The purpose of audio reproduction devices such as loudspeakers and headphones is to convert electrical audio signals to sound power. The figure below shows a diagram of a common type of loudspeaker called the moving-coil or dynamic loudspeaker. This speaker consists of a permanent magnet which is mounted on soft iron pole pieces, a voice (moving) coil which will act as an electromagnet, and a loudspeaker cone which is connected to the voice coil. The audio signal has been previously amplified (in terms of both voltage and power) and is applied to the voice coil. The voice coil is mounted on the center portion of the soft iron pole pieces in an air gap so that it is mechanically free to move. It is also connected to the loudspeaker cone, and as it moves, the cone will also move. When audio currents flow through the voice coil, the coil is moved back and forth proportionally to the applied AC current. As the cone (diaphragm) is attached to the voice coil, it also moves in accordance with the signal currents and, in so doing, produces sound waves.
Standard impedance values for such speakers are 4, 8, 16, and 32 ohms. Other impedance values may be obtained but those listed are by far the most common.
An electromagnet may be used in place of the permanent magnet to form an electromagnetic dynamic speaker. However, in this instance, sufficient DC power must be available to energize the field electromagnet. The operation otherwise is much the same as that of the permanent magnet type.