Electromagnetism & electronics

Loudspeaker driver

Electroacoustic · Moving coil

How does an electrical signal in a wire become a voice you can hear across the room?

Inside a loudspeaker a small coil of wire sits next to a strong magnet. When the music signal flows through the coil, the magnet pushes the coil forward and back many times a second. The coil is glued to a paper cone, and the cone pushes the air into the sound waves that reach your ears.

A moving-coil loudspeaker is a linear motor: the audio current in a voice coil sitting in a radial magnetic field feels an axial force F = Bli, which drives a light cone that pushes on the air. The cone, suspension and air load form a mass–spring–damper system summarised by a few Thiele–Small parameters, and only about 1% of the electrical power becomes sound.

Key facts

Motor force

F = B l i (force factor Bl ≈ 3–25 T m)

Gap flux density

≈ 0.8–1.5 T

Nominal impedance

4, 6 or 8 Ω (DC resistance ≈ 70–85% of nominal)

Efficiency (direct radiator)

≈ 0.3–2%

Sensitivity

≈ 82–92 dB SPL at 1 W, 1 m (hi-fi); 95–100+ dB (professional)

Woofer resonance f_s

≈ 20–60 Hz

Parts you can explore (11)

• Basket (frame)
• Magnet & pole piece
• Voice coil
• Cone
• Dust cap
• Surround
• Spider
• Terminal tabs
• Tinsel leads
• Magnetic field
• Sound waves

In the 3D model

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