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James Lehman
 
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"Nigel Thomson" wrote in message
...
I found a formula for calculating speaker inductance but its a little
unclear to me, is this right?

\ /---------------
\/ M * M - Re * Re / 2Pi
------------------------
frequency

so its the root of (M*M-Re*Re),
divided by (2*Pi),
divided by the frequency?

Where (M) is the impedance Magnitude.

It says, obtain (M) like this:

* Connect an amplifier and frequency generator set to 1000Hz and put a
10 ohm resistor on the output.

* Adjust the gain on the amplifier and frequency generator until you
get 10 V.A.C.


By doing these steps you have created a (somewhat) calibrated measuring
system. You know that 1KHz into a 10 ohm, purely resistive load will yield a
voltage reading of 10V.



* Remove the resistor and connect the speaker.


Replace a resistive load with a load that is mostly resistance, but some
reactance; the speaker.


* Set the frequency at the drivers highest usable frequency.


If you think of the voice coil as a simple coil, it should have a constant
value for its inductive reactance in Henries. Voice coils are not simple
coils. They are suspended in a strong magnetic fields and they move. But,
you are looking for a single target value for the inductive reactance of the
coil. So, why not take the measurement where it is likely to have more
effect on the whole system? That should be the top end of the portion of the
audio spectrum for which this device is going to be used. Try putting in the
frequency of the crossover point to the next driver.


* The voltmeter reading is the driver's impedance magnitude (M) for
this frequency.


That means that M is just a numerical factor, that is a voltage reading that
is used to derive the real value in Henries.


What is the "drivers highest usable frequency" anyway? ZMax?


No. Zmax is the maximum linear piston motion of the coil before it leaves
the magnetic influence of the pole plate and plug.

Its my understanding that the inductance should be obtained at 1000Hz,
is this what I want?


I don't think you will be able to measure the difference between 1000Hz and
5000Hz. Only 5000Hz will give you a different reading on the meter. Try it!


http://www.akrobiz.com/speakers/

~James. )


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