Overview
A loudspeaker, commonly called a speaker, converts electrical signals into acoustic signals. For example, a radio receives electromagnetic waves transmitted through the air; after demodulation and amplification by internal circuitry, the result is an audio electrical signal. These electrical signals must be converted by a loudspeaker before they become sounds audible to the human ear.
Moving-coil loudspeaker structure
Speakers are classified by their operating principles. The moving-coil speaker, also called a dynamic speaker, is one of the most common types. Its basic structure places a coil, called the voice coil, inside the magnetic field of a permanent magnet. When an audio current passes through the coil, the coil produces a magnetic field that varies with the audio current. This varying magnetic field interacts with the field of the permanent magnet, producing forces that cause the coil to vibrate.
The voice coil is attached to the speaker cone, so the cone moves with the coil. The cone movement causes the surrounding air to vibrate, producing the sound we hear. The moving-coil speaker has a relatively simple principle and structure, and it is feasible to build one to better understand its operation.
Magnet and vibration system
A typical speaker consists of a magnet assembly and a vibration system. The magnet assembly includes the permanent magnet and an iron pole piece. The vibration system includes the cone (diaphragm), the spider (centering support), the voice coil, and the dust cap. When an audio current flows through the voice coil, the coil generates a time-varying magnetic field. The permanent magnet provides a static magnetic field. The interaction between the time-varying field of the coil and the static field of the magnet produces forces on the coil, causing motion perpendicular to the direction of current in the coil. Because the voice coil is mechanically coupled to the cone, the cone vibrates and drives the air to produce sound.
The larger the current delivered to the voice coil, the greater the magnetic force and the larger the cone excursion, resulting in higher sound pressure. High-frequency sound mainly comes from motion near the center of the cone, while low-frequency sound mainly comes from motion near the cone edge. A cone with a softer surround and a larger diameter tends to produce better low-frequency performance.
Electromagnetic induction
Moving-coil speakers are an application of electromagnetic induction. They use the electromagnetic interaction between a current-carrying coil and a magnetic field to convert electrical current into mechanical motion and thus into sound.
The working principle of a moving-coil speaker is based on electromagnetic effects. When current flows through the coil, forces are produced in the magnetic field that cause the vibration system to move, driving the cone or diaphragm and producing sound.
