After hard magnetic materials are magnetized, there is still remanence. The strength and direction of the remanence depend on the strength and direction of the magnetism during magnetization.
Audio - recording tapes are composed of a base, an adhesive, and a magnetic powder layer. The base is generally made of polycarbonate or vinyl chloride. The magnetic powder is fine powder of r - Fe₂O₃ or CrO₂ with strong remanence.
During recording, the current corresponding to the sound change, after being amplified, is sent to the coil of the recording head. This causes a concentrated magnetic field to be generated in the gap of the magnetic head core. As the current of the coil changes, the direction and strength of the magnetic field also change accordingly. When the tape passes through the gap of the magnetic head at a constant speed, the magnetic field passes through the tape and magnetizes it. Since the tape has corresponding remanence after leaving the magnetic head, its polarity and strength correspond to the original sound. As the tape keeps moving, the sound is continuously recorded on the tape.
During playback, the recorded tape passes through the playback magnetic head gap at the same speed as during recording, closely attached to it. The magnetic head core is made of a high - permeability ferrite soft - magnetic material, which has very little resistance to magnetic flux. Therefore, the audio - recorded remanent magnetic flux on the tape can easily form a loop through the magnetic head core. The remanent magnetic flux on the tape induces an induced electromotive force in the playback magnetic head coil that has the same variation law as the remanent magnetic flux. After being amplified by the playback amplifier, it is sent to drive the speaker, and the audio signal recorded on the tape is restored to the original sound.
