How a planar magnetic speaker works
- Source
- Amplifier
- Speaker
- Room
A planar magnetic speaker has no cone and no box. The signal runs through thin traces bonded to a large film, the film hangs between rows of magnets, and the whole sheet moves at once. A narrow strip beside it, like a ribbon, plays the treble.
With nothing behind it, the panel plays from its back as well as its front, and the back wave is the front one turned upside down: it is a dipole. Where the two meet at the panel’s edges they cancel, so the panel is quiet at its sides, and its bass fades once the wavelength outgrows the panel.
When pairing: low sensitivity and a flat 4 Ω load want a powerful, current-capable amplifier, and the panel needs real space from the wall behind it, where its back wave goes.
The sum line is what reaches you. Below about 104 Hz the back wave wraps round the panel’s edges and cancels the front, so the bass falls away; a bigger panel moves that point lower. The dotted line is the dynamic speaker.
When pairing: the panel’s bass leans on the room and the wall behind it, and many owners add a subwoofer.
Nearly flat: 4.0 to 4.6 Ω across the whole range. The traces are close to a plain resistor, and the crossover is built so the amplifier sees the same load at every frequency. The dotted line is the dynamic speaker, with its humps.
When pairing: a steady 4 Ω asks for current everywhere, not just in a dip. Choose an amplifier rated into 4 Ω, ideally one whose power nearly doubles from 8 Ω to 4 Ω.
This generic 85 dB panel needs about 10 W for a loud 95 dB at 1 m; the 87 dB dynamic speaker needs 6 W.
When pairing: low sensitivity and a low, flat impedance want a powerful, current-capable amplifier with plenty of headroom.
Motor
Crossover
Structure
How the numbers work
Every number on the page comes from one small model of a generic panel, not a product. The dotted lines come from the dynamic speaker’s model on the other page.
- The panel: 0.36 m by 1.2 m. The back wave reaches a listener in front late, by the path round the edge. The model takes that path as twice the radius of a disc of the same area, 0.74 m. On axis the front minus the back,
|1 − e^(−jkD)|, rises 6 dB per octave in the bass and averages √2 above; the model keeps the smooth version, a first-order high-pass at 104 Hz, where the two meet. - The film: a mass on a light suspension, resonant at 40 Hz with Q 1. Below that the bass falls faster again.
- Crossover: first order at 1.6 kHz. A 0.40 mH coil in series with the film and a 24.9 µF capacitor in series with the strip, both legs 4 Ω. With those values it is a constant-resistance network, so the amplifier sees 4 Ω at every frequency. Each leg’s share in the readout is of power, which sums to 100% in a first-order network.
- Impedance: that 4 Ω plus the film’s small motional hump at its resonance (0.6 Ω, small because a planar motor has a weak force factor). It stays between 4.0 and 4.6 Ω.
- Sensitivity: 85 dB at 1 W and 1 m, a typical figure for a panel this size, chosen, not measured.
- The sound waves: a slice at the height of the panel’s top. Each ring is the set of points the same distance from the panel, a straight run across the face and a quarter circle round each edge. The back’s crests sit half a ring spacing behind the front’s, because the film pushes one side as it pulls the other. A mask fades both to nothing near the panel’s plane, where they cancel. Ring spacing and speed are the dynamic page’s; strength is the sum line’s level.
- Simplifications: one path length stands for the panel’s whole edge, so the bass corner is smooth where a real panel ripples; the room, the floor and the wall behind are not in the model, and they matter more for a dipole than for a box.