Western Electric 618A Instructions For Use Manual

Western Electric

1VIICROPHQNE

No. 618A, Moving

Coil Type

and Associated Apparatus

INSTRUCTIONS

FOR

USE

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Moving

Coil

Type

Microphone

and

Associated Apparatus

No. 6

i

8A

Instructions

for Use

GENERAL

In radio broadcasting, sound recording

and public address systems a

microphone is needed which has

the utmost

fidelity of pick -up,

inherent quiet-

ness, stability and mechanical ruggedness. These requirements

are met by

the Western Electric No. 618A Moving

Coil

Type Microphone. This

micro-

phone

has a low electrical impedance and may

be

connected

to

its

amplifier

by means of a suitable transformer; it may be used at

a considerable distance

from

an associated amplifier.

The microphone may

be

mounted in

a

suspension, table,

or floor

type of

mounting; it is shown in Figure 1.

DESCRIPTION

No.

618A Microphone

The

microphone has

a thin duralumin diaphragm of

low

mechanical stiff-

ness, which

is clamped around

its

outer

edge.

To

this

diaphragm

is attached

a coil of

aluminum ribbon. Motion of

the diaphragm causes the coil to

move in

the magnetic

field in which it is located. The

magnetic field is

maintained

by

a permanent magnet

made from

a

high grade cobalt

steel alloy.

The

use

of

the permanent

magnet eliminates

the

necessity for

supplying a polarizing:

voltage

to the microphone.

Motion of the

coil

in

the magnetic field, caused by sound pressures acting

on

the

diaphragm,

induces

a voltage in the coil which is proportional to the

velocity

with which

it moves. A number of air chambers and slot openings

connecting

them

have

been associated with the diaphragm in order to obtain

substantially

uniform response

over a

frequency range

of

from

35 to

9500

cycles

per second.

One of these acoustic elements,

in

addition to exerting

a

control

on the motion of

the diaphragm, allows air to be transferred

from the

front

to the back of

the diaphragm.

This

eliminates effects due to

changes in

barometric

pressure.

[1]

Figure 1 --No.

618A -13

Moving Coil

Type

Microphone

The design and principles

of operation

of the

No. 618A

Microphone are

such that

it is not affected

appreciably

by any changes

in temperature

likely

to be encountered

in its use.

The electrical impedance

of the

No. 618A

Microphone at

low frequencies

is

approximately

28 ohms with a phase

angle

of nearly zero

degrees.

The

impedance

rises gradually

to about 38

ohms with a phase

angle of about

20

degrees at

10,000 cycles per second.

The efficiency

of this microphone

is

higher than that

of the conven-

tional

form of condenser

microphone.

For the condition

of a person

speaking

with normal conversational

intensity at a

distance

of 3 feet

from the micro-

phone, an

output

level

of

-84 db

relative to a

zero level of 6

milliwatts

is

representative.

A speaker

need not be directly

in front of a No.

618A

Microphone.

If

he

talks toward it,

he may even be at

right angles to

the front of

the microphone

without

great

loss

of

efficiency.

The same

is true of

orchestral

instruments

and other

program sources.

Nos.

17A, 18A -13

and 19A -13 Microphone

Mountings

The

No.

17A Type Mounting

illustrated in

Figure 2

is intended for suspen-

sion

from

a boom.

In this mounting the

face of the

microphone

may be

ad-

justed in a horizontal

plane.

The No. 18A -13 Mounting

illustrated

in Figure 3

is

a table

type

mounting

and has an oxidized bronze

finish. It has a

fixed overall

height

of 13

inches

[2]