Peakoverload
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Are Speaker Magnets A Problem For Pi?

Fri Aug 16, 2019 6:03 pm

I'm going to be putting a Pi into an old radio and it looks like I may have to mount the Pi directly behind the speaker. Is the magnet in the speaker likely to cause damage to the Pi? It's a 6.5" full range driver. If it is likely to do damage, does anyone know roughly how far away it would need to be mounted? Space is somewhat limited.

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rpdom
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Re: Are Speaker Magnets A Problem For Pi?

Fri Aug 16, 2019 6:09 pm

Magnets should have no noticeable effect on the Pi.

drgeoff
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Re: Are Speaker Magnets A Problem For Pi?

Fri Aug 16, 2019 6:15 pm

Very unlikely but in theory a super strong magnetic field could saturate the transfomers in the ethernet jack and cause loss of ethernet connectivity.

W. H. Heydt
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Re: Are Speaker Magnets A Problem For Pi?

Fri Aug 16, 2019 8:07 pm

If you discover a problem, mount a sheet of steel between the speaker and the Pi.

bjtheone
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Re: Are Speaker Magnets A Problem For Pi?

Fri Aug 16, 2019 9:17 pm

W. H. Heydt wrote:
Fri Aug 16, 2019 8:07 pm
If you discover a problem, mount a sheet of steel between the speaker and the Pi.
Magnetic fields are tricky to shield. A simple sheet will just cause the field to wrap around the sides. This is based on spending much money and engineering effort to try and shield CRT monitors from a large transformer. Having said that I doubt it will actually be an issue.

W. H. Heydt
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Re: Are Speaker Magnets A Problem For Pi?

Fri Aug 16, 2019 10:15 pm

bjtheone wrote:
Fri Aug 16, 2019 9:17 pm
W. H. Heydt wrote:
Fri Aug 16, 2019 8:07 pm
If you discover a problem, mount a sheet of steel between the speaker and the Pi.
Magnetic fields are tricky to shield. A simple sheet will just cause the field to wrap around the sides. This is based on spending much money and engineering effort to try and shield CRT monitors from a large transformer. Having said that I doubt it will actually be an issue.
I don't think it will be a problem either, but if it is, even partial shielding could make it good enough to get by. If it were a serious problem, the obvious solution would be a set of Helmholz coils. The advantage here is that the OP is dealing with a static magnetic field.

trejan
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Re: Are Speaker Magnets A Problem For Pi?

Fri Aug 16, 2019 10:31 pm

Similar to what drgeoff said but a very strong magnet could cause the inductor in the power supply circuitry to saturate. It'll cause the SMPS circuitry to brown out as it won't be able to supply any significant current. I doubt a speaker magnet will cause this to happen though. You usually need something very strong like neodymium and hold it close to the inductor.

Paul Hutch
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Re: Are Speaker Magnets A Problem For Pi?

Fri Aug 16, 2019 11:20 pm

bjtheone wrote:
Fri Aug 16, 2019 9:17 pm
W. H. Heydt wrote:
Fri Aug 16, 2019 8:07 pm
If you discover a problem, mount a sheet of steel between the speaker and the Pi.
Magnetic fields are tricky to shield. A simple sheet will just cause the field to wrap around the sides. This is based on spending much money and engineering effort to try and shield CRT monitors from a large transformer. Having said that I doubt it will actually be an issue.
Mu-Metal is what you want to use for low frequency magnetic shielding.
http://www.mu-metal.com/

W. H. Heydt
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Re: Are Speaker Magnets A Problem For Pi?

Fri Aug 16, 2019 11:35 pm

trejan wrote:
Fri Aug 16, 2019 10:31 pm
Similar to what drgeoff said but a very strong magnet could cause the inductor in the power supply circuitry to saturate. It'll cause the SMPS circuitry to brown out as it won't be able to supply any significant current. I doubt a speaker magnet will cause this to happen though. You usually need something very strong like neodymium and hold it close to the inductor.
Strongest magnetic field I ever encountered was 37.5Kgauss...but that was between the field poles for the LBL 188" cyclotron...

Peakoverload
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Re: Are Speaker Magnets A Problem For Pi?

Sat Aug 17, 2019 5:15 pm

Thanks all for the replies and reassurances. I shall give it a go and hope it all works as planned.

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Burngate
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Re: Are Speaker Magnets A Problem For Pi?

Sat Aug 17, 2019 6:31 pm

Just as an addendum, it's perhaps worth noting that the magnetic devices on board the Pi were designed to be as efficient as possible, which means they don't spray magnetic fields around if at all possible.
The corollary is that stray magnetic fields around them have little effect.

