Bruel & kjaer 4111 User guide

4111

INSTRUCTIONS

AND

APPLICATIONS

Condenser

Microphone

Type

4111

Specially

designed

for

measurement

and

calibration

applications,

this

Condenser

Microphone

features

very

high

stabilit

y, a

high

linea

r

frequency

response

together

with

small

dimension

s.

BHUEL

&

KJJEK

Naerum,

Denmark

.

Phone

Naerum

500.

Telegrams:

BRUKJA,

Copenhagen

Beat frequency oscillators

Low frequency oscillators

Deviation

test

bridges

Strain gauge apparatus

Frequency measuring bridges

Distortion measuring bridges

Heterodyne voltmeters

Audio

f~uency

spectrometers

Frequency analyzers

Level

recorders. a. c. and d. c.

Automatic frequency response

recorders

Automatic

A.

F.

spectographs

Logarithmic potentiometers

Noise level potentiometers

Recording paper

Polar diagram recorders

Vacuum tube voltmeters

Wide range vacuum tube

voltmeters

Diode voltmeters

D.

C.

voltmeters

Megohmmeters

~icrophone

amplifiers

Impact sound generators

Acoustic standing

wa.,.

apparatus

Artificial ears

Condenser microphones

Microphone calibration

apparatus

Vibration pick-ups

Integration networks

Rotary selectors

Universal selectors

Freqllency response tracers

Conductivity meters

Electronic counters

Counting

rate

meters

Impulse generators

Portable dose

rate

meters

Portable counting

rate

meters

G.

M.

tubes·& accessories

CONTENTS

Page

Description

Connections

9

Measurements

of

Sound

Pres

sure

Level

. . . . . . . . . . . . . . . . . .

13

Noise

Measurements

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . • • .

20

Control

of

Frequency

Response

and

Sensitivity

of

Condenser

Microphone

4111................

23

Calculation

of

Sensitivity

. . • . . . . . . . . . . . . . . . . • . . . . . . . . . . .

29

DESCRIPTION

The

Condenser

Microphone

type

4111

is

particularly

designed

for

mea-

surement

applications

and

has

therefore

a

very

high

stability

and

com-

paratively

small

dimensions.

It

is

built

up

of

a

cartridge

directly

con-

nected

with

a

cathode

follower

tube,

making

it

possible

without

any

dif-

ficulty

to

use

long

cables

between

the

microphone

and

the

following

amplifier.

The

polarization,

anode

and

filament

voltages

to

the

ca-

thode

follower

and

cartridge

are

all

taken

from

the

amplifier,

which

is

why

only

Microphone

Amplifier

type

2601

and

Frequency

Analyzer

type

2105

or

2109

can

be

used.

Fig.

1.

Photo

of

Condenser

Microphone

type

4111.

All

the

above

mentioned

apparatus

is

proviC::.ed

with

a

special

built-in

rectifier,

w

hich

delivers

d.

c.

current

for

filament

to

the

cathode

fol-

lo

wer

in or

der

to

retain

a

low

hum

level.

T he

cartridge

is

mounted

in

the

end

of

a

tube

(diameter

36

mm,

length

9u

mm),

which

also

contains

the

cathode

follower

with

componeq.ts.

2

Fig.

2

shows

an

exploded

view

of

the

instrument.

The

other

end

o.

the

tube

tapers

and

continues

as

a

self-supporting

goose

neck

with

a

length

of

400

mm.

This

terminates

in

a

7-pin

plug,

directly

fit-

ting

the

socket

marked

"Condenser

Microphone"

on

the

amplifier

or

analyzer.

It

is

thus

possible

to

mount

the

microphone

directly

on

the

above

mentioned

3

pieces

of

apparatus.

By

bending

the

goose

neck

the

microphone

can

then

be

placed

in

any

desired

position.

Fig.

2.

Explodea

view

of

Condenser

Microphone

type

4111.

The

sensitivity

of

the

microphone,

about

2

mV

per

fbar,

is

compara-

tively

high,

particularly

when

taking

into

acc..ount

its

small

dimens-

ions

and

high

re~onance

frequency.

The

accurate

sensitivity

of

each

individual

microphone

as

well

as

the

total

frequency

response

charac-

teristic

are

shown

on

a

recording

in

the

back

cover,

made

individually

for

each

r."licrophone

delivered.

