The next means of classification of headphones is based on the technical way that they produce sound.
There are three basic types of headphone drivers: dynamic, balanced armature, and planar. Planar has subcategories but all are similar in operating concept, so we won’t spend time unpacking them.
You’ll find dynamic drivers in all of the three main types of headphones. If you have ever looked at a speaker, you have seen a dynamic driver. In a typical headphone, electrical signals arrive at the dynamic driver all around its circumference causing it to move in and out to create the sound you hear. The dynamic drivers in most headphones are fairly small in diameter. One driver will typically cover the entire frequency range. Dynamic drivers are known for their outstanding bass response and warm sound.
Balanced armature is only found in in-ear headphones. These were originally developed for voice applications in the 1920s and are quite popular in hearing aids due to their small size. This design puts a small arm inside a coil of wire surrounded by magnets. Another arm is placed at a 90-degree angle and is centered in a tiny diaphragm. This allows manufacturers to design their drivers to cover a specific frequency range well, however, the overall range is somewhat limited. A single in-ear headphone can have as many as eight different drivers, creating a sound that is very clear and detailed.
Planar-type drivers are normally only found in open-back, over-ear designs. Planars have some subcategories but all of them share one thing in common: a relatively large, ultra-thin driver that does not come in contact with anything that moves. Planar speakers have been a choice of music enthusiasts for over 50 years, and the same holds true in the headphone world. The driver in a planar headphone weighs almost nothing and is extremely fast in response. The downside is that it can be somewhat hard to drive, creating the need for a headphone amp. However, a nice pair of headphones with planar-type drivers provides an incredible soundstage with amazing detail.
Electrostatic drivers work by applying a static electric charge onto an ultra-thin film, which causes it to “float” freely between two perforated metal plates where opposing forces then act on the film. These forces “cling” and “repel” the film, causing the entire diaphragm to move all by itself.
Similar to planar drivers, electrostatic systems use an ultra-thin film that doesn’t make contact with anything that moves. One area where these differ from planar drivers is in how thin. The film in an electrostatic headphone is so thin it weighs less than the air around it, and has nothing to resonate. It’s called "static" because the electric charge on the film doesn’t move. With nothing to resonate with and no energy to store, this technology provides the most accurate transient responses available with virtually no distortion.
The downside with these is they require special high-voltage power supplies. This usually means they’ll need an amplifier that plugs into the wall. They can also be some of the most expensive headphones on the market. The upside is they lack distortion and they provide the best possible sound from headphones money can buy.