Drivers are the unit, or units, that produce sound in earphones. Made up of magnets, diaphragms, voice coils, and other components.
The most common type of headphone drive mechanism. Dynamic drivers produce sound by transferring signals directly to the diaphragm. Larger diaphragms may be used to enhance sound quality.
Dynamic drivers in IEM’s work in a similar way to the drivers found in over-ear headphones and larger speakers. The diaphragm is mounted on a voice coil to which a current is applied. The entire section is then either attracted or repelled from a permanent magnet which moves the diaphragm and produces sound. Because they are able to move more air, bass reproduction is better than balanced armature drivers, however they cannot move as fast, meaning intricate details (including those within the bass range) may be lost.
Because powerful bass and ample sound pressure can be produced, dynamic drivers are used not only in headphones but also in many other styles, such as in-ear canalphones and regular and clip-on earbuds.
Electrostatic drivers consist of a thin, electrically charged diaphragm, typically a coated PET film membrane, suspended between two perforated metal plates (electrodes). The electrical sound signal is applied to the electrodes creating an electrical field; depending on the polarity of this field, the diaphragm is drawn towards one of the plates. Air is forced through the perforations; combined with a continuously changing electrical signal driving the membrane, a sound wave is generated. Electrostatic headphones are usually more expensive than moving-coil ones, and are comparatively uncommon. In addition, a special amplifier is required to amplify the signal to deflect the membrane, which often requires electrical potentials in the range of 100 to 1000 volts.
Due to the extremely thin and light diaphragm membrane, often only a few micrometers thick, and the complete absence of moving metalwork, the frequency response of electrostatic headphones usually extends well above the audible limit of approximately 20 kHz. The high frequency response means that the low midband distortion level is maintained to the top of the audible frequency band, which is generally not the case with moving coil drivers. Also, the frequency response peakiness regularly seen in the high frequency region with moving coil drivers is absent. The result is significantly better sound quality, if designed properly.
Electrostatic headphones are powered by anything from 100v to over 1kV, and are in proximity to a user’s head. The usual method of making this safe is to limit the possible fault current to a low and safe value with resistors.
Isodynamic (a.k.a Orthodynamic)
Isodynamic drive headphones have drivers with flexible plastic film diaphragms into which a flat voice coil (spiral or serpentine in shape– see photos below) is embedded or glued on for even distribution of the drive force. That is, the diaphragm is driven more or less equally over its entire surface at once, obviating the need to make the diaphragm rigid so that it can therefore be as lightweight and agile and nonresonant as possible. Isodynamic diaphragms are either lightly tensioned or corrugated to supply restoring force.
Magnetic assemblies (bar magnets or perforated disc magnets) on both sides of the diaphragm supply the magnetic field against which the voice coil’s varying magnetic field reacts, which makes the diaphragm vibrate.
Isodynamic diaphragms are more massive than electrostatic diaphragms, but the equal-force drive principle allows them to move very quickly and in a coherent, uniform way, so provided there is sufficient mechanical damping, an isodynamic headphone gives excellent transient response and a sound quality not unlike that of an electrostatic