dynamic mics and speaker coils?

[andychamp]

Just curious if anyone has ever looked into the phenomenon and if there is any info about it out there...
Not worried about the implications for "correct" micing, just the physics involved.
When close-micing a guitar or bass speaker with a dynamic mic, it occured to me that the magnetic activity between the speaker coil and its magnet might have some influence on what's happening inside the mic capsule, considering the size difference between the two arrays...
Possibly even generate some sound that is NOT caused by the air movement between the two?

[winky dinglehoffer]

I'm no expert, but I think that you'd have to indulge in some pretty peculiar mic placements for there to be more than a negligible effect.

[digitaldrummer]

Ribbon mics have magnets. Ribbon mics have been around for a long time (longer than me). Speakers have magnets. Also been around a long time. I think as long as your not trying to mic the back of an open cabinet and getting really really close (and if so, why?) I think it's probably safe. I've certainly never experienced any adverse effect.

[Scodiddly]

At one point I was fooling around with sticking a tape head to the magnet in a guitar speaker. It did pick up sound but I guess it wasn't interesting enough to pursue any further. In general the real action happens deep inside the magnet, in the narrow gap where the voice coil lives.

[Nick Sevilla]

Well, to have an interaction between the magnets, basically only the speaker magnet would be the one attracting the microphone body. It would also likely stop or greatly slow down the voice coil inside the mic or the ribbon mic could be destroyed.
You could always try some experiments, knowing full well that the likelihood of the microphone getting damaged would be good.
YMMV.

[Scodiddly]

OK, so let's think about what's happening in a speaker.

Alternating current flows through the voice coil, generating a magnetic field. That field reacts with the constant field of the magnet, causing the voice coil to move - sometimes quite forcefully. So how much of a magnetic field would be radiated out from that combination? The field generated by the coil is localized inside the magnet, which itself is made of ferrous materials so that it can be a magnet as well. Likely there would be very little radiated energy from the voice coil, as it is both contained with a form of shielding and also has its own field largely neutralized by being able to move somewhat freely.

A dynamic mic has the same basic thing happening only in reverse.

[jimjazzdad]

A needle hanging from a piece of thread is a way to explore the magnetic field around your speakers. I was checking out my studio monitors (concerned about the proximity of my reel to reel), and there wasn't much happening until I was an inch or so away from the woofer voice coil. I could see there might be a bit of magnetic pull when you jam your 57 right up against your guitar amp speaker but, hey, its just a 57...who cares?

[frans_13]

I think it's like Scodiddly wrote - the magnetic field stays mostly where it is and you would have to get very, very close for it to make a noticeable contribution besides that of the energy transfer picked up by the membrane/coil of the mic. On top of that, some mics have a humbucking coil so they do not pick up stray fields, be it from a power transformer or something else close by.
Of course you can experiment and see what you get. As he also mentioned a pickup head from a tape recorder ...i have a naked guitar pickup with just a pice of cable hanging off it i can use to get signal from everything that vibrates.. or to whick i stick magnetic tape. I once tried it on snares .. a speaker grille... etc. etc. - not useful so for, but i try stupid and crazy stuff all the time, now and then it leads to good things.

[floid]

Pretty sure the inverse square law would negate any observable effect in most real world setups.
But I wonder, maybe if you took a large speaker coil, just the coil, and placed a mic right up inside it.... it would essentially become a (probably very inefficient) transformer.