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I need to mount a speaker next to a magnetic compass. Any suggestions?

I have a becker radio which comes with a traditional 4 ohm moving coil magnetic speaker. Due to space limitations I don't really have much choice about where to mount it, and it's going to be so close to a compass it's likely to interfere with it. So far my research has suggested that a piezo speaker (no magnet!) is NOT a like-for-like swap.

What are my options? I can't really change the design of the radio, is there a driver-circuit or converter that will allow a piezo speaker to be driven off a radio designed for a moving coil magnetic speaker?

My only other option is embark on physically moving the speaker, which (because it's in an aircraft) will require paperwork (a far more daunting challenge than anything engineering!)

What do others do? - the aircraft is a glider, and most glider pilots use GPS to navigate.

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  • \$\begingroup\$ Check first and see if it interferes with the compass. Some (many?) speakers have shielded magnets. \$\endgroup\$ – JRE Mar 13 at 12:21
  • \$\begingroup\$ Shielding the speaker is liable to be the best choice - if the existing shielding is in fact inadequate. If the speaker does have some effect it may be able to be 'swung' as is done traditionally with ships to correct for compass bias - but this may run you afoul of the dread paperwork. I'd assume that this problem is already extremely well known and dealt with by others. What do others do? You could indeed produce a magnetic drive to piezo-sound-better unit, and they probably exist, but trying the abve first may well be enough. \$\endgroup\$ – Russell McMahon Mar 13 at 13:19
  • \$\begingroup\$ One becker radio specifies 30cm between compass and speaker: soargbsc.com/Becker3201.pdf \$\endgroup\$ – Scott Seidman Mar 13 at 13:24
  • \$\begingroup\$ Just in general, keep in mind that your load is changing from an inductive load to a capacitive load. Your amplifier is going to need to compensate for that. \$\endgroup\$ – yhyrcanus Mar 13 at 14:59
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Shielding the speaker is liable to be the best choice - if the existing shielding is in fact inadequate. Mu metal works magic but iron sheet may work well enough.

If the speaker does have some effect it may be able to be 'swung' as is done traditionally with ships to correct for compass bias - but this may run you afoul of the dread paperwork. Compass swinging is already a part of regulatory requirements for many aircraft (maybe not yours?).
This page may help -
https://www.aviationpros.com/aircraft/article/10385554/aircraft-compass-swing

NZ regulatory requirements and methods

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I'd assume that this problem is already extremely well known and dealt with by others. What do others do?

You could indeed produce a magnetic drive to piezo-sound-better unit, and they probably exist, but trying the above first may well be enough.

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    \$\begingroup\$ Won't iron sheet or mu metal distort a compass all by itself? \$\endgroup\$ – Scott Seidman Mar 13 at 13:26
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    \$\begingroup\$ @ScottSeidman Yes, but. It's a matter of distance and field strength. Shielding a nearby strong magnet with a shield in close proximity to the magnet will have far more affect on the field than the shield will have on a large but distant weaker field (read VERY large and VERY distant here). Also, mumetal performs tasks indistinguishable from magic [tm] in that its effects diminish under very strong or very weak fields "Go figure" :-). Ordinary high permeability iron is less magic but still benefits from the square law reduction with distance affect, | \$\endgroup\$ – Russell McMahon Mar 14 at 12:49
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    \$\begingroup\$ I'm not totally comfortable with that answer BUT I think it's what you will see :-). Also see: wiki2.org/en/Mu-metal \$\endgroup\$ – Russell McMahon Mar 14 at 12:49

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