Brüel & kjær 4131/32 User guide

One-inch

Condenser Microphones

Microphone

Cartridges Type

4131/32

Cathode Followers Type

2612/2613,

2630

Accessories

Precision

condenser

microphones

for

measurement

purpose

. Each

microphone

is

individually

cali-

brated.

Frequency

range

of

calibration

20-

20ooo

c/

s.

The

4132

fulfils

the

requirements

of

the

American

Standard

A.S.A. Z. 24.8-1949.

BHUEL

&

KJ~H

·

Nrerum,

Denmark

.

ey[;'

80

05 00

.

~

BRUKJA,

Copenhagen

. Telex;

5316

4131.4132

I

/

One-inch

Condenser Microphones

AUGUST

1962

Contents

Condenser Microphones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Description

of

the

Equipment

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Principle

of

the

Condenser

Microphone

. . . . . . . . . . . . . . . . . . . . . . . . . . 3

Definition

of

Free-Field

and

Pressure

Response

. . . . . . . . . . . . . . . . . . . . 6

Random

Incidence

Response

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 'l

Description:

Microphone

Cartridges

4131-4132

. . . . . . . . . . . . . . . . . . . . 7

Cathode

Followers

2612-2613 . . . . . . . . . . . . . . . . . . . . . . . . 9

General Characteristics:

1.

Sensitivity

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

13

2.

Frequency

Response

.......

.

......

..

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

14

3.

Phase

Characteristics

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

15

4.

Free

Field

Corrections

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

15

5.

Directional

Characteristics

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

16

6.

Dynamic

Range,

Noise

and

Distortion

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

18

7.

Equivalent

Volume

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

19

8.

Influence

of

the

Polarization

Voltage

. . . . . . . . . . . . . . . . . . . . . . . . . . 19

9.

Temperature

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

10.

Static

Ambient

Pr

ess

ure

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

21

11.

Magnetic

Fields

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

12.

Vibrations

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

22

13.

Humidity

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

23

Modification

of

the

pressure

equalization

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

24

Comparison

Type

4132 -

Type

WE

640

AA

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

25

Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Random

Incidence

Corrector

UA 0055

..

.

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

26

Wind

Screen

and

Nose

Cone,

UA 0050, UA 0051

...•................

28

Microphone

Stands

UA 0026,

UA

4122

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

31

Extension

Cabl~s

AO

0027-28-29,

AR 0001

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

31

Adaptor

UA

0030

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

32

Flexible

Extension

Rod

UA

0039

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

32

Couplers

DB

0138-160-161

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

32

Two-Channel

Selector

4408

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

33

Microphone

Power

Supply

2801

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

,

...

34

Cathode

Follower

2630

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

.

........

35

Microphone Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

(Pistonphone

4220,

Microphone

Calibration

Apparatus

4142,

Electrostatic

Actuator

UA

0023,

Noise

Source

4240)

Cathode

Followers

Part Numbers

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

41

Condenser Microphones

General.

Description

of

the

Equipment.

A

comp

l

ete

microphone

consists

of

a

microphone

cartridge

and

a

cathode

follower

for

impedance

conversion,

allowing

long

cables

and

relatively

low

input

impedance

amplifiers

to

be

used

between

the

microphone

and

the

measuring

instrument.

The

microphone

cartridge

is

screwed

onto

the

housing

of

the

cathode

follower

making

a

small,

rugged

unit.

Fig. 1.

Microphone

Cartridge

Cathode Follower

L - / Gooseneck Connecting Plug

----

-

I~

rrr

I2J:s;miliilliiiiiii\liil\\ili\il\iLliiiliiiliiilii

IIIli

:tl

I 6

-

---

-=---

1

59203

Fig.

1.

Complete

Microphone,

consisting

of

Microphone

Cartridge

Type

4131

or 4132

and

Cathode

Follower

Type

2612.

