Moving Coil Headphone Transducer

A moving coil headphone transducer, also known as a dynamic headphone, converts electrical energy into mechanical energy to produce sound. It relies on Faraday’s law of electromagnetism, which describes how a current flowing through a coil produces electromotive force (EMF). The assembly involves an extremely thin and light coil attached to a very light diaphragm. It has to be very light so that it can vibrate fast. The coil rests inside a magnetic channel where a powerful magnet produces the magnetic field and a soft iron frame directs it into the channel. The force produced is proportional to the amount of current flowing through the coil and therefore a fast varying audio signal will make the coil move, which in turn makes the attached diaphragm move. This fast movement (vibration) of the diaphragm produces sound in the air.

The physics is vey simple, and in most cases, the total mass of the diaphragm and the coil assembly are critical. The lighter they are, the faster they can vibrate, and this affects their higher frequency response. Modern developments use various techniques to reduce their weight. For example, one technique is the use of aluminium wire with copper cladding. Since aluminium is very light, this reduces the weight of the coil. They are also able to make the diaphragm lighter by using advanced polymer materials. Usually, headphones with a higher frequency response will cost more as they use materials that cost more to produce.

In this particular design, the iron frame focuses the magnetic field into the channel so that there are more lines of flux going through it.

Two of the most widely used technologies to produce the magnetic field are neodymium, and samarium cobalt magnets. A powerful magnet usually has a greater flux density, which results in better sound definition. The alternative, which also has the same effect, is to have more coil windings, however; that has the disadvantage of making the diaphragm heavier, and introducing a higher load resistance to the amplifier output.