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Easy Nickel Vas Method
The Easy Nickel method gets its name from the fact that United States Nickels, that happen to weigh 5.00 grams, have not changed
in weight since 1936. Furthermore, a resonably accurate TS model can be obtained by simply placing a driver on a firm surface
(but not blocking a vented pole piece) and placing a known mass on its surface.
Is The Easy Nickel Method Accurate?
Two controversies are often discussed. The first is that the up/down cone motion orientation allows the suspension to sag under
the influence of gravity. The second is that the nickels are merely 'stuck' to the cone by their own weight. Though convenient,
these two question should be addressed:
- Cms non linearity can be measured directly with one of the extended Woofer Tester tests. Newtons law of gravity F=M*A
is then used to compute the gravitational force acting on Mms. Finally, the spring force equation F=X/Cms allows the
computation of the displacement X. If Mms is extremely heavy, Cms is large or Cms is highly non linear, then the value
of Vas might be suspect. In this case the delta compliance 'sealed test box' method or delta mass using modeling clay
method should be used to calculate Vas.
Note:
Box compliance will dominate driver Cms in most sealed and vented box systems. The 'sealed box' setting in the simulator
will show this value as Kbox. Kbox is determined by Vbox and Sd and is the same for Sealed and Vented boxes.
- Since the Nickels are simply sitting on the cone they are free to move around and bounce. The usual problem is that the
tester's drive level is sufficiently high that the downward stroke accelerates downward faster than the force of gravity
can hold the mass to the surface. In this case, simply lower the drive level. Another possability (especially if a lot
of Nickels are required) is to glue or tape together stacks of Nickels. You will want to measure the final weight in
this case.
Is it Possible to Minimize the Effect (how much test mass is needed)
The industry accepted rule of thumb is that the test mass should be approximately 50% that of Mms and produce a 25% shift in Fs.
The assumption is that this will not depress the suspension too badly and yet provide enough Fs shift to make a reasonably
accurate measurement. The only other solution(s) are to mount the driver sideways and use a sticky clay mass (and maybe destroy
the cone) or to use the delta compliance box method.
The Woofer Tester Solution
The Woofer Tester is far more accurate than any typical sunset technology signal generator, amplifier and voltmeter
measurement setup. It is important to remember that when the LDCB was written, there was no woofer tester that had the kind of
accuracy that could even begin to show the effects of things like non-linear Cms compression.
Basically, this means the old general rule of thumb is probably far too conservative. The data below shows how the resulting
Thiele-Small model changes with a wide range of test masses. In our case, Cms stabilized with a test mass that was considerably
less than 1/2 that of the suggested test mass.
Credence 18" Fs&Q Baseline
----------------------------
Re 6.6430
Fms 17.7849
Qes 0.4448
Qms 10.6031
Qts 0.4269
Zmax 164.9975
Vas(L) BL(N/A) Cms(uM/N) Kms(N/m) Mms(g) TestMass(g)
----------------------------------------------------------------
347.2 25.95 198.45 5038.9 403.53 24.95
356.2 25.62 203.56 4912.3 393.39 50.06
357.8 25.56 204.49 4890.0 391.60 74.94
357.1 25.59 204.07 4900.1 392.41 100.0
357.1 25.58 204.09 4899.5 392.37 125.2
357.7 25.56 204.44 4891.5 391.72 150.4
358.0 25.55 204.60 4887.4 391.39 175.7
359.5 25.50 205.44 4867.4 389.80 201.0 <- sugg test
360.3 25.47 205.90 4856.7 388.93 226.3 <- mass here
360.7 25.46 206.15 4850.7 388.45 251.6
361.3 25.44 206.49 4842.6 387.80 277.1
361.8 25.42 206.76 4836.5 387.32 302.1
363.3 25.37 207.63 4816.2 385.69 327.1
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