Audio Engineering

734 Chapter 25

signals feeding two front-positioned loudspeakers to give an all-round effect, with sounds
appearing to come from behind the listener as well as in front ( Figure 25.1 ). Surprisingly,
the system worked, but only when the listener’s head was held within a very limited space
in front of the speakers. Predictably, the idea did not catch on.

25.3 Headphone Types ..................................................................................................

25.3.1 Moving Iron

Early headphones relied on many turns of very fi ne wire wound on to a magnetic yoke
held close to a stiff disc made of a “ soft ” magnetic alloy such as Stalloy ( Figure 25.2 ).
A permanent magnet pulled the thin disc toward the yoke with a constant force and
audio signals fed to the coil caused this force to vary in sympathy with the input. They
were very sensitive, needing hardly any power to drive them, and were very poor in
sound quality due to the high mass and stiffness of the diaphragm, which caused major
resonances and colorations—not to mention distortions due to magnetic nonlinearities.

Currently used in telephone receivers, they are not found today in any application
requiring high-quality sound. That is reserved for other more advanced techniques.

Input buffer

Input buffer

Left input Left ouput

Right input Right ouput

Filter

Filter

Filter

Filter

Delay

circuit

Delay

circuit

Figure 25.1 : Block diagram of a headphone cross-blend circuit with delay.