Handbook for Sound Engineers

504 Chapter 16

provide room for its movement. The cementing flat is
bonded to the face plate. A stiff hemispherical dome is
designed to provide adequate acoustical capacitance.
The coil seat is a small step where the voice coil is
mounted, centered, and bonded on the diaphragm.

Early microphones had aluminum diaphragms that
were less than 1 mil (0.001 in) thick. Aluminum is
light-weight, easy to form, maintains its dimensional
stability, and is unaffected by extremes in temperature
or humidity. Unfortunately, being only 1 mil thick
makes the diaphragms fragile. When it is touched or

otherwise deformed by excessive pressure, an

aluminum diaphragm is dead.

Mylar™, a polyester film manufactured by the

DuPont Company, is commonly used for diaphragms.

Mylar is a unique plastic. Extremely tough, it has high

tensile strength, high resistance to wear, and outstanding

flex life. Mylar™ diaphragms have been cycle tested

with temperature variations from 40°F to +170°F

(40°C to +77°C) over long periods without any

impairment to the diaphragm. Since Mylar™ is

extremely stable, its properties do not change within the

temperature and humidity range in which microphones

are used.

The specific gravity of Mylar™ is approximately 1.3

as compared to 2.7 for aluminum so a Mylar™

diaphragm may be made considerably thicker without

upsetting the relationship of the diaphragm mass to the

voice-coil mass.

Mylar™ diaphragms are formed under high tempera-

ture and high pressure, a process in which the molecular

structure is formed permanently to establish a dimen-

sional memory that is highly retentive. Unlike

aluminum, Mylar™ diaphragms will retain their shape

and dimensional stability although they may be

subjected to drastic momentary deformations.

The voice coil weighs more than the diaphragm so it

is the controlling part of the mass in the diaphragm

voice-coil assembly. The voice coil and diaphragm mass

(analogous to inductance in an electrical circuit) and

compliance (analogous to capacitance), make the

assembly resonate at a given frequency as any tuned

electrical circuit. The free-cone resonance of a typical

undamped unit is in the region of 350 Hz.

If the voice coil were left undamped, the response of

the assembly would peak at 350 Hz, Fig. 16-33. The

resonant characteristic is damped out by the use of an

acoustic resistor, a felt ring that covers the openings in

the centering ring behind the diaphragm. This is analo-

gous to electrical resistance in a tuned circuit. While

Figure 16-31. A simplified drawing of a dynamic
microphone. Courtesy Shure Incorporated.

Figure 16-32. Omnidirectional diaphragm and voice coil
assembly.

Pole piece

Voice coil

Magnet

Diaphragm

Hinge point

Spacer

Cementing

flat

Tangential compliance section

Dome

Coil seat

Voice coil

7/8"

B. Top view

A. Sectional view

Figure 16-33. Diaphragm and voice-coil assembly response

curve.

Frequency—Hz

Damped

Undamped

40 50 100 200 500 1 k 2 k 5 k 10 k 20 k

+ 10

0



10