4, Dallas TX
Details
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Compliance and Break In
Compliance non-linearity is a mechanism that is measurable with the Woofer Tester using the
"Compression & Cms Linearity" test in the options pulldown menu. The test requires a
complete precision low power TS test that will later be used as a baseline to begin.
The baseline measurement establishes the drivers moving mass, and a few other constants
that are then used in subsequent tests. For example, mass must be constant, therefore
Fs variation with drive level must be related to changes in the suspension. This process
is then used at a number of test levels resulting in a graph like the one shown below. Note
here the number of parameters that can be plotted against each other.
Analysis
Fs in the example below varies between a maximum of 84hz at uA drive levels to a minimum of 58hz at what would be considered the drivers usable Xmax. At this point Fs begins to climb steeply as the suspension bottoms out. Not surprisingly, different test set ups and methodologies will yield different results if the same test conditions are not used. There is also a slight kink at the WT2 and WTP test boundaries. This is mainly caused by the drivers suspension and voice coil 'settling in' as the power was decreased from the initial very high drive level.
Example graphs for 4" Mid Bass - XY Plot of Fs versus Idrive (Click Images to Enlarge)
Impedance Plots for three Fs/Q Sweeps
Conclusions
The data clearly shows Fs increases as Idrive is decreased from 278mA to 200uA (currents >3mA
are measured with the Woofer Tester Pro). Mechanically speaking the effect is that of
'stiction' where it takes a finite amount of energy to break free and is mainly caused by the
heat cured resins used to stiffen this spider. The outer suspension can also be highly
non-linear
Test Methodology Details
The data taken above uses a continuous sine wave output to excite the driver at
each test frequency. Though slow, this allows the woofer tester hardware and
software to maximize the amount of energy at one particular frequency and reject
signals that are not of interest. The result is a fairly accurate, noise free
and repeatable measurement even when Idrive is low. An alternative method would
be to use a broad band 'Step' or 'MLS' drive signal and then use an FFT to
extract impedance and phase for a broad range of frequencies. Step and MLS
excitation are however limited by the fact that the energy at any one particular
frequency are not concentrated leading to a low signal to noise ratio. A further
limitation is that FFT frequency resolution is proportional to FFT size. This
again either leads to poor frequency resolution or even more signal degradation.
Note: The Step/MLS signal with FFT method is not without its merits and can be quite
useful as a 'quick' production test. If this would be useful to you, please contact
Smith & Larson Audio for more information
Automated Cms Non-Linearity Test
If precision baseline data on a driver is known, this and a precision impedance and
phase test can be used with the appropriate math to compute a full Thiele-Small
parameter list for any drive level. The Cms-Vs-Idrive compression test is
found in the Options pulldown menu when the TS Control window is active. The
test itself can take some time to complete, but the results speak for themselves. The following table is a compilation of results using the low power 'WT2' port and the
new high power test port of the Woofer Tester Pro.
The following extended range data is from a 4" mid-bass covering the full range of
drive levels available with the Woofer Tester 2 and Woofer Tester Professional (WT-Pro)
Note: Some values like Xmm are calculated, but do correlate well with actual values.
;------------------------------------------------------------------------------------------------;
; Power Compression Test: Date:Wed Feb 07 19:38:24 2007
;------------------------------------------------------------------------------------------------;
; These constants are derived from the baseline low level test
; Revc = 5.9578 ohms
; Mms = 4.4155 grams
; Diam = 82.5500 mm
; Sd = 5352.0971 m^2
;------------------------------------------------------------------------------------------------------------------------------------;
; WT-Pro High power mode ;
; Idrive Fs Qes Qms Qts Vas BL Cms Kms Rms PctEff Zmax Watts Xmm ;
;------------------------------------------------------------------------------------------------------------------------------------;
323.260mA 72.075 0.7082 2.6557 0.5591 4.494 4.1016 1104.2989 905.552 0.7530 0.2232 28.3000 14.047E+00 5.466E+00