Speakers should be designed similarly, but are often not.
The magnetic field from its magnet should only pass through the speaker coil, but putting a screwdriver close to one shows that the designers weren't successful

trejan
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Re: Are Speaker Magnets A Problem For Pi?

Sat Aug 17, 2019 6:41 pm

Burngate wrote:
Sat Aug 17, 2019 6:31 pm
Speakers should be designed similarly, but are often not.
The magnetic field from its magnet should only pass through the speaker coil, but putting a screwdriver close to one shows that the designers weren't successful
Yeah. I doubt these will be shielded. One of the features for "computer speakers" is magnetic shielding. Not that useful now but it was an important feature when CRTs were common as you didn't want a wavy image from the speakers either side.

bjtheone
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Re: Are Speaker Magnets A Problem For Pi?

Sat Aug 17, 2019 7:16 pm

This was my issue. Some genius in facilities decided to move the pcb layout department to right over the main building transformer. This was back in the day of teeny tiny CRT monitors. Said layout group had very very expensive large Sony monitors. We (the engineering support group said it would be an issue) but what do we know. Fast forward to an entire department with nasty flicker in monitors they stared at 12+ hours a day. Rather than admit they messed up they brought in shielding experts and decided to install a giant metal plate under the carpet. Much moving and installing later it was proven that the field happily leaked around the edges, making the situation even worse for the people near the edges.

Our solution was to buy them $20K LCB monitors.

Heater
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Re: Are Speaker Magnets A Problem For Pi?

Sat Aug 17, 2019 7:47 pm

Those were the days. When was involved in getting graphics drivers integrated into a PCB CAD package I had a huge colour CRT monitor to test with. If you turned it around a bit on the desk the colour went all shades of green as it's orientation w.r.t. the Earth's magnetic field had changed.

trejan
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Re: Are Speaker Magnets A Problem For Pi?

Sat Aug 17, 2019 9:08 pm

I don't miss those days at all. Anything bigger than 17 inches was backbreaking to move and the "can't unsee it now you've noticed it" horizontal support wires on an aperture grille display were awful.

carbone
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Re: Are Speaker Magnets A Problem For Pi?

Sat Aug 17, 2019 9:26 pm

trejan wrote:
Sat Aug 17, 2019 9:08 pm
I don't miss those days at all. Anything bigger than 17 inches was backbreaking to move and the "can't unsee it now you've noticed it" horizontal support wires on an aperture grille display were awful.
Only a problem with cheap Sony Trinitron and out of patent copies. Quality monitors and TVs used a shadow mask.

trejan
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Re: Are Speaker Magnets A Problem For Pi?

Sat Aug 17, 2019 9:44 pm

carbone wrote:
Sat Aug 17, 2019 9:26 pm
Only a problem with cheap Sony Trinitron and out of patent copies. Quality monitors and TVs used a shadow mask.
From what I remember, it was the other way around. High end monitors were mostly aperture grille. These were monitors for graphics/video work though so it might be different for other areas.

carbone
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Re: Are Speaker Magnets A Problem For Pi?

Sat Aug 17, 2019 10:26 pm

trejan wrote:
Sat Aug 17, 2019 9:44 pm
carbone wrote:
Sat Aug 17, 2019 9:26 pm
Only a problem with cheap Sony Trinitron and out of patent copies. Quality monitors and TVs used a shadow mask.
From what I remember, it was the other way around. High end monitors were mostly aperture grille. These were monitors for graphics/video work though so it might be different for other areas.
I worked in field service in CAD Unix workstations and Sony Trinitron were entry level (because of the lines you describe ). Philips PIL or crts produced under their licensed were high level, they used an in line gun with vertical pixels but a shadow mask rather than a grill.

Paul Hutch
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Re: Are Speaker Magnets A Problem For Pi?

Sun Aug 18, 2019 12:46 pm

Burngate wrote:
Sat Aug 17, 2019 6:31 pm
Just as an addendum, it's perhaps worth noting that the magnetic devices on board the Pi were designed to be as efficient as possible, which means they don't spray magnetic fields around if at all possible.
The corollary is that stray magnetic fields around them have little effect.

Speakers should be designed similarly, but are often not.
The magnetic field from its magnet should only pass through the speaker coil, but putting a screwdriver close to one shows that the designers weren't successful
You are making an apples and oranges comparison. The magnetics in ethernet jacks are transformers which are very weak AC electromagnets which you can relatively easily and inexpensively be designed and manufactured to limit stray fields. A loudspeaker uses a powerful DC permanent magnet where field containment can be very difficult and expensive. Since the vast majority of loudspeakers are used in applications where the stray magnetic field is of no consequence, most have had zero engineering or manufacturing costs added to limit the stray field.

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