Introduction
The moving-coil loudspeaker is the most widely used loudspeaker type. It appears in many electronic products, from radios to Bluetooth speakers and public-address systems, wherever electrical waveforms need to be converted into audible sound.
Principle of Operation
Moving-coil loudspeakers operate on the magnetic effects produced by an electric current in motion. When a current flows through a conductor, it generates a magnetic field. Winding the conductor into a coil increases that effect.
If the coil is placed within a stable magnetic field supplied by a fixed magnet, the two fields interact. Opposite poles attract and like poles repel. As a result, the current through the coil produces forces that attract or repel the coil relative to the fixed magnetic field; the force magnitude is proportional to the current.
When the coil is mechanically attached to a large diaphragm, those movements are transferred efficiently to the air as sound waves. This is the basic concept of the moving-coil loudspeaker.
Cross-Section and Key Components
Typical moving-coil loudspeakers follow a standard format, though electrical and mechanical dimensions, parameters, and materials vary significantly.
A typical cross-section shows these main parts:
- Magnet: Provides the fixed magnetic field that interacts with the coil. Magnets are typically ferrite or strong neodymium.
- Chassis/frame: Usually circular or sometimes oval, the frame forms the structural base of the speaker and supports other components.
- Diaphragm or cone: A cone or diaphragm at the front transfers the coil's motion to the air. Cones may be made from fabric, plastic, paper, or lightweight metals. The outer edge of the diaphragm is attached to the frame and often has corrugations to allow easier movement.
- Surround: The surround lets the main portion of the diaphragm move freely and linearly.
- Dust cap (dome): Protects the voice coil from dust and dirt.
- Voice coil and former: The voice coil is the key electromechanical element. It receives current from an audio amplifier; the resulting magnetic field interacts with the permanent magnet to produce forces that move the coil and the mechanically attached diaphragm, generating sound.
- Spider (suspension): A flexible corrugated support that holds the voice coil in place while allowing axial movement. The spider helps keep the coil centered, maintaining a narrow gap around the coil for maximum efficiency and preventing contact with the magnet that would cause audible scraping.
Speaker unit showing the suspension and related parts.
Design Considerations
Understanding design limits, trade-offs, and common issues is useful when designing or using moving-coil loudspeakers.
Doppler distortion: When a speaker reproduces high and low frequencies simultaneously, movement from low-frequency tones can introduce Doppler effects on high-frequency tones.
Cone construction: Cone design is a trade-off. The cone must be rigid to avoid distortion from bending, yet low in mass to minimize inertia so the speaker can respond accurately to signal changes. New materials have helped improve this balance in recent years.
Cone suspension: How the cone or diaphragm is suspended significantly affects performance. The suspension must keep the cone aligned while allowing free axial movement. Many speakers use additional suspension elements near the coil to ensure centering. Older designs used an adjustable spider fixed to the pole piece to allow manual centering if misalignment occurred.
Cone resonance: The cone resonance frequency is commonly listed in speaker specifications. Below this frequency, output falls at approximately 12 dB per octave, so a lower resonance frequency generally improves low-frequency response.
Conclusion
Moving-coil loudspeakers are precision electro-mechanical devices that balance many competing requirements. Careful design of magnets, coil geometry, cone materials, and suspension yields speakers that perform well across a wide range of applications.
