measureing linear displacement with inductance meter does not work

I had the idea that it might be possible to measure the linear displacement of an unknown driver by hooking up the voice coil to an inductance meter and displacing the membrane statically. I thought that if one end of the coil left the gap, the inductance decreases.

Turns out the inductance increases for any inward movement, even if unrealistically far, and decreases for any outward movement. My interpretation is that this is due to the center pole piece filling more or less of the coil area or its stray field. The air gap itself does not seem to have much influence.


Your interpretation is good, you are right. This phenomenon causes high frequencies to be modulated by large displacement caused by low frequencies, which is clearly measurable with a suitable microphone. High quality drivers uses a copper ring on the center pole to cancel this inductance variation.

Regards, Pierre Lacombe.
Hi Eric

I can confirm your guess and Pierre's Statement. This is one usual solution to make tunable inductors for RF purposes (although not with solid iron cores).

Apart from copper -rings or -platings, a T - shaped polepiece will also reduce this effect a little (althogh primarily intended to reduce 2nd order distortion by increasing the SYMMETRY of the magnetic field).

The inductance modulation problem is one reason that would speak for driving loudspeakers by a current source rather than a voltage source (OTOH at the lower end you definitely need voltage drive unless you use MFB, sigh.......).



Good, good. You finally did the experiment and verified the issue for yourself. I am happy. :)

If you want to reduce the modulation of the voice-coil inductance along the stroke, rather than keeping the length of the center polepiece the same and making the top part into a "T', extending the length of the center polepiece usually works better (or you could combine the two to get a + shaped polepiece). The reason is that it keeps a decent amount of iron within the coil at all times (ie, even when the coil is moving outwards).

BTW, apologies for my last post in the "Barkhausen noise in drivers and inductors" thread.

I saw this "Inductance Meter" thread, assumed that it was initiated by someone else, and added a message to the "Barkhausen" thread. Didn't realise that in both cases, it was you.

I was posting at 6 in the morning, so my brain was half-asleep. Sorry!

regards, jonathan carr
More Experiments....

Good, good. You finally did the experiment and verified the issue for yourself. I am happy.

Yes I have done the experiment in the past of inverting the amplifier input and inverting the speaker connections at the same time (same acoustic AP) and observed a difference.
The motional modulation of the VC inductance can affect the behavior of the amplifier IME, but differently according to the polarity of the load.


Anybody tried removing the dust cap and fitting a cylindrical magnet to the end of the central polepiece ? - magnetised/magnetic phase plug ?
I have also seen an old speaker with a copper disc glued to the end of the central polepice but did not get to hear it - perhaps this is a cheap mod worth trying too.
Eric (mrfeedback & capslock)

I believe that adding a cylindrical magnet to extend the length of the center polepiece would have value in stabilizing the voice coil inductance along its stroke. I haven't tried this with a magnet, but I have extended the length of the iron comprising the center polepiece, and this definitely is beneficial.

Regarding the gluing of a copper disc to the end of the center polepiece, I am not so sure. I am fairly certain that you will get some reduction in harmonic distortion, but unless you can design and place a number of conductive caps and rings at strategic locations along the polepiece system, I have some doubts that you will see a similarly dramatic effect on the voice-coil inductance.

Before you proceed any further, I think you should first establish your general direction - is adding a conductive cap/ring going to increase or decrease the amount of iron that the voice coil thinks that it sees?

regards, jonathan carr