How does a loudspeaker make sound? Current pushes a coil back and forth beside a magnet. The coil moves the cone, which pushes nearby air. The gold air marker moves around one place; the pattern of compression travels away.
Try two small comparisons
- Reverse the current and watch the coil force and cone motion.
- Follow one air marker. Does it travel away, or move back and forth nearby?
Look closer: observations and controls
Why does the cone follow an electrical signal?
The permanent magnet stays fixed. The yoke, central pole and top plate guide flux into a narrow gap. Current in the voice coil produces axial force there; reversing current reverses that force.
The coil is wound on a bobbin bonded to the cone and dust cap. They move as one assembly. The surround and spider connect this assembly to the fixed basket, allowing axial travel while providing elastic support. Flexible leads preserve the electrical connection as it moves.
Conversion also works backward. Motion of the coil generates back EMF, so its current depends on motion as well as resistance. The film slows the mechanism to make this interaction visible.
Inspect a connection, then change one input
- Examine the magnetic gap and scrub phase. The magnetic field stays fixed while current and force change sign together.
- Inspect the suspension. Its moving endpoints follow the same displacement; its fixed endpoints remain on the basket.
- Hold peak voltage constant and compare frequencies. Each response point is a settled sinusoidal result, not a transient frequency sweep.
- Increase damping to reduce the excursion peak, then inspect the instantaneous power balance.
Ranges have explicit names and support arrow keys, Home and End. Reset restores voltage, frequency, phase, damping and inspection view. There is no component-generated test tone.
Calculations, sources and scope
One reciprocal force factor
The declared constant-parameter model is
u = Ri + L di/dt + Blv
M dv/dt = Bli − Kx − Dv
dx/dt = v
Defaults are R = 6 Ω, L = 0.5 mH, Bl = 4 N/A, M = 12 g, K = 1200 N/m and D = 0.8 N·s/m. These are teaching parameters, not measurements of a commercial driver. Mass and suspension properties represent the combined moving assembly.
The same force factor gives (Blv)i = (Bli)v. With
E = ½Li² + ½Mv² + ½Kx²
ui = Ri² + Dv² + dE/dt
resistive and mechanical losses are nonnegative, while stored energy can rise or fall. The release shot sets an ideal voltage source to zero and leaves the coil circuit closed through its resistance. It does not disconnect the leads or demonstrate energy recovery into a battery.
For sinusoidal peak phasors,
Zₑ = R + jωL
Q = K − Mω² + jωD
X = BlU / [ZₑQ + jω(Bl)²]
I = UQ / [ZₑQ + jω(Bl)²]
Displacement, impedance, current and power use that same solution. The undamped mechanical natural frequency is about 50.3 Hz; the full response also depends on coupling, damping and inductance. The displayed excursion curve is not an SPL curve.
What the air illustration means
The final shot is a one-way, one-dimensional plane-wave kinematic illustration. It uses the cone motion as a boundary condition: ξ(s,t) = x(t − s/cₐ), with cₐ = 343 m/s. Material points oscillate near their rest positions while compression, −∂ξ/∂s = v(t − s/cₐ)/cₐ, travels outward.
Air loading is not fed back into the driver, and a finite cone’s three-dimensional radiation is not solved. Air displacement has its own magnification and color is normalized by compression phase. These are not CFD, SPL, radiation efficiency or distance-attenuation predictions. Mechanical dissipation is not identified with total radiated sound power.
Geometry and limits
The cutaway is a meridional slice of an axisymmetric structure. Phone shots reproject the assembly or focus on the gap and one suspension side. Mechanical travel is enlarged eight times, with all connected endpoints derived from one displacement. Flux lines show circuit topology, not a magnetic finite-element solution; winding sections are grouped schematically.
This small-signal piston model omits magnetic nonlinearities, frequency-dependent lossy inductance, coil heating, cone breakup, the enclosure and coupled spatial acoustics.
Sources: KLIPPEL modeling, COMSOL driver anatomy, COMSOL transient analysis, and Caltech sound waves.