Sabtu, 08 Agustus 2015

Headphone Driver Types


There are many Headphone Driver Types, the most common is Dynamic Driver, below is the list of other Drivers:


1. Moving Coil Drivers (Dynamic driver)
Moving coil drivers, or just regular dynamic drivers, the regular part of that nomenclature deriving from the fact that the vast majority of sound drivers in use today are this type, work by passing an electrical current through a coil of metal, called the voice coil and commonly made of copper or aluminium, which is located inside a matching sized cylindrical channel bored into a permanent magnet. The magnet exerts a continuous magnetic field on the voice coil, which is attached to a cone or dome of rigid material, called the diaphragm. The diaphragm is commonly paper for loudspeakers and plastic for headphones, but other materials such as Kevlar or metal can be used. As an electrical signal is passed through the coil, its magnetic field is altered, it has become an electromagnet. Differences in polarity between the permanent magnet and the charge of the voice coil will cause the two to attract or repel one another. As the coil is repelled it is pushed away from the permanent magnet causing the diaphragm to move outward. An opposite electromagnetic charge in the coil will cause the opposite effect and the diaphragm will move inwards. At rest (when no signal is passed) the voice coil does not have any charge and the diaphragm will sit at its rest position. This pushing and pulling of the diaphragm, precipitated by the changes in the charge of the coil is what produces the sound.

2. Electrostatic Drivers.
Electrostatic drivers work by suspending a sheet of ultra thin Mylar plastic, held under tension and coated in an electrically conductive material, between two perforated metal plates called stators. The Mylar diaphragm has a high biasing voltage applied over it and by inducing changes in the charge field applied to the stators, the Mylar is pulled toward one stator while being pushed away by the other, and vice versa for motion in the opposite direction. This means that because the driver is producing a mirror motive force, the audio output from an electrostatic driver is dipole, meaning that as much sound radiates outward from the driver in one direction as does in the other. Electrostatic drivers require their own special kind of amplification due to the power requirements needed to drive them.

3. Air Motion Transformers
AMT is the acronym for Air Motion Transformer. Invented by Dr Oscar Heil in 1973, an and known also the AVT or Air Velocity Transformer, the AMT moves air by pushing against itself rather then trying to push air directly. Comprising of a folded Mylar sheet, bonded to a series of Aluminium struts in a high intensity magnetic field, the diaphragm pushes back and forward against itself in a similar physical motion pattern to when an accordion is squeezed in and out to pump air though the reed chambers, albeit over an exceedingly smaller motive range. The result is a dipole driver with an extraordinarily rapid response rate, caused both by the extremely low mass of the Mylar driver, and by the far smaller motion range it undergoes on each “swing” compared to an dynamic driver. In this technical respect, it shares characteristics with the electrostatic driver. The air movement is roughly five times bigger than the movement of the membrane; therefore also the velocity of the air is five times larger and hence the name.

4. Balanced Armatures
With a balanced armature driver you still have a permanent magnet and a voice coil, however the voice coil is not fixed to the diaphragm. In a balanced armature driver the coil is, to scale, longer than the coil on a dynamic driver and is fixed in position just like the permanent magnet. The changes in magnetic field do not move the coil, rather they move an arm (this is the armature part of the driver) which is suspended within the centre of the voice coil. The interaction of the magnetic fields between the permanent magnet and the voice coil cause the armature to piston backwards and forwards. The motion generated in the armature by this system is only very small though, so to have the armature drive the diaphragm in a piston motion like with a dynamic driver is very inefficient. Instead then of pushing the diaphragm directly, the extruding end of the armature is connected by a hinge to a perpendicular driving rod. Audio signals are passed down the drive rod from the armature and the drive rod manipulates the diaphragm from a central axis. Balanced armatures are today only utilised for IEMs insofar as I am aware because they are easier to miniaturise well while preserving sound quality, however they have been used in the past for loudspeakers.

