Using a multimeter as a gaussmeter

[J M Fahey]

In general analysis, yes.

But here we have a section of the whole magnetic circuit, "section" being the dictionary definition, , as in "cutting across and looking at what we find".
Since radially cutting the magnetic circuit in any angle will yeald the same result, any section tells the whole story.

[Helmholtz]

The illustration of the section shows a moderate flux line density in the the center pole piece (less than in the airgap). This is not in line with the high B values in the center which can only be explained by a 360° integration.

[Joe Gwinn]

Remember itīs VERY VERY hard and can only be worked with diamond grinding wheels

Diamond wheels will have very short lives if run at normal grinding speed, especially if dry. If you see sparks, the diamond wheel will soon wear out, because diamond (a form of carbon) dissolves in iron.

A better choice would be CBN (Cubic Boron Nitride), which can be used to turn or grind hard steel at astounding speeds. The chips fly off incandescent. Now this requires modern high-speed spindles, but one need not be that dramatic.

https://www.youtube.com/watch?v=-VzyESxaHwY

and

https://www.youtube.com/watch?v=nPVr3LiJQ8Y

And then there are ceramic cutting tools:

https://www.youtube.com/watch?v=c1K5QjR3hyU

The above is hard-steel machining, just to show that it can be done. One can also get CBN grinding wheels.

CBN tooling is expensive, but very effective where it is needed.

[Joe Gwinn]

Because results are completely different.
Sweeping a coil through the gap gives you the TOTAL flux, which you divide by gap surface to calculate AVERAGE flux density .

With the added uncertainty that sweep also passes through fringe areas and wasted flux (which may be as much as 50% of total) so:

a) in principle, value will be artificially higher than real.
Not good in a *measurement* instrument.

b) itīs an end to end average, you have no clue about flux at a given point.

c) while a small (say 1mm by 1 mm) Hall sensor will give you accurate results, measured "there and nowhere else" so you can, among other things, optimize voice coil length for maximum efficiency by mapping gap flux.

An approximation to that can be done using a single turn coil (for precision) mounted on a rig which allows *exact* 1 mm sweeps (easier said than done) so you again integrate and divide but in 1mm steps.

Reasonable on a Lab environment where you are writing a Paper or something, slow clumsy for everyday testing of production speakers, even if you take 1 in 100 aside for Quality Control.

What I do is "waste" time once doing the detailed mapping and once determined the strong/useful area scan just that in future tests sliding coil along a turned nylon cylinder with adjustable "stops".
But inserting a thin probe and watching a display certainly looks easier

I see. Thanks for the explanation.

It occurs to me that some kind of gradiometer may be useful here. This would consist of a voice coil having two very short coils next to one another, connected in series opposing. when drawn out of the magnet, the voltage versus position curve will tell the gradient of the magnetic field. This will allow one to tell the maximum field, and ignore the fringing fields. One can also use three coils, the flanking coils having half the turns of the central coil, wired 0-+-0.

[J M Fahey]

Yes, something like that is done to simplify/quicken day to day checking.

You still have to "map" the field end to end at least once because it is still strong a couple mm either side of the area defined by plate thickness so search coil must be somewhat longer.

Of course, making a voice coil longer than plate thickness soon meets diminishing returns, while weight increases steadily, so (like on everything else) itīs a compromise, personal choice.

No big deal in woofers because coil is longer than any useful flux area anyway, but Guitar speakers always sacrifice "Bass" to Sensitivity, any day of the week.

Unless you prefer one of those monstrosities called "Powered Guitar Cabinet" ... 600 to 1000W RMS to play a Club

Well, at least it costs only $299

https://www.sweetwater.com/store/det...guitar-cabinet

EDIT: rereading the fine print, not sure you can even play a Club on its own, itīs being offered as a personal monitor while sending Guitar to "Front Mix"

That said, those guys with 524 presets in their Simulators or 42 pedals in a bed mattress sized pedalboard will LOVE it.
Imagine!!!! Getting exact same sound from Bedroom to .... ugh!! .... Stage !!!!

[Joe Gwinn]

Yes, something like that is done to simplify/quicken day to day checking.

You still have to "map" the field end to end at least once because it is still strong a couple mm either side of the area defined by plate thickness so search coil must be somewhat longer.

This does not sound right. See next.

Of course, making a voice coil longer than plate thickness soon meets diminishing returns, while weight increases steadily, so (like on everything else) itīs a compromise, personal choice.

No big deal in woofers because coil is longer than any useful flux area anyway, but Guitar speakers always sacrifice "Bass" to Sensitivity, any day of the week.

If one has a lathe, one can make the gradiometer coils very short by using many turns of #44 wire wound into a pair of grooves cut into a VC tube made of plastic, with the wire shellaced into place. Or use bondable wire, and simply wet it with alcohol, or heat it with a hot air gun.

Unless you prefer one of those monstrosities called "Powered Guitar Cabinet" ... 600 to 1000W RMS to play a Club

My ears hurt already.

[J M Fahey]

If one has a lathe, one can make the gradiometer coils very short by using many turns of #44 wire wound into a pair of grooves cut into a VC tube made of plastic, with the wire shellaced into place. Or use bondable wire, and simply wet it with alcohol, or heat it with a hot air gun.

Oh, definitely.
More turns, more sensitivity, thatīs for sure.

But people in each trade tend to use the wire they handle every day; you mention guitar pickup type wire (so I imagine where youīre coming from ); a typical gauge covering many bases in VC design is much thicker, 0.20 to 0.16 mm tops so AWG32 to AWG34 .

Any thinner and it becomes hard to wind the **perfect** 2 layer coils needed, no bumps, misalignment, free space or superpositions allowed.

[Joe Gwinn]

Oh, definitely.
More turns, more sensitivity, thatīs for sure.

But people in each trade tend to use the wire they handle every day; you mention guitar pickup type wire (so I imagine where youīre coming from ); a typical gauge covering many bases in VC design is much thicker, 0.20 to 0.16 mm tops so AWG32 to AWG34 .

Any thinner and it becomes hard to wind the **perfect** 2 layer coils needed, no bumps, misalignment, free space or superpositions allowed.

If the gradiometer coils are thin enough (compared to diameter), perfect layer winding is not needed.

The metric of thin here is the diameter of the smallest circle into which the winding cross-section will fit, divided by the diameter of the voice coil. If one approximates the geometry of a single turn close enough, only turns count matters.

[J M Fahey]

in VC design is much thicker, 0.20 to 0.16 mm tops so AWG32 to AWG34 .
Any thinner and it becomes hard to wind the **perfect** 2 layer coils needed

I am talking perfect winding in voice coils where itīs essential.

Yes, a few turns of very thin wire used for measuring might be scrambled.

But since each speaker needs its special diameter coils anyway, rather than custom turning a special winding core in a lathe, common practice is to simply take a standard voice coil that fits, which by definition is available on the shop , and unwind it so only 1 or 2 mm worth of winding are left.

Since itīs 2 layers anyway and typical diameter is some 0.20 to 0.18mm, 10 to 20 turns are left, which is more than enough for good sensitivity.