Jessica Robinson, of St Mary's School, asks :-

How do sound tape machines work?

Geoff Barnes, a physicist at Massey University, responded.

Sounds are waves of tiny changes in air pressure which vibrate your ear drum. Sound waves can also vibrate a sensor in a microphone which can send electrical signals to an amplifier.

When a microphone is connected to a tape recorder and the RECORD button is pushed, weak electrical currents from the microphone are amplified and sent to the recording head of the magnetic tape recorder. This head is a coil of wire wrapped around an iron core having the approximate shape of a horseshoe with a small gap between the two ends.

Current in the coil generates a magnetic field which bulges around the gap and penetrates the magnetic coating on the tape, producing a line of very tiny magnets. These are retained in the coating as the tape slides past the head. The loudness and frequency of the sound sensed by the microphone is related to the strength and frequency of the magnetism recorded in the tape coating.

When the tape is rewound and the PLAY button pushed, the magnetic pattern on the moving tape produces current in the coil of the head. This current is amplified and sent to a loudspeaker. The speaker cone vibrates to produce a sound similar to that previously sensed by the microphone.