In the case of headphones, as in the case of other speakers, there are also various systems of sound conversion.
Electromagnetic
The design principle dates back to the early days of audio technology and was used for both loudspeakers and headphones. Typically, a steel diaphragm is moved, which radiates the sound directly. The current from the amplifier flows through two coils pre-magnetized with a permanent magnet, similar to an electromagnet, whose magnetic poles are located at a short distance behind the sheet steel diaphragm. Premagnetization is essential for sound reproduction, because without it, the diaphragm would oscillate at twice the frequency (the iron diaphragm would be tightened once at the positive and then again at the negative half-wave). Due to the premagnetization, the diaphragm receives a magnetic (as well as mechanical) preload, to which the active alternating voltage is added or subtracted.
Such transducers were used, for example, in early radio technology or military communications; they were also used in Morse headphones and telephone receivers. Their impedance was usually 2 to 4 kΩ (kiloohms). They could therefore be used without output transformers in tube amplifiers or in detector receivers.
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Due to the poor playback quality (very high distortion factor, severely limited frequency range, etc.), electromagnetic transducers are usually no longer used today.
Electrodynamic
As with a dynamic loudspeaker, the sound conversion arises from the movement of a coil fed by the sound signal in a magnetic field: A moving coil in the center of the diaphragm is located in the narrow air gap of a strong permanent magnet mounted on the headphone frame and, when the sound frequency voltage is applied, becomes longitudinal due to the interaction of constant and variable magnetic fields. vibrations that are transmitted to the surrounding air via the diaphragm.
To avoid excessive partial vibration and for reasons of material inertia, the driver diaphragm is made of lightweight material with high rigidity, such as cellulose, plastic or metal.
The electrodynamic diaphragm drive is the most widely used today, it offers cost-effective production, uncomplicated operation and also delivers the highest reproduction quality with the current state of the art and correspondingly high design effort.
Isodynamic or orthodynamic or magnetostat
The design of the elements of the transducer is similar to that of an electrostatic push-pull loudspeaker, in which a constantly electrically charged flat diaphragm moves between the perforated stators controlled by the high-voltage sound frequency.
In the case of the isodynamic transducer, on the other hand, the diaphragm is driven by the electrodynamic effect of a regular sound frequency voltage, which is fed to a conductor path (a flat coil) that is glued or vapor-deposited in a spiral shape from the center of the diaphragm to the outside. The evenly clamped diaphragm moves in a homogeneous magnetic field between the pole plates of two opposing permanent magnets or several magnetic rods arranged over a flat surface when the NF voltage is applied.
An appropriate efficiency of the isodynamic drive systems can only be achieved with relatively large and therefore heavy magnetic disks with high magnetic energy density, which results in an unusually high weight and unfavorable wearing comfort of the headphones.
Balanced armature transducers
The balanced armature transducer (“BA”) design is primarily intended to increase electrical efficiency by eliminating the load on the diaphragm, as is characteristic of many other types of propulsion. The BA principle consists of a permanent magnet and a movably mounted armature that is precisely centered in its magnetic field. In the center of the magnetic field, there is no resulting force on the armature, hence the term “balanced”. Now, when current flows through the coil of the armature, the armature magnetizes so that it is easily moved in one direction or another. The diaphragm is attached to the armature with a drive and then generates sound waves.
The design is not mechanically stable, and the armature, attracted by the permanent magnet, would stick. Therefore, a relatively rigid diaphragm with a high restoring force is required to keep the armature in “balance”. Although this has a negative impact on efficiency, this design can produce sound from little power better than any other.
Today, they are usually used in ear canal headphones (in-ear earbuds) and hearing aids due to their small size and low impedance. They are usually limited to the human hearing range (about 20 Hz to 16 kHz) and require a higher seal than other types of drivers to deliver their full potential. High-end models can use multiple BA drivers to better represent the sound spectrum. With the help of a passive crossover, these are combined to form an overall sound image. There are also models that combine BA drivers with classic coil diaphragm drivers for the bass range.
Electrostatic
Functional principle of an electrostatic push-pull transducer, control with step-up transformer
The principle of the electrostatic drive works like a condenser microphone operated in reverse. The drive mechanism consists of a thin, conductively coated polyester membrane film, which is constantly electrically charged with the positive bias (bias or EHT) of a high-voltage cascade and is clamped between two perforated metal plates – the stators. For an adequate efficiency of the diaphragm drive, a high drive voltage is necessary, which is provided by a special step-up transformer, which is connected directly to the low-impedance loudspeaker outputs of a regular audio amplifier with sufficient output power on the primary side.
Newer model variants work together with specially designed headphone amplifiers without the transformer, which are unfavorable in terms of transmission, which must be operated with very high symmetrical supply higher voltages to generate an appropriate volume level. The high-voltage sound frequency is fed to both perforated stators and moves the pre-charged diaphragm foil over the entire surface due to the changing conditions of the electric field.
Despite the relatively simple design of the transducer, the electrostatic headphones are usually more expensive than a model with an electrodynamic drive due to the higher operating costs.
Due to its extremely thin and therefore light membrane, which is often only a few micrometers thick, the frequency range of electrostatic headphones usually extends far beyond the audibility limit of approx. 20 kHz; the almost inertial reaction of the diaphragm makes the transducer extraordinarily true to impulse, which benefits a high resolution of the spatial information of the signal. In addition, the isoplanar vibration behavior of the diaphragm offers a minimum of partial vibrations and, with an optimal design, thus contributes to the particularly natural and clear sound result of the electrostatic headphones.
Image credit: https://soundnews.net/headphones/full-size/the-ultimate-hifiman-susvara-review/
Although electrostatic headphones are usually operated with voltages from 100 V to over 1 kV and are located on the head of the listener, their use is still considered safe due to the extremely low currents. Such systems are appreciated by audiophiles, but they are more complex to manufacture and correspondingly higher in price.
Piezoelectric
Piezoelectric Transducers typically have high efficiency and low sound quality (hardly any bass reproduction, pronounced resonances). The impedance is high (frequency-dependent, several kiloohms). They were used, for example, as so-called crystal earphones for detector receivers.
