Headphones Transducer technology-Moving-coil Electrostatic and Electret

Headphones use various types of transducers to convert electrical signals into sound. This passage introduces the three types: moving-coil, electrostatic, and electret.

Moving-coil

The moving coil driver, more commonly referred to as a "dynamic" driver, is the most common type used in headphones. It consists of a stationary magnet element affixed to the frame of the headphones, which sets up a static magnetic field. The magnet in headphones is typically composed of ferrite or neodymium. A voice coil, a light coil of wire, is suspended in the magnetic field of the magnet, attached to a diaphragm, typically fabricated from lightweight, high stiffness-to-mass-ratio cellulose, polymer, carbon material, paper, or the like. When the varying current of an audio signal is passed through the coil, it creates a varying magnetic field that reacts against the static magnetic field, exerting a varying force on the coil and causing it and the attached diaphragm to vibrate. The vibrating diaphragm pushes on the air to produce sound waves.

Electrostatic

A thin, electrically charged diaphragm, typically a coated PET film membrane, is suspended between two perforated metal plates in electrostatic drivers (electrodes). The electrical sound signal is applied to the electrodes, resulting in the formation of an electrical field. Depending on the polarity of this field, the diaphragm is drawn toward one of the plates. A sound wave is produced when air is forced through the perforations and combined with a constantly changing electrical signal driving the membrane. Electrostatic headphones are typically more expensive than moving-coil headphones and less common. Furthermore, a special amplifier is required to amplify the signal to deflect the membrane, which frequently requires electrical potentials ranging from 100 to 1,000 volts.

The frequency response of electrostatic headphones typically extends well above the audible limit of approximately 20 kHz due to the extremely thin and light diaphragm membrane, which is often only a few micrometers thick, and the complete absence of moving metalwork. Because of the high-frequency response, the low-mid band distortion level is kept at the top of the audible frequency band, which is not always the case with moving coil drivers. Furthermore, the frequency response peakiness that is commonly observed in the high-frequency region with moving coil drivers is absent. Electrostatic headphones that are well-designed can produce significantly better sound quality than other types.

Electrostatic headphones require a voltage source generating 100 V to over 1 kV and are on the user's head. Since the invention of insulators, there is no actual danger. They do not need to deliver significant electric current, which further limits the electrical hazard to the wearer in case of fault.

Electret

An electret driver functions along the same electromechanical means as an electrostatic driver. The electret driver, on the reverse side, has a built-in permanent charge, whereas electrostatics have the charge applied to the driver by an external generator. Electrostatic and electret headphones are uncommon. Originally, electrets were also less expensive and had the lower technical capability and fidelity than electrostatics. From 2009 to 2013, patent applications were approved demonstrating that by using different materials, such as a "fluorinated cyclic olefin electret film," frequency response chart readings could reach 50 kHz at 100 dB. When these new, improved electrets are combined with a traditional dome headphone driver, headphones that are worthy of joining the Hi-Res Audio program can be produced, according to the Japan Audio Society. Patents in the United States 8,559,660 B2. 7,732,547 B2.7,879,446 B2.7,498,699 B2.