It is difficult to find a more common technical object than headphones. The variety of shapes, colors and design solutions is sometimes amazing. But the fact that "what is inside" is already known much less. It is this gap that I want to fill in and tell about what kinds of headphones are “from the inside”, why they are invented, and how they differ.
I’ll just make a reservation that I’ll mainly consider the current state of the headphone market, with a little insight into the history of the issue, since the headphone history goes back to the 19th century and is worthy of perhaps a separate material.
To begin with, it is worth discussing the headphone form factors, so that later you can make references to which emitters are used where.
In English, everything is much simpler, there the main form factors are separated by names. Headphones that are worn on the head are called headphones, inserted into the ear canal - earphones, earbuds such as standard Apple-earbuds. There are also terms for the separation of two categories of headphones: circumaural - headphones, ear cushions that cover the entire ear, supra-aural - overhead headphones, in which the ear cushions are usually made in the form of pads that cling to the ears.
In the Russian language there is no such order, and therefore there is a strong disorder and vacillation. For example, “monitor” headphones are often referred to as circumaural headphones, although sometimes “monitor” headphones are called flat frequency response, used to control sound quality during recording. Some people call “earbuds” or “inserts” all small headphones, others refer this term to erabuds. Also confusing is the term “overhead”, sometimes referring to all headphones, sometimes only to supra-aural.
Since in this article it is necessary to clearly separate the full-size headphones from the in-channel headphones, I will call them, making reservations if necessary.
The most popular scheme of emitters used in headphones is the well-known dynamic emitters with a moving coil.
The scheme of work is quite simple. A permanent magnet is fixed to the headphone case, creating a static magnetic field. Usually, ferrite magnets are used in frankly cheap headphones, in most middle and high-level models, neodymium has become the norm (although some manufacturers still emphasize neodymium magnets as an outstanding feature of their models).
In a static magnetic field there is a coil of wire through which alternating current passes, modulated by a sound signal. A change in current in a conductor leads to a change in the electromagnetic field surrounding it. In turn, the interaction of two magnetic fields (constant from the magnet and alternating from the coil) causes the coil to move. The coil is attached to a thin membrane mounted on an elastic suspension, and the movement of the coil causes the membrane to vibrate, which is transmitted through the air and perceived as sound.
Membranes are made of different materials, and this, of course, affects the sound. Cheap synthetic headphones use different synthetic polymer films, in the solutions of more expensive Mylar, cellulose and other materials are very popular. Some top models of headphones use titanium, although now there are FA-003Ti, offering speakers with a titanium membrane at an affordable price.
Here is an example of a typical dynamic headphone emitter.
This scheme with minimal changes is used in almost all modern headphones of any form factors, although there are sometimes surprising members of the family. For example, the AKG K1000, which is essentially not two headphones, but two small speakers suspended from the head. Thanks to the suspension, they can be rotated relative to the ears, adjusting the virtual scene, and due to the used emitters, a full-fledged amplifier for acoustic systems is required for their rocking. Now these headphones are discontinued, but in the secondary market they are sold at a price of about $ 1,300.
Despite the high prevalence, dynamic emitters have a number of disadvantages. The first of them is sometimes a low response rate to a change in sound, especially in the low-frequency region. This is especially true for headphones-inserts and earbuds. Since radiators with a diameter of 4 mm and somewhere up to 14 (usually from 7 to 10) are used there, it’s very difficult to make such a membrane play equally well at low frequencies and high frequencies. This led to the emergence of dynamic models with two emitters, the most popular with us are Fischer Audio Tandem, Ritmix RH-145 Pro and Brainwavz R1.
Also emitters of similar design suffer because of the non-uniformity of the magnetic field in which the coil moves, and the membrane itself with its suspension also in the real world adds unevenness and unpredictability to the sound.
Finally, another factor is the fairly large linear dimensions of the dynamic emitters. Anyway - to make a compact round membrane will not work.
An attempt to solve all these problems was the emergence of emitters with a balanced (balanced) anchor. They are “armature”, from their English name “balanced armature”.
These radiators were invented in the 20s of the last century, and because of their small size, they almost immediately began to be used in medicine for hearing aids. During World War II, they were also actively used in military telephones, which, thanks to the “fittings”, provided sound 20–40% louder than dynamic counterparts.
The device of these emitters is quite simple. As it is easy to guess from the name, they are based on a U-shaped anchor plate with a coil located on its axis. This plate is suspended (balanced) in a magnetic field in such a way that it can rotate in it. Then everything is simple: the sound signal is applied to the coil, a magnetic field arises, the anchor deviates from its usual position. Through the connection, the movement of the anchor is transmitted to the membrane, which gives the sound.
