Here is another epic video by The Action Lab YouTube channel, this time literally "hearing" magnetic domains change when an external magnet changes the internal magnetic field of a piece of iron.
Magnetic Domains is a region of uniform magnetization within a magnetic material. Typically these domains are in the micrometer size range (10-6 m).
Materials such as iron have their magnetic domains naturally in a random direction, which thus cancels out the net magnetization of all the magnetic domains. And placing a magnet near it will be in attraction because the nearby region will shift its magnetic domains to align with the magnet.
In the video, The Action Lab host demonstrates the Barkhausen effect to hear the magnetic domains change. This is done by wrapping a coil of wire around the magnet and connecting the output to an amplifier and loud speaker.
Because most materials have defects, when magnetic domains change, the domain may get caught on the defect and then "snap" past it, creating a sudden change in the magnetic field. This results in sending a pulse of current into the wires, which the amplifier and loud speaker make it loud enough to hear.
Note that "H" in this context is the applied magnetic field.
This means that magnetizing a material is not a smooth process, but rather has many discontinuous jumps.
Note that J refers to the magnetization, and B is magnetic flux density or the total magnetic field of the material.
Also in the video, he demonstrates that the sound only comes during the magnetization but not when the iron bar has already been magnetized. That is, once magnetized, bringing the same magnetic pole close to the iron bar doesn't produce any sound since the domains aren't changing.
Pretty epic stuff!
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Interesting! The stepped square-wave like results are unexpected. I've done some cool recordings with magnetic coil pickups and running them through filters, Holding them next to discman CD players and shaking them, picking up the fridge frequencies, etc