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Using magnets to measure change in mass for T/S


DS-21

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Josh, I remember you posting somewhere (AVS perhaps) about using magnets on each side of the cone to measure Vas.

 

Very elegant, as it allows adding mass without risk of greasy stains on the cone from putty or whatever.

 

Alas, I can't find your post.

 

So, any recommendations for magnet size/strength/quantity? Also, have there been any cone materials for which you method did not work?

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I am using some ceramic magnets that I found at a hobby store that just happen to weigh 6 grams a piece and have a firm grip on each other through a heavy cone. They are about the size of a nickel and maybe 8 or 9mm thick. I had a bunch of small disc neo mags that I was going to use but I didn't have quite enough to come up with the mass needed for the biggest drivers and since the price had gone through the roof buying the rest was rather expensive. The other problem with neo is that the bond can be too strong making removal of them very difficult without damaging the cone especially with lighter cones or smaller drivers. You don't want too much strength and you need a decent amount of mass to it as well. 2 6 gram neo mags are going to be rather difficult to get apart. So you want a small to medium size with a decent amount of attraction to each other such that it will be strong enough to bond well through even the thickest cone materials but still easy to remove and because you want to try and distribute the weight around the cone somewhat evenly. Also try to get the mags as close to the cone center as feasible so as to prevent potential cone flexing from the inertia of the added mass during the test. 300-350g in total should be enough for even the biggest heaviest drivers. I've not had any trouble with any cone materials other than aluminum(Or other metals) which gets small scratches in it from the magnets being attached and detached. You can use some cheap cloth in order to prevent that. You will need a digital scale like they sell in pharmacies to weigh the magnets. Weigh a bunch at once and divide by the number of them to get a more accurate average weight. Don't forget to include the cloth or whatever if you are going to use it as well. Also I found that the magnetic field proximity actually seems to alter the digital scales reading ever so slightly so you may want a 3 or 4" object to distance the magnets somewhat from the scale electronics. You will need to weigh it and subtract it out. Use as much signal level as you can get away with running to take the measurements especially for sub drivers. Somewhere near 1V to 3V is a good range to be in but it can get loud and annoying at that level. Use the longest, slowest measurement that is feasible for better noise rejection and accuracy. Always take at least 2 or 3 measurements for averaging and consistency checking. Clamp the driver by the motor horizontally to some sort of heavy immobile object. This has an effect on the shape and magnitude of the impedance peak (Sharper, higher impedance) which in turn will affect the apparent BL product (Increased) and other associated parameter calculations (Lower qes, etc.). You may see some features in the impedance trace which are related to the resonance of the driver and object it is clamped to but these are usually obvious.

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Thanks for the detailed explanation! Looks like it's off to the hobby store for me, because I don't want to waste any more time cleaning silly putty or duct seal residue off cones. :)

 

Fortunately, I have a postal scale.

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  • 2 weeks later...

I am using some ceramic magnets that I found at a hobby store that just happen to weigh 6 grams a piece and have a firm grip on each other through a heavy cone. They are about the size of a nickel and maybe 8 or 9mm thick. I had a bunch of small disc neo mags that I was going to use but I didn't have quite enough to come up with the mass needed for the biggest drivers and since the price had gone through the roof buying the rest was rather expensive. The other problem with neo is that the bond can be too strong making removal of them very difficult without damaging the cone especially with lighter cones or smaller drivers. You don't want too much strength and you need a decent amount of mass to it as well. 2 6 gram neo mags are going to be rather difficult to get apart. So you want a small to medium size with a decent amount of attraction to each other such that it will be strong enough to bond well through even the thickest cone materials but still easy to remove and because you want to try and distribute the weight around the cone somewhat evenly. Also try to get the mags as close to the cone center as feasible so as to prevent potential cone flexing from the inertia of the added mass during the test. 300-350g in total should be enough for even the biggest heaviest drivers. I've not had any trouble with any cone materials other than aluminum(Or other metals) which gets small scratches in it from the magnets being attached and detached. You can use some cheap cloth in order to prevent that. You will need a digital scale like they sell in pharmacies to weigh the magnets. Weigh a bunch at once and divide by the number of them to get a more accurate average weight. Don't forget to include the cloth or whatever if you are going to use it as well. Also I found that the magnetic field proximity actually seems to alter the digital scales reading ever so slightly so you may want a 3 or 4" object to distance the magnets somewhat from the scale electronics. You will need to weigh it and subtract it out. Use as much signal level as you can get away with running to take the measurements especially for sub drivers. Somewhere near 1V to 3V is a good range to be in but it can get loud and annoying at that level. Use the longest, slowest measurement that is feasible for better noise rejection and accuracy. Always take at least 2 or 3 measurements for averaging and consistency checking. Clamp the driver by the motor horizontally to some sort of heavy immobile object. This has an effect on the shape and magnitude of the impedance peak (Sharper, higher impedance) which in turn will affect the apparent BL product (Increased) and other associated parameter calculations (Lower qes, etc.). You may see some features in the impedance trace which are related to the resonance of the driver and object it is clamped to but these are usually obvious.

 

Damnit josh makes paragraphs!

 

:P

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