How we hear music
The needle, or stylus, is an artificial sapphire or diamond with a rounded or elliptical tip. The groove (shown 1000 times enlarged) is a different shape each side, one for the right-hand stereo signals and one for the left-hand signals.
The needle vibrates as it runs along the uneven groove walls, setting up electrical signals in the pick-up head. The signals are amplified and then converted to sound by cones (diaphragms) vibrated by electromagnets in the loudspeakers.
In a moving-magnet type of pick-up head, the needle is linked to a magnet. As the needle vibrates, the magnet's movements induce electrical current in two wire coils, creating the signals fed to the two stereo speakers.
The stereo groove causes the moving magnet to induce different signals in each coil. If only the outer groove wall contains a signal, then only the coil corresponding to the right-hand speaker will produce current. Or if only the inner groove wall contains a signal, only the left-hand speaker will receive current.