For 10 points, name this effect demonstrated through the presence of audible "pops"  when a ferromagnetic material is hooked up to a speaker, named after its German discoverer.
The Sixtus-Tonks effect occurs in Wiegand sensors due to this effect.  In thin films, it is thought to be due to the creation of a vortex near the edge of the film, though its nature is more commonly studied using the zero-temperature random field (*) Ising model.  Its namesake developed a microwave-generating triode vacuum tube with Kurz.  It occurs when a slowly varying magnetic field causes discontinuous namesake "jumps" in magnetic flux density due to irreversible motion of domain walls.  Its 1919 discovery was the first experimental verification of magnetic domains, and it emits a namesake "noise."  For 10 points, name this effect demonstrated through the presence of audible "pops"  when a ferromagnetic material is hooked up to a speaker, named after its German discoverer.
The Sixtus-Tonks effect occurs in Wiegand sensors due to this effect.  
{Barkhausen} effect [accept {Barkhausen} avalanche, pulsation, jump; accept {Barkhausen} noise before given] (4) [DW]