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Cassette tape players use a read head where the magnetic field from the tape is directed through an induction coil. The induced current in the coil is then amplified and sent to the speakers. Now I'm wondering if magnetic tape technology could also work with a permanent magnet that vibrates from the changes in magnetic field in the tape (for playback) or that is made to vibrate externally to create a changing magnetic field on the tape (for recording).

Obviously this is just a theoretical question as I'm not going to use a permanent magnet tape player for anything. I am just wondering if this would be possible with any level of sound quality.

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No. The magnetic flux density available on a tape is way to small to move a magnet with any significant mass enough to produce audible sound.

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  • $\begingroup$ Do you know what typical magnetic fluxes on a tape are, or what the maximum achievable flux for tape technology is? $\endgroup$ – JanKanis Aug 4 '17 at 14:29
  • $\begingroup$ en.wikipedia.org/wiki/Orders_of_magnitude_(magnetic_field) I'm guessing you are at least 3 or 4 orders of magnitude too small to make the faintest sound. I suppose if you genericise the notion of what you mean by "tape" to something 10^4x thicker you might have something. $\endgroup$ – agentp Aug 4 '17 at 14:35
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    $\begingroup$ I'd also suspect if the permanent magnet were were close enough to the tape to be effected, it would erase the tape. $\endgroup$ – Eric Shain Nov 2 '17 at 17:47
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Magnetic force microscopy should have sufficient resolution. The principle of operation here is that an external force causes a very small (few micrometers) magnetic probe to vibrate, and then the magnetic field from the sample (e.g. the tape) will cause tiny changes to the amplitude or phase of the vibration. Certainly not practical for a cassette player, and maybe not exactly what you were after, but it should work.

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