What the driver in headphones is and whether its size matters
What the driver is
The driver is the speaker inside headphones: the part that turns the electrical signal into sound. It has a thin diaphragm that vibrates very fast and moves the air in front of it, and that moving air reaches the ear as sound.
When the box says "10 mm driver", the figure is the diameter of that diaphragm. In small earbuds, drivers range from about 6 to 18 mm; in over-ear headphones, around 40 mm; in portable speakers, more.
The dynamic driver
Most headphones use dynamic drivers: a diaphragm attached to a coil that moves in a magnetic field. They are robust, give full bass and can sound very good at any price. There are other types too, such as balanced armature drivers, small and precise, often used in professional in-ear monitors.
TESNA headphones use dynamic drivers, from 8 mm in the Free7 to 18 mm in the Open2 and 40 mm in the Pods Ultra, which have a bio-cellulose diaphragm.
Does size matter?
A bigger driver can move more air, which helps bass, especially in headphones that do not seal the ear. That is why open-ear buds usually have bigger drivers: the Open2 have 18 mm, the GOG100 16.2 mm, to make up for not closing the ear.
In in-ear buds, which seal the canal, a small driver can give full bass because the air has nowhere to escape. The Free7, with an 8 mm driver, have a complete sound precisely because the tip closes the ear.
What matters more than millimetres
How the sound is tuned, the bud's housing and, above all, the fit in the ear change the sound more than driver size. An in-ear bud with a tip that is too small loses almost all its bass, however big the driver.
So two pairs with drivers of the same size can sound very different. The figure on the box is a hint, not a guarantee of quality.
Special drivers
Some headphones use drivers with a particular design. The Ultra have an 11 mm coaxial driver, where two parts of the sound come from the same point, for clearer sound. The S7 have 17 mm directional drivers that aim the sound at the ear so people nearby hear less.
The Pods Ultra have 40 mm drivers with a bio-cellulose diaphragm, a light, stiff material that helps keep the sound clean at high volume.
In speakers
In speakers, drivers are much bigger, because they have to fill a room or a garden, not just an ear. The Pod Pro has two 45 mm drivers and a passive radiator, a diaphragm with no motor that vibrates along with the drivers and strengthens the bass. The Pod mini has a 52 mm full-range driver.
With speakers too, placement matters: on a hard surface, near a wall, the bass sounds fuller.
How to actually choose
Do not choose headphones by driver size alone. First choose the type, in-ear, open-ear or over-ear, by where you wear them, then look at fit, and only then at the technical details. If you like strong bass, in-ear or over-ear headphones give it more easily than open-ear ones, whatever the millimetres.
For a comparison of the types, our guide to in-ear, open-ear and over-ear headphones explains the differences.
The driver and volume
A good driver sounds clean at low volume too, not just loud. If you have to turn the volume up a lot to hear the bass, the problem is usually the fit in your ear, not the driver: a larger or smaller tip, as needed, can change the sound a lot. With open-ear buds, surrounding noise covers part of the bass, so in quiet places they sound fuller than in traffic.
Very high volume tires both the driver and the ear: the sound starts to distort and hearing tires faster. Good sound at moderate volume is healthier than loud, distorted sound. Our guide to safe volume explains why.
Every model is in the shop, under true wireless earbuds and over-ear headphones.