MOVING-IRON HEADPHONES
The so-called magnetic type (moving iron) headphones use the same basic principle as was originally adopted for telephone earpieces or receivers. As shown in Fig. 3.6 a thin flexible soft iron diaphragm is supported by the casing just clear of the tips of two soft iron pole-pieces which are attached, in turn, to a small permanent magnet. Magnetic attraction causes the diaphragm to be distended slightly towards the pole tips, while still leaving a clearance of something less than a half-millimeter.
The incoming audio current passes through coils wound around the pole-pieces. In the presence of an audio signal the varying magnetic field created by the current interacts with that from the permanent magnet, varying the pull ...
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