That's true only for DC magnet fields. At AC it depends on the conductivity of the material and the thickness of it.
Thick mu-metal + thick copper is extremely effective at blocking magnetic interference. Just don't saturate the mu-metal, though.
Hi,
I have a suggestion that will eliminate ones for all where is the problem. Take the speaker in your car drive away and see if the speaker still hum. If still hum then it is in the speaker. If does not hum then the problem has to be in your home. Simple test. Also try to download from Google play store the Ultimate EMF detector PRO application in your cellphone and use it to read how much electromagnetic field it is around your home.
I have a suggestion that will eliminate ones for all where is the problem. Take the speaker in your car drive away and see if the speaker still hum. If still hum then it is in the speaker. If does not hum then the problem has to be in your home. Simple test. Also try to download from Google play store the Ultimate EMF detector PRO application in your cellphone and use it to read how much electromagnetic field it is around your home.
No, that's exactly how they work. The VC magnet field tries to modulate the magnet flux which is a thing we don't want (it causes distortion). The shorting copper ring builds up an opposing field (from the induced eddy currents) that reduces the modulation (the same principle is found in transformers using a copper "belly band" to reduce stray field). VC field itself is not altered when the copper demodulation ring is not too close to the VC -- that's why it is usually placed down low in the gap, close to the back plate.
A copper ring/cap in the gap is different, it has less effect in reducing flux modulation but reduces VC inductance in that it forms a shorted secondary of an air core transformer.
Leads will not affect the situation if this is magnetic coupling.
A voltage induced across the coil from a magnetic field, with adequate energy to make sound from a speaker is a serious thing. Further:
- The meter / scope is high impedance and won't load it down.
- The hypothesis of magnetic coupling will indeed induce a voltage in the loop mage by the meter, but will be very (very) low impedance, and won't effect the hypothesised energy going to the speaker. (Do not mistake electrostatic coupling for magnetic - they are very different.)
I await the measurements with bated breath.
A voltage induced across the coil from a magnetic field, with adequate energy to make sound from a speaker is a serious thing. Further:
- The meter / scope is high impedance and won't load it down.
- The hypothesis of magnetic coupling will indeed induce a voltage in the loop mage by the meter, but will be very (very) low impedance, and won't effect the hypothesised energy going to the speaker. (Do not mistake electrostatic coupling for magnetic - they are very different.)
I await the measurements with bated breath.
Sufficiently thick copper or aluminum is a very effective shield for 50/60 Hz fields. I have built EMI shields for electron microscope rooms using 1/4" aluminum. It is more effective than iron or mu-metal in most situations. Large ferromagnetic shields tend to saturate by the earths magnetic field alone and therefore become ineffective. The best are multilayered using both ferromagnetic and eddy current shielding.
could it be that the magnetic fields [of the speaker magnet and "charged" coil] in their excited state create a forced resonance with each other? turning the coil 90 degrees or just removing it from the speaker box might help.
alternatively get a lead safe and put the coil in there
does the same hum happen when everything is disconnected and the coil is then wired to the driver (de-fielded coil, no amp in the picture)?
alternatively get a lead safe and put the coil in there
does the same hum happen when everything is disconnected and the coil is then wired to the driver (de-fielded coil, no amp in the picture)?
could it be that the magnetic fields [of the speaker magnet and "charged" coil] in their excited state create a forced resonance with each other? turning the coil 90 degrees or just removing it from the speaker box might help.
alternatively get a lead safe and put the coil in there
does the same hum happen when everything is disconnected and the coil is then wired to the driver (de-fielded coil, no amp in the picture)?
I'll try that. Waist deep in grading papers . Maybe later tonight.
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