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5 - ELECTROMAGNETIC AND ELECTROSTATIC TRANSDUCERS

5.1 - THE ELECTROMAGNETIC TRANSDUCER

The electrodynamic moving coil microphone - “A conducting wire moving in a magnetic field creates an electromotive force in the wire” - the discovery of electromagnetic induction, described by Faraday in 1831, and its quantification by Lenz in 1833, led the way to the development of the electromagnetic transducer. However, it wasn’t until the development of the electronic amplifier in the early 1900s, that this type of transducer could be used as part of the recording chain. Most microphones with electromagnetic coupling are called electrodynamic if they generate an electromotive force by moving a conductor in a magnetic field - moving coil and ribbon microphones are examples of this type of transducer.

In an electrodynamic moving coil microphone, a light-weight diaphragm is rigidly attached to a coil, sometimes called the ‘voice coil’. The mechanical suspension of the diaphragm and voice coil, in the form of corrugations around the diaphragm, allows theoretically only for a piston-like motion of the voice coil in a strong magnetic field produced by a permanent magnet. The permanent magnet is made from an alloy of steel called Ticonal or more recently Neodymium, containing aluminium, cobalt and nickel. A cross-section of a moving coil electrodynamic microphone capsule is shown in Figure 15.



Figure 15 - Section through an Electrodynamic Moving Coil Microphone Capsule
Variations in acoustic pressure set in motion the diaphragm and coil, the movement of the coil in the mainly uniform magnetic field surrounding the coil generates an electromotive force which is proportional to the velocity of movement. Figure 16 shows the component parts of an electrodynamic moving coil microphone capsule.



Figure 16 - The Diaphragm and the Moving Coil Assembly
of an Electrodynamic Moving Coil Microphone (Beyer)