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Thread: Using a multimeter as a gaussmeter

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    It strikes me that one can use a voice coil as the test coil, and you know how many turns you wound.
    Yes, see posts # 11,17, 26.

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    Quote Originally Posted by Helmholtz View Post
    Yes, see posts # 11,17, 26.
    You are right. So, I don't understand why the continuing angst about how best to measure the field in the gap.

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    You are right. So, I don't understand why the continuing angst about how best to measure the field in the gap.
    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

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    Out of curiosity: Are airgap flux densities in excess of 1 to 1.5 Tesla efficiently achievable using low carbon steel (1006 to 1010) pole piece material or would this require something like "pure" ARMCO iron?

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  5. #40
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    No need for special steels, low carbon rules.

    SAE1008 is standard, sometimes 1018 is used because thatīs all thatīs stocked at sellers.

    For polepieces 12L14 cold rolled round bar stock is popular because it contains minuscule lead droplets which help break swarf into small scales or pieces while soft malleable low carbon steel can produce looong (up to 5 meter) sharp edged coils which to boot clog machines or at least get in the way, big time.

    The slight magnetic properties loss is acceptable.

    Big problem is not permeability or saturation, but "impure" iron keeps *some* residual magnetism which is opposite polarity from main magnet and so is a guaranteed flux killer.

    Remember tools magnetize easily and Alnico is just tool quality Steel alloy which *happens* to keep residual magnetism more than others.

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

    back to question: yes, up to 15kG (1.5 Tesla) is achievable without much trouble, and up to 20kG being very careful, of course admitting some losses.

    Only acceptable in HF drivers; a 20kG woofer would be prohibitive.

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    Thanks.
    I just figured it would require more than doubling magnet weight to double airgap flux density using low carbon steel, as pole parts would be in "partial saturation" at least above 1 Tesla.

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  7. #42
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    Yes, you are going uphill and slope becomes steeper all the time.

    In this case "saturation" behaves different.

    Saturated iron does not "stop" magnetic flux, it simply has less and less permeability, it slowly stops being the "preferred path" compared to air or vacuum, it stops "focusing", waste increases, and flux is less focused where you need it and spreads outside.

    So to increase it a little you just increase magnet mass a lot.

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    Yeah, that's what I expected.

    The behaviour you describe is typical for beginning saturation. Flux cannot be stopped anyway as there is no magnetic insulator. It can be locally shielded or compensated but that doesn't mean stopping the flux of the magnet. Every flux line that leaves the north pole must re-enter at the south pole as flux lines are closed loops.

    Surprized that you seem to use alnico (remembering that you built a strong magnetizer for ceramic magnets). I figure that with alnico it is necessary to magnetize with the magnet mounted in its final assembly to avoid partial self-demagnetization of the free magnet.

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    Last edited by Helmholtz; 08-19-2019 at 03:18 PM.
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  9. #44
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    Oh, sorry, I only use Ceramics.

    I mentioned Alnico as an example , people think of it as "a magnet, period" while itīs actually more than that, a VERY HARD steel.

    Iīm adding a FEMA graph of magnetic flux intensity in a speaker magnet assembly; this one reaches very high levels ... but compare the huge magnet size to the tiny VC gap.

    It also shows magnetic lines going outside, what people experiences when sticking a screwdriver to a speaker back and which is actually 100% waste, and saturation *inside* the system.

    Surprisingly the bottleneck is not the gap itself but inside the pole piece




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    Surprisingly the bottleneck is not the gap itself but inside the pole piece
    Flux (lines') density must be highest where flux carrying cross section is lowest. But as it is a 3-dimensional problem, a 2-dimensional illustration may be misleading.

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  11. #46
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    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.

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    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.

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    Last edited by Helmholtz; 08-20-2019 at 02:22 PM.
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  13. #48
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    Quote Originally Posted by J M Fahey View Post
    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.

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    Quote Originally Posted by J M Fahey View Post
    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.

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  15. #50
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    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 !!!!

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    Last edited by J M Fahey; 08-20-2019 at 05:04 PM.
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  16. #51
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    Quote Originally Posted by J M Fahey View Post
    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.

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  17. #52
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    Quote Originally Posted by Joe Gwinn View Post
    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.

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    Quote Originally Posted by J M Fahey View Post
    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.

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  19. #54
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    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.

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