The

microphones

may

be

directly

connected

to

the

different

B&K

measuring

in

strumen

ts

which

are

provided

with

a

"Condenser

Microphone"

input

socket

fitting

the

microphone

connecting

plug.

Stabilized

plate

and

heater

voltages

for

the

cathode

follower

and

polarization

voltage

for

the

cartridge

are

avail-

able

on

this

seven-poled

socket.

The

microphones

may

also

be

used

with

other

equipment

when

operated

from

a

Microphone

Power

Supply

Type

2801

which

provides

the

necessary

voltages

in

the

same

conditions

as

the

measur-

ing

instruments,

or

with

the

Battery-driven

Cathode

Follower

Type

2630

(see

Accessories).

For

free-field

and

room-acoustic

measurements,

the

micro-

phones

should

be

placed

remotely

from

the

measuring

instruments,

since

these

would

often

disturb

the

sound

field

because

of

their

non-negligible

dimensions.

Different

types

of

microphone

stands

and

extension

cables

are

available

for

this

pu~pose

,

see

Accessories.

An

examp

le

of

measuring

equip-

ment

is

shown

on

Fig.

2.

Principle

of

the

Condenser Microphone.

A

condenser

microphone

consists

essentia

lly

of

a

thin

metallic

diaphragm

mounted

in

close

proximity

of

a

rigid

back

plate.

Diaphragm

and

back

plate

3

2812

,

UA

0028

4131

.

(4

132

)Microphone cartridge

2612

Cathode

follower

stand

Measuring

amplifier

159Z01

Fig.

2.

Typical

measuring

arrangement

for

sound

measurements:

one-inch

microphone

equipped

with

"goose-neck"

cathode

follower

connected

to

a

B & K Microphone

Amplifier.

are

electrically

insulated

from

each

other

and

constitute

the

electrodes

of

a

capacitor

.

See

Fig.

3.

Housing

and

insulator

form

with

the

diaphragm

a

closed

chamber,

which

is

only

in

communication

with

the

outside

for

slow

static

ambient

pressure

variations.

When

the

microphone

is

exposed

to a

sound

pressure,

the

diaphragm

is

submitted

to

an

alternating

force

proportional

to

the

pressure

and

the

diaphragm

area.

The

consequent

move-

ment

of

the

diaphragm

varies

the

capacity,

and

these

variations

are

trans-

duced

into

an

AC

voltage

component

if

a

constant

charge

is

present

between

the

electrodes.

The

charge

is

obtained

by

means

of

a

stabilized

DC

polari-

zation

voltage,

and

it

remains

constant

as

long

as

the

charging

time

constant

of

the

circuit

is

much

long

er

than

the

period

of

the

sound

pressure

variations.

It

is

possible

by

careful

design

to

maintain

the

proportionality

of

the

AC

output

voltage

to

the

sound

pressure

within

a

wide

frequency

range

and

an

extended

dynamic

range.

The

widest

linear

frequency

range

for

the

pressure

response

is

obtained

if

the

resonance

of

the

mechanical

system

(diaphragm)

is

critically

damped.

This

is

realized

by

adjusting

the

mechanical

damping

of

the

air

cushion

between

electrodes

by

means

of

holes

in

the

back

plate.

The

low

frequency

limit

of

the

lin

ear

range

is

set

by

the

"cut-off"

(or

time

constant)

of

the

microphone

cartridge

circuit.

Referring

to

the

schematic

diagram

of

Fig.

4

the

cut-off

frequency

is

equal

to:

2

.n

(C

+ C + C ) Rt

Re

S 1 Rt +

Re

4

Since

the

semitivity

of

th

e

microphon

e is

determined

by

the

re

lative

variation

of

capacity

L1

C/

(C

+

Cs

+ Ct),

the

total

parallel

capacity

should

be

made

as

sma

ll

as

possibl

e.

The

first

amplifier

stage

is

therefore

built

in

the

same

housing

as

the

microphone.

