Handbook for Sound Engineers

1424 Chapter 37

with the sleeves if they get lost, stolen, or are in need of
repair. A final option, for users of the ear-bud type of
earphone, is a rubber boot that fits over the earphone.
This option typically has the poorest isolation.

If there is ever a problem with a universal earphone,
another set can be substituted with no negative reper-
cussions. A custom molded earphone does not allow for
this kind of versatility; if one needs repair, the only
alternative is to have a backup to use in the interim.

IMPORTANT NOTE: There are several brands

of custom molded earplugs with internal filters

that have relatively flat frequency response and

different levels of attenuation. Although it may be

physically possible to make universal earphones

fit into the plugs with the filter removed, this is not

advised. The location of the earphone shaft in the

ear canal is crucial to obtaining proper frequency

response, and most earplugs will prevent them

from getting in the proper position. Once again,

custom molded earplugs are NOT an acceptable

alternative to custom sleeves.

37.6.2 Earphone Transducers

The internal workings of earphones vary as well. There
are two basic types of transducer used in earphone
design—dynamic and balanced armature.

The dynamic types work on the same principle as
dynamic microphones or most common loudspeakers. A
thin diaphragm is attached to a coil of wire suspended in
a magnetic field. Diaphragm materials include Mylar (in

the case of dynamic microphones) or paper (for loud-

speakers). As current is applied to the coil, which is

suspended in a permanent magnetic field, it vibrates in

sympathy with the variations in voltage. The coil then

forces the diaphragm to vibrate, which disturbs the

surrounding air molecules, causing the variations in air

pressure we interpret as sound. The presence of the

magnet-voice coil assembly dictates a physically larger

earphone. Dynamic transducers are commonly used in

the ear-bud types, but recent technological advances

have allowed them to be implemented in universal

designs. They are also found in some custom molded

earphones.

Originally implemented in the hearing aid industry,

the balanced armature transducer combines smaller size

with higher sensitivity. A horseshoe-shaped metal arm

has a coil wrapped around one end and the other

suspended between the north and south poles of a

magnet. When alternating current is applied to the coil,

the opposite arm (the one suspended in the magnetic

field) is drawn towards either pole of the magnet, Fig.

37-13. The vibrations are then transferred to the

diaphragm, also known as the reed, usually a thin layer

of foil. Balanced armature transducers are similar to the

elements used in controlled magnetic microphones. In

addition to the increased sensitivity, they typically offer

better high-frequency response. Achieving a good seal

between the earphone and the ear canal is crucial to

obtaining proper frequency response.

A further subdivision occurs with the use of multiple

transducers. Dual transducer (dual driver) earphones are

the most common. Another example of a loudspeaker

Figure 37-12. Shure SCL5 universal earphone. Courtesy
Shure Incorporated.

Figure 37-13. Dynamic and balanced armature transducer.

Courtesy Shure Incorporated.

Pin N Magnet

S

Coil

Moving

end

Diaphragm

Armature

Fixed end

Diaphragm

Voice coil

Magnet