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 neodymium 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. The fast movement (vibration) of the diaphragm produces sound in the air.
The most critical aspects of the assembly is the total mass of the diaphragm and the coil. The lighter they are, the faster they vibrate, which affects their higher frequency response. All modern developments have been to reduce their weight. For example, one improvement was in 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 utilise materials that are more advanced.
The soft iron frame directs the magnetic field into the channel so that there are more lines of flux going through it.
Many high-end headphones employ the neodymium magnet, which is more powerful and has a greater density of flux.