The Barkhausen Effect

The Barkhausen effect demonstrates the existence of magnetic domains in a ferromagnetic material. If the North pole of a magnet is moved across a buncle of iron wires, which pass through a solenoid connected to a speaker as shown below, the speaker produces a sound. The sound is produced as the magnetic domains in the coil align themselves with the magnetic field of the magnet.

No effect is observed by moving the North pole of the magnet across the coil again, since the domains are aligned with the field. If the South pole of the magnet is used, the sound is heard again as the domains align themselves in the opposite sense.