Fig.

3

shows

an

arbitrary

example

of

such

a

calibration

curve.

Both

the

sensitivity

(ex,pressed

in

mV/fbar

(==dynes/

em

2)

or

decibels

below

1

volt/fbar)

and

the

frequency

response

are

valid

for

the

sound

waves

of

a

free

sound

·

field

impinging

normal-

3

ly

on

the

microphone

membrane,

with

the

protective

grid

removed.

(A

correction

for

the

influence

of

this

grid,

which

under

routine

mea-

surements

should

be

employed

in

all

cases,

follows

on

p.

5 ).

The

straight

curve

from

15

to

20.

ooo

c/s

is

the

response

of

the

cathode

follower

alone

(with

a

capacity

of

5.

ooo

pF

across

the

input).

The

factor

k

is

the

distance

of

the

membrane

above

the

edge

of

the

car-

tridge,

which

is

a

necessary

datum

when

checking

the

sensitivity

by

:neans

of

the

Electrostatic

Actuator

type

4113

(see

p.

24).

11

..

w,_

;

,......_,.

,.......

.,.

IIP4_,.

c

adlo4e

t.ll

--

~h

lctl

utr

l

4te

It

c.ca4

.

*···

·

-

~

.

~

~~~.::~:f!J;:r

;

;;~yr.:.~

;

~

:

.~~

:

-~;;

~

M.

POCMt

!.mettor:

l$4!.

DM•

11

1:6 •

$S

SI

I"·

...

Ief8

.

Fig.

3.

Example

of

an

individual

calibration

·

curve

for

the

microphone

fitted

with

a

specified

cartridge

and

with

a

specified

polarization

voltage.

The

microphone

is

adjusted

so

that

its

frequency

response

characte-

ristic

is

as

flat

as

possible

for

zero

angle

of

incidence.

The

fre-

quency

range

is

from

20

c/s

to

16.ooo

c/s,

of

which

the

range

30

c/s

to

10

kc/s

is

linear

to

within

±2

db.

The

f.i

rst

resonance

fre-

quency

is

about

11

kc/s.

When

a m i

crophone

is

placed

in

a

free

sound

f~~ld,

a

pressure

in-

crease

on

the

microphone

membrane

is

got,

co¢pared

with

the

sound

4

pressure

of

the

free

travelling

sound

wave,

on

account

of

the

scat-

tering

effect

of

the

microphone

at

higher

frequencies,

dependent

on

the

wave

length

of

the

sound

and

the

dimensions

of

the

microphone.

This

pressure

increase

for

normally

impinging

sound

waves

is

given

in

db

in

fig.

4.

In

order

to

obtain

a

flat

response

in

a

free

sound

field,

i.e.

a

constant

electrical

output

of

the

microphone

versus

fre-

quency,

for

a

constant

free

field

sound

pressure

prior

to

the

intro-

duction

of

the

microphone

in

the

field,

the

frequency

characteristic

of

the

microphone

measured

under

constant

pressure

conditions,

where

a

constant

sound

pressure

is

applied

to

the

diaphragm,

must

decrease

for

frequencies

higher

than

Z.ooo

c/s.

d6

1D

s

0

-s

-10

0 50

100

200

1000

ell

2k

5k

IOk

Fig.

4.

Pressure

increase

on

microphone

diaphragm

due

to

diffraction

around

a

Condenser

Microphone

type

4111

mounted

in

a

free

sound

field

{with

protective

gricJ

removed

and

with

the

sound

wa

v

es

i

mpinging

normally).

The

actuator

measurement

as

described

on

page

24

for

the

determi-

nation

of

the

frequency

characteristic

simulates

such

constant

pres-

sure

conditions.

Therefore,

if

that

measuring

procedure

should

be

employed

for

checking

the

microphone,

fig.4

should

be

added

alge-

braically

to

the

resulting

recorded

curve

in

order

to

obtain

the

free

5

field

response

of

the

microphone.

-

-----

- - - -- -

--

-

J 0

()

0

()

0 u

()

u 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 c

~

-f-

-

·-

db~

=

=\

:_

..

-

--!

- - . -

_-,- _

__:

_

~

-10

'b·/

-15 0 50

100

200 500

1000

CIS

2k 5k

10k

15k

20

'J 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 c

Fig.

5.

