FAQ
 
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)
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                                   Impedance Plots for three Fs/Q Sweeps
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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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