5. Orthodynamic Drivers
Orthodynamic drivers, sometimes called isodynamic drivers, are comprised of a thinly pressed disc made of tightly coiled fine aluminium wire affixed to a mylar sheet or more commonly an printed circuit. This pressed disc is the diaphragm and is usually slightly ridged or pleated in order to give it greater strength and to make it flex more readily in the direction of motion. The diaphragm is then sandwiched between two magnets which have the same polarity facing each other. The magnets are trying to repel from each other and so the whole assembly must be clamped together. The orthodynamic driver is not unlike a magnetic version of an electrostatic driver. Except that we don’t need a special amplifier for them. Indeed, orthodynamic drivers can be easier on amplifiers than moving coil drivers. Orthodynamic diaphragms, like electrostatic ones, can be made much lighter compared to dynamic diaphragms because they don’t need the same degree of structural rigidity. Also like an electrostatic driver an orthodynamic is a planar driver, so the motive force is applied over the entire diaphragm at once. Orthodynamic headphones have been out of fashion with headphone manufacturers for decades now and gods knows why because they can sound just marvellous. Fostex still makes some, but they are the lesser sons of their greater sires. They have their devoted legions of fans of course

6. Ribbon Drivers
Ribbon drivers have similarities with orthodynamic drivers, but only to the degree that dynamic and balanced armatures share similarities. Both of them are planar-magnetic drivers in that they both use a flat diaphragm manipulated by a magnetic field to generate sound. A ribbon driver though, rather than being a flat pressed coil in between two opposing sandwiched disc magnets like an orthodynamic, is a single solid Aluminium strip, held pressed in between two rows of bar magnets. As with the orthodynamic driver, changes in the magnetic field manipulate the Aluminium to generate sound. Oscar Heil’s original Air Motion Transformers were an augmentation of this ribbon technology. The only headphone which I know of that uses a ribbon driver is the ribbon prototype built by Head-Fi member Setmenu. Problems with Ribbon Drivers include exceptionally low impedances, often well below 1 ohm in some cases.

7. Piezoelectric Drivers
With everything above, the generation of sound is achieved by the manipulation of a diaphragm by a separate motive generator. In each case, a piece of plastic or metal is manipulated by a magnetic or electrostatic field generated by a mechanism external to the diaphragm. With a piezoelectric driver, the diaphragm itself is the motive generator. Electricity is passed through the diaphragm and the physical material of which the diaphragm is composed then alters in shape with the variance of the electrical signal. The piezoelectric effect was originally discovered the other way around when it was established that certain materials would generate electricity when they were squeezed.
n theory, this is one of the best possible ways to generate an audio signal; however the practical application is never as simple as the theory behind it. A driver comprised of a piezoelectric panel, as employed by Pioneer in the SE-700 does not have a lot of excursion, meaning that it cannot “swing” very far. As a result, low frequency sound becomes difficult to reproduce. Any planar drive system has a similar problem when compared to a moving coil system; however it is not possible to increase panel size for greater low frequency reproduction, as can be accomplished with electrostatic or magnetostatic speakers, obviously because one can only make a panel so big for a headphone before impracticalities set in. We have all seen the size of the Jansen panels used in the Jecklin Float electrostatic headphone. As well as this you cannot, unlike with an electrostatic system, tinker with driver design parameters such as strator spacing or bias voltage. In designing the H2, TakeT have resolved the low frequency problems of limited excursion by borrowing from the Heil design book and making a piezoelectric air motion transformer. An increase in plotted surface area and a preservation of the transient speed of a planar drive system, but without the need for the extraordinarily powerful and subsequently heavy magnets of traditional AMT drivers. They have also resolved the problems with the film of the Pioneer SE-700. The Pioneers film was too wobbly, which exacerbated its problems with bass reproduction. The H2 film is much stiffer; it has to be in order to maintain a Heil wave shape. Will these changes in design over its grandfather be of real sonic merit though?

8. Plasma Driver
contoh cans: PlasmaSonic


Source:
Headfi & Audiophile Indonesia

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