Sony APM-2 (Unknown)

Flat diaphragm planar magnetics in a foam-filled cabinet - this obscure Sony experiment sounds like nothing else they ever made.

Overview

The Sony APM-2 isn't one of the company's best-known speakers, and it doesn't turn up in glossy brochures or vintage catalogs. It's a shadow model, barely documented, that slipped through the cracks of Sony's otherwise well-archived speaker lineage. What little is known is interesting, though. It's one of the few times Sony used square planar magnetic drivers with a flat diaphragm, a big departure from the cone-based APM series like the APM-22ES that followed. These aren't dynamic drivers with paper cones flapping in a magnetic field. They're rigid, flat panels suspended in a magnetic array, which promises piston-like motion and ultra-low distortion. Whether this was a prototype, a limited regional release, or an engineering test bed, the APM-2 stands apart from Sony's mainstream speaker output.

It uses a bass reflex enclosure, so there's a port somewhere doing its job. The internal construction is where things get curious. The cabinet is foam-filled, which suggests Sony cared about damping internal reflections, possibly to keep the delicate planar drivers from being colored by cabinet resonance. That kind of attention to enclosure behavior is more typical of high-end audiophile designs than experimental one-offs. And despite the lack of specs, the bi-wire capable terminals hint that the APM-2 was meant for enthusiasts. They give separate connections for highs and lows, likely to preserve signal integrity to those sensitive planar drivers.

Here's the catch: nobody knows when it was made, how much it cost, how loud it plays, or how big it is. No frequency response, no impedance, no sensitivity rating. The archives are silent. It doesn't even show up alongside the better-known APM-22ES, which came later and used conventional (though advanced) cone drivers. The APM-2 looks like a prototype that slipped out the back door, a lab experiment that got a model number and terminals but never a datasheet.

Specifications

Key Features

Square Planar Magnetic Drivers

The APM-2's drivers are square planar magnetic units with a flat diaphragm, a technology more associated with brands like Magnepan or certain high-end Japanese manufacturers. The idea is simple: replace the cone with a thin, rigid sheet suspended in a magnetic field, driven across its surface rather than at a single point. In theory, this eliminates breakup modes and gives cleaner midrange and treble. The "flat diaphragm" aspect suggests minimal flex, aiming for what Sony might have called "accurate pistonic motion" long before they trademarked APM for their cone drivers. Whether it worked as intended is unknown, but Sony did try this in a product labeled APM (Accurate Pistonic Motion). This might be the origin of the APM concept, even if it never made it to marketing materials.

Foam-Filled Cabinet

The cabinet isn't just sealed or ported. It's foam-filled. That's unusual for a speaker with planar drivers, which typically rely on open or lightly damped enclosures. Filling the box with foam kills internal standing waves, but it also adds mass and damping, possibly to stop the cabinet from resonating and smearing the transient response. It could also be a way to control rear-wave energy from the planar driver, which often radiates both forward and backward. Without knowing the density or layout of the foam, or how much space it occupies, that's guesswork. Was it a full fill? Strategic baffling? Either way, it shows Sony was thinking about enclosure interaction, a level of detail that didn't always make it into their mass-market designs.

eBay Listings

SONY APM-20AV SPEAKER TESTED WORKS

$850

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Related Models

• Sony apm-20av (1980s)
• Sony apm-22es (1984-1986)
• Sony apm-55w (1980s (exact years uncertain))
• Sony apm-66es (1983-1985)
• Sony apm-8 (1982-1984)