Corrections

to

be

added

to

the

calibration

curve

of

fig.

3

to

find

the

sensitivity

and

frequency

response

under

a)

free

field

measurements,

l)

degree

(normal)

incidence

with

protective

grid,

b)

free

field

measurements,

90°

(grazing)

inc

ide

nee,

with

protective

grid,

c)

free

field

measurements,

random

incidence,

with

protec-

tive

grid.

The

frequency

response

of

fig.

3

was

valid

in

a

free

sound

field

with

normally

impinging

sound

and

protective

grid

removed.

Fig.

5,

a,

b

and

c

show

three

corrections

which

should

be

algebraically

added

to

this

curve

or

to

the

corresponding

curve

valid

for

the

microphone

de-

livered,

and

placed

in

the

back

cover

.

in

order

to

get

the

right

ca-

libration

curve

under

the

following

measuring

procedures:

1)

Free

field

measurements,

0

degree

incidence

with

protective

grid.

Fig.

5a

indicates

the

diffe

renee

in

sensitivity

for

the

microphone

with

and

without

grid.

6

2)

Free

field

measurements,

90

degrees

incidence

with

protective

grid.

Fig.

Sb

indicates

.the

difference

in

sensitivity

for

the

micro-

phone

with

grid

under

90

degrees

incidence

and

without

grid

for

nor-

mal

incidence.

3)

Measurements

in

diffuse

sound

fields

with

random

incidence.

Fig.

Sc

indicates

the

difference

in

sensitivity

for

random

incidence

with

grid,

compared

with

0

degree

incidence

without

grid.

These

coJ.·rections

are

only

dependent

on

the

geometrical

form

of

the

microphone,

and

may

therefore

be

assumed

constant,

and

used

for

finding

the

corresponding

sensitivity

for

any

microphone

delivered,

for

which

the

free

field

0

degree

incidence

frequency

response

is

given.

"

'J

0 Cl 0 0 0 0 0

lJ

0 r l 0

()

0 0 0 0 0 0 0 0 0 U

rJ

0

Cl

D

()

0

t-1-t--

dbr

-H

- - -

-

r-

- - I

.:1

I I

-

t--t

--

0o 50

100

200 50(1

1000

CIS

u u u 0 0 0 0 0 0 0 0 0 0 0 0 0 2k

0 0

l

ll..I""""""~.LU,;l--1~

~.

•.

CI,

l

It

li

5k

10k

15k

20

0 u 0 u 0

()

(I (I

l 1 U

()

l'

Fig.

6.

The

corrections

of

fig.

5,

a,

b,

c,

performed

on

the

arbitrary

example

of

a

calibration

curve

of

fig.

3.

The

result

of

these

three

corrections

carried

out

on

the

curve

of

fig.

3

is

shown

in

fig.

6,

where

curve

a

represents

the

free

field

0°

incidence

sensitivity

as

a

function

of

frequency,

curve

b

the

free

field

7

90°

incidence

sensitivity,

curve

c

the

random

sensitivity.

The

directional

characteristics

(as

recorded

with

the

aid

of

the

Polar

Diagram

Recorder

type

2370)

of

Condenser

Microphone

4111

are

given

in

fig.

7

for

frequencies

up

to

9

kc/s.

Once

again,

the

geometrical

form

of

the

microphone

determines

the

shape

of

these

curves

principal-

ly,

so

that

they

can

be

used

for

any

type

4111

microphone.

The

curves

are

taken

with

the

protective

grid

mounted

on

the

cartridge.

Fig.

7.

Di

'

rectional

characteristics

of

Condenser

Microphone

type

4111

with

protective

grid

mounted.

The

sensitivity

s

mV/fbar

as

indicated

on

the

graph

in

the

back

cover

is

the

electrical

output

.after

the

cathode

follower

for

a

free

sound

wave

of

pressure

level

l

fbar,

impinging

perpendicularly

on

the

dia-

phragm.

This

output

results

initially

from

the

voltage

as

generated

by

the

cart

ridge,

which

is

attenuat<::d

in

two

ways.

First

there

is

a

loss

of

sensitivity

owing

to

the

input

capacity

of

the

cathode.

fol-

BRUEL & KJAER 4111 User guide

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