The dynamic speaker has not changed in decades. The frame is made of stamped iron or aluminum. The permanent magnet is a ceramic ferrite material composed of iron oxide, strontium and a ceramic binder. Cones, surrounds and star wheels are made of adhesive-treated paper. The voice coil consists of a plastic winding tube and a coil wound with thin and insulated copper wire.
The most common amplified speaker is the dynamic speaker. It consists of a frame, a permanent magnet, a soft iron core, a voice coil and a cone. The frame supports the cone and permanent magnet assembly. The voice coil consists of an insulated wire wound on a plastic spool. One end of the spool is attached to the cone and the body of the spool slides over the soft iron core.
The wire from the voice coil is connected to the audio amplifier. When the audio signal from the amplifier is applied to the voice coil, an electromagnetic field is generated around the voice coil. This causes the voice coil to move back and forth along a soft iron core that contributes to, or resists, the magnetic field generated by a permanent magnet. The movement of the voice coil causes the connected cone to vibrate and produce sound.
There are four main types of amplified speakers: full-range, tweeter, mid-range and woofer. Full-range amplified speakers can reproduce most of the audio spectrum. However, a single amplified speaker cannot accurately reproduce the entire audio range of the human ear. Therefore, other amplified speakers are designed to overcome the limitations of full-range speakers.
Tweeter speakers are designed for higher audio frequencies or highs in the 4-20 kHz range. They are small, typically about 2 inches (5.1 cm) or less in diameter. Midrange speakers can reproduce sound in the 1,000 Hz to 10 kHz frequency range. They range from 2 inches to 8 inches (5.1-20.3 cm) in diameter. Subwoofers reproduce bass or very low sounds in the 20-1,000 Hz frequency range and subwoofers can be extended down to 3 Hz. Subwoofers range from 4" to 15" (10.2-38.1 cm) in diameter, with the most common diameter being 10-12 inches (25.4-30.2 cm).