This scheme provides several advantages at once. Firstly, it is compact, the anchor emitters are made in a very small package, closed on all sides, except for a small sound guide. Due to this, headphones with such emitters can be made very small and use several emitters in one earpiece (about this below). In addition, due to the closed design, the sound of such emitters depends less on the shape and materials of the headphone casing. Secondly, the reinforcement is free from distortion caused by the shape of the diffuser due to the large mass of the armature itself. Thus, these emitters bypass the same dynamic in sensitivity, volume and clarity of sound. Also, usually (although not always), the reinforcement has a more even frequency response.
As usual, there are no pluses without minuses, a minus of these emitters is a narrower frequency range in comparison with “dynamos”. Manufacturers overcome this in many ways. Some tune emitters, achieving the maximum range (for example, one of the recognized market leaders in Phonak hearing aid with their Audeo headphone line). Other manufacturers combine several emitters in one earphone that are responsible for different parts of the frequency range. Manufacturers of the emitters themselves simplify this task by releasing dual models in one package. The arms race has reached incredible heights, and now there are headphone models with 4 drivers for one ear (Sony, Westone, and others), and in custom headphones made from ear casts, there are generally popular models from 6 to 10 drivers.
By the way, I wrote a review on AppleInsider about custom multi-driver headphones.
Recently, many companies have produced hybrid headphones in which a dynamic emitter for low frequencies is combined with a reinforcement for medium and high frequency ranges. So far I have listened to only one model, but, one has to admit, the results are interesting.
In headphones, it often happens that everything new is well forgotten old. The following types of radiators also happened: isodynamic and orthodynamic (sometimes the name “planar” is also encountered). About 30 years ago they were very popular, even in the USSR several models were produced with such emitters under the TDS brand, which now lovers of high-quality sound are looking for and restoring.
With the departure of the 80s, these emitters somehow “forgotten”, and gave way to a dynamic palm, but the last 5-7 years have seen a return of interest to them, they have firmly taken their places in the pantheon of top-end expensive models, primarily due to the efforts of companies HiFiMan and Audeze.
The principle of operation and iso-and orthodynamic models are the same, it differs only in the shape of the emitter itself. A conductive spiral film is deposited on a thin membrane, the membrane itself is placed between two magnetic grids. Then everything is traditional - a current is supplied to the spiral, the electromagnetic field of the coil interacts with the field of magnets, the membrane oscillates, sound is produced.
Here is the membrane of one of the most popular models of isodynamic headphones - Fostex T50RP.
Orthodynamic emitters work on the same principle, but are round.
In general, isodynamic headphones have a higher level of detail, but usually require a good amplifier to unlock their capabilities.
Another little-known type of radiator is electrostatic. The humor of the situation is that the first such models appeared in the 80s of the 19th century, but since the methods of electrical reproduction of signals were not yet available, they were used mainly for experiments with ultrasound. At that time, powerful magnets didn’t learn how to do it; therefore, the electrostatic technology gave the best results compared to dynamic emitters. Already in the early 20th century, with the advent of tube amplifiers, the first electrostatic sound reproduction systems used to sound theaters began to appear.
With the advent of normal ferromagnets, dynamic emitters burst forward, since electrostatics requires a very thin membrane, which was impossible to produce at that time.
Electrostats began to return to widespread use sometime since the 1950s, mainly in the form of large acoustic systems. In 1960, the Japanese company Stax released its first model of electrostatic SR-1 headphones, and now Stax and electrostatic headphones have become almost synonymous, although other companies also produce electrostatic, including Koss, AKG and Sennheiser (their Sennheiser Orpheus system is unhealthy $ 12,000)
How does this masterpiece of audiophile industry work? The membrane of the thinnest polymer is located between two grid electrodes coated with an insulating varnish. An acoustic signal is sent to these electrodes, although, unlike other types of emitters, with a very large voltage (from 100 to 1000 volts), an electric field arises, which causes membrane oscillations. Through the holes in the electrodes the sound comes out. Due to the need for high voltage, electrostatic headphones require special amplifiers (or special power-supply devices, which are essentially boost transformers).
Electrostatic headphones are expensive (some are simply unrealistic), require special equipment and are not designed for portable use (although there are exceptions). Instead, they have a large dynamic range: from 20 Hz to 20 KHz, and even more, very flat frequency response, small distortion, especially in the HF area, where the rest of the headphones are not so good, and a lot of other advantages.
Sometimes in the secondary market there are used electrostatic headphones in good condition, and this is the way to get them for sane money. For example, I bought very cheap vintage “gold” Stax SR-5 with an energy generator, and am very pleased with the purchase.
Here are the main types of emitters used in modern headphones. Some exotic solutions such as plasma emitters, piezoelectrics, and a number of others did not come into consideration, but they are perhaps worthy of a separate consideration if I gather enough material about them.
The article is based on materials .
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