223.151mA 64.959 0.5767 2.5501 0.4704 5.532 4.3147 1359.5056 735.562 0.7067 0.2470 32.2999 8.720E+00 4.699E+00
152.412mA 60.551 0.5011 2.6315 0.4209 6.367 4.4692 1564.6610 639.116 0.6384 0.2651 37.2465 5.409E+00 3.952E+00
106.730mA 58.646 0.4679 2.8181 0.4013 6.787 4.5517 1667.9518 599.538 0.5773 0.2749 41.8428 3.348E+00 3.219E+00
78.623mA 58.249 0.4527 2.9380 0.3922 6.880 4.6118 1690.7919 591.439 0.5500 0.2822 44.6246 2.066E+00 2.540E+00
60.769mA 58.600 0.4485 2.9642 0.3896 6.798 4.6471 1670.6013 598.587 0.5485 0.2866 45.3323 1.274E+00 1.972E+00
48.131mA 59.362 0.4515 2.9510 0.3916 6.624 4.6619 1627.9412 614.273 0.5581 0.2884 44.8997 783.901E-03 1.520E+00
38.418mA 60.348 0.4555 2.9190 0.3940 6.410 4.6794 1575.1771 634.849 0.5736 0.2906 44.1333 482.523E-03 1.165E+00
30.785mA 61.493 0.4612 2.8829 0.3976 6.173 4.6943 1517.0696 659.166 0.5918 0.2924 43.1956 296.798E-03 890.970E-03
24.585mA 62.805 0.4714 2.8852 0.4052 5.918 4.6929 1454.3596 687.588 0.6039 0.2923 42.4241 182.595E-03 682.382E-03
19.671mA 64.368 0.4823 2.8840 0.4132 5.634 4.6968 1384.5839 722.239 0.6192 0.2927 41.5835 112.304E-03 519.925E-03
15.649mA 66.045 0.4983 2.9315 0.4259 5.352 4.6804 1315.1634 760.362 0.6251 0.2907 41.0044 69.108E-03 397.902E-03
12.372mA 67.845 0.5168 3.0126 0.4411 5.072 4.6581 1246.3248 802.359 0.6248 0.2879 40.6850 42.525E-03 304.863E-03
9.724mA 69.855 0.5380 3.1284 0.4591 4.784 4.6325 1175.6329 850.606 0.6195 0.2848 40.5995 26.163E-03 233.421E-03
7.560mA 71.757 0.5576 3.2774 0.4765 4.534 4.6121 1114.1066 897.580 0.6074 0.2823 40.9771 16.110E-03 179.372E-03
5.813mA 73.712 0.5778 3.4788 0.4955 4.296 4.5918 1055.8002 947.149 0.5879 0.2798 41.8258 9.923E-03 138.123E-03
4.383mA 75.634 0.5982 3.7667 0.5162 4.081 4.5714 1002.8217 997.186 0.5571 0.2773 43.4706 6.094E-03 106.641E-03
3.346mA 77.069 0.6144 4.0063 0.5327 3.930 4.5535 965.8282 1035.381 0.5337 0.2752 44.8084 3.773E-03 83.065E-03
2.544mA 78.344 0.6256 4.2220 0.5449 3.803 4.5495 934.6455 1069.924 0.5148 0.2747 46.1628 2.314E-03 64.347E-03
1.943mA 79.456 0.6402 4.4416 0.5595 3.698 4.5294 908.6650 1100.516 0.4963 0.2722 47.2936 1.418E-03 50.064E-03
;------------------------------------------------------------------------------------------------------------------------------------;
; WT Low power mode ;
; Idrive Fs Qes Qms Qts Vas BL Cms Kms Rms PctEff Zmax Watts Xmm ;
;------------------------------------------------------------------------------------------------------------------------------------;
2.990mA 77.656 0.6156 4.0440 0.5343 3.871 4.5662 951.2922 1051.202 0.5327 0.2767 45.0945 3.051E-03 74.000E-03
2.346mA 78.667 0.6265 4.2186 0.5455 3.772 4.5557 927.0002 1078.748 0.5173 0.2754 46.0743 1.961E-03 58.893E-03
1.841mA 79.598 0.6354 4.3875 0.5550 3.684 4.5505 905.4224 1104.457 0.5033 0.2748 47.0977 1.262E-03 46.894E-03
1.445mA 80.448 0.6442 4.5567 0.5644 3.607 4.5433 886.3942 1128.166 0.4898 0.2739 48.1000 810.412E-06 37.358E-03
1.134mA 81.027 0.6509 4.6863 0.5715 3.556 4.5362 873.7841 1144.447 0.4797 0.2731 48.8540 514.829E-06 29.676E-03
889.595uA 81.609 0.6572 4.7995 0.5780 3.505 4.5305 861.3533 1160.964 0.4717 0.2724 49.4667 325.033E-06 23.481E-03
698.117uA 82.101 0.6630 4.8876 0.5838 3.463 4.5242 851.0653 1174.998 0.4660 0.2716 49.8789 203.520E-06 18.516E-03
547.836uA 82.500 0.6664 4.9790 0.5878 3.430 4.5235 842.8493 1186.452 0.4597 0.2715 50.4696 128.315E-06 14.657E-03
429.897uA 82.805 0.6698 5.0627 0.5915 3.405 4.5204 836.6540 1195.237 0.4538 0.2712 50.9888 80.649E-06 11.602E-03
337.348uA 83.111 0.6731 5.1070 0.5948 3.380 4.5175 830.5044 1204.088 0.4515 0.2708 51.1573 49.991E-06 9.110E-03
264.735uA 83.322 0.6760 5.1726 0.5979 3.362 4.5137 826.3109 1210.198 0.4469 0.2704 51.5460 31.256E-06 7.199E-03
207.745uA 83.436 0.6757 5.1631 0.5975 3.353 4.5177 824.0421 1213.530 0.4483 0.2708 51.4819 19.199E-06 5.628E-03
163.015uA 83.551 0.6791 5.2016 0.6007 3.344 4.5094 821.7794 1216.872 0.4456 0.2699 51.5890 11.871E-06 4.429E-03
127.917uA 83.569 0.6794 5.2031 0.6009 3.343 4.5090 821.4226 1217.400 0.4456 0.2698 51.5841 7.308E-06 3.474E-03
100.372uA 83.684 0.6789 5.1949 0.6004 3.333 4.5139 819.1672 1220.752 0.4469 0.2704 51.5480 4.493E-06 2.717E-03
78.754uA 83.702 0.6773 5.1794 0.5990 3.332 4.5196 818.8115 1221.282 0.4484 0.2711 51.5181 2.763E-06 2.128E-03
61.795uA 83.818 0.6795 5.2910 0.6022 3.323 4.5153 816.5632 1224.645 0.4395 0.2706 52.3468 1.756E-06 1.699E-03
48.489uA 83.836 0.6764 5.2222 0.5988 3.321 4.5263 816.2087 1225.177 0.4454 0.2719 51.9578 1.065E-06 1.319E-03
38.030uA 83.951 0.6789 5.0895 0.5990 3.312 4.5211 813.9676 1228.550 0.4576 0.2712 50.6232 622.106E-09 1.004E-03
29.837uA 84.059 0.6804 5.2415 0.6022 3.304 4.5189 811.8727 1231.720 0.4449 0.2710 51.8548 401.794E-09 808.715E-06
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