A loudspeaker (or loud-speaker or speaker) is an electroacoustic transducer which converts an electrical audio signal into a corresponding sound.
A loudspeaker consists of paper or plastic moulded into a cone shape called ‘diaphragm.’ When an audio signal is applied to the loudspeaker’s voice coil suspended in a circular gap between the poles of a permanent magnet, the coil moves rapidly back and forth due to Faraday’s law of induction. This causes the diaphragm attached to the coil to move back and forth, pushing on the air to create sound waves.
Faraday’s law of induction is a basic law of electromagnetism predicting how a magnetic field will interact with an electric circuit to produce an electromotive force (EMF)—a phenomenon called electromagnetic induction. For details on Faraday’s Law of Induction: click here
Voice coil, usually made of copper wire, is glued to the back of the diaphragm. When a sound signal passes through the voice coil, a magnetic field is produced around the coil causing the diaphragm to vibrate. The larger the magnet and voice coil, the greater the power and efficiency of the loudspeaker.
The coil is oriented co-axially inside the gap; the outside of the gap being one pole and the centre post (called as the pole piece) being the other.
The gap establishes a concentrated magnetic field between the two poles of the permanent
magnet. The pole piece and backplate are often a single piece, called the pole plate or
yoke, as shown in Fig. 1.
As pulses of electricity pass through the electromagnet coil, its magnetic field direction rapidly changes, causing the cone to vibrate. The cone structure amplifies these vibrations, pumping sound waves into the surrounding air and towards your ears. That’s how sound comes from a loudspeaker.
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