Audio Engineering

Headphones 735

25.3.2 Moving Coil

Moving-coil headphones ( Figure 25.3 ) work in exactly the same way as moving-coil
loudspeakers. A coil of wire, suspended in a radial magnetic fi eld in an annular magnetic
gap, is connected to a small radiating cone.

When an alternating audio signal is applied to the coil, the coil vibrates axially in
sympathy with the signal, recreating an analogue of the original wave shape. The cone
converts this into corresponding fl uctuations in air pressure, which are perceived as sound
by the listener’s nearby ears. Figure 25.4 shows a cut-away view of a Sennheiser unit.

The major difference, of course, between moving-coil headphones and loudspeakers is
that the former are much smaller, with lighter and more responsive diaphragms. They
can consequently sound much more open and detailed than loudspeakers using the
moving-coil principle. They are usually also much more sensitive, which can mean that,
in addition to reproducing detail in the signal that is inaudible through loudspeakers, they
can also reproduce any background noise more clearly, particularly power amplifi er hiss,
which is not reduced when the volume is turned down. This is not peculiar to moving-coil
headphones and can occur with any sensitive headphone.

‘Stiff’ diaphragm

made of ‘soft’ iron

Permanent magnet

Coils ‘Soft’ iron pole pieces

Input

N

S

Figure 25.2 : Moving-iron headphone. The current fl owing in the coil either strengthens or

weakens the force on the soft iron diaphragm. AC audio signals thus vibrate the diaphragm

in sympathy.