A

cathode

follower

stage

is e

mployed,

because

s

mall

parallel

capacity

requires

high

R1

and

Re

in

order

to

obtain

a

satis-

factory

low

limiting

frequency.

Dia

Back

plate

Hole for

static

pressure

equalization

Fig. 3.

Schematic

co

nstruction

of

a

condenser

microphone

cartridge.

Mechanical

iH

.·

..

Cartridge

Electrical

Mass

Mechamcal Damping

Cathode

Follower

162161

Polarization

Circuit

C =

Polarized

cartridge

capacity

AC

= Variation of capacity

Cs

=

Stray

capacity

of

connection

to

cathode

follower

Ri,Ci

= Input impedance of

cathode

follower

Fig.

4.

Simplified

diagram

of

the

Condenser

Microphone.

(The

electrical

leakage

in

the

cartridge is neglected).

Definitions

of

Free-field

and

Pressure

Response.

The

Free-field

Response

of

a

microphone

is

the

ratio

of

the

1:.m.s.

output

voltage

to

the

r.m.s.

sound

pressure

existing

in

the

free

field

at

the

micro-

phone

location

with

the

microphone

removed

from

the

sound

field.

The

Pressure

Response

of

a

microphone

is

the

ratio

of

the

r.m.s.

output

·

voltage

to

the

r.m.s.

sound

pressure,

uniformly

applied

over

the

diaphragm.

The

two

definitions

coincide

for

a

microphone

having

negligible

dimensions

with

respect

to

the

sound

wavelength.

In

the

case

of

the

B&K

one-inch

microphones

this

is

practically

fulfilled

up

to

about

1400 c/s,

where

the

wavelength

attains

ten

times

the

diameter

of

the

microphone

(

i.

e. 240

mm).

The

difference

in

response

is

then

less

than

a

small

fraction

of

a

decibel.*)

Free-Field

Response="t

Pressure

Response=

I;

vo~ts~L___Mi-crophone~IC2~~-------c;;....--~

Wavelength

Sound

pressure

p

0

______

_

0° incidence

f62f60

Fig. 5.

Definitions

of

Free-field

and

Pressure

Response.

At

higher

frequencies

the

diffractions

of

the

sound

waves

on

the

microphone

produce

an

appreciable

change

in

the

resulting

sound

pressure

acting

on

the

microphone

diaphragms

as

illustrated

on

Fig.

5.

The

difference

p1

-

po

, C{\lled

free-field

correction

,

depends

on

the

orientation

of

the

microphone

w1th

respect

to

the

direction

of

propagation

of

the

sound

and

on

the

external

dimensions

of

the

microphone

(in

particular

those

of

the

front

and

of

fitted

protective

grids

or

"nose

cones").

The

free-field

behaviour

of

a

microphone

is

thus

described

by

means

of

a

set

of

free-field

corrections

curves

for

various

incidences,

which

have

been

measured

on

a

model

of

the

microphone

(see B&K

Tethnical

Review

Nos.

1 & 2-1959),

and

which

should

be

added

to

the

pressure

frequency

curve

of

the

microphone

in

each

particular

case.

See

Figs.

13

and

14.

For

~nicrophones

intended

for

ffee-field

work

it

is

possible

to

give

the

diaphragm

resonance

such

a

damping

that

the

normal

incidence

free-field

*)

In

the

particular

case

of

the

90°

incidence

(

parallel

to

diaphragm)

the

difference

in

response

remains

negligible

up

to

10

kc

/s.

6

•

corrections

are

compensated

for

up

to

frequencies

well

above

the

resonance

frequency,

in

order

to

obtain

the

flattest

possible

frequency

response.

Random

Incidence

Response.

In

the

important

case

of

indoor

sound

measurements,

where

a

great

part

of

the

sound

pressure

is

caused

by

sound

waves

which

have

been

reflected

in

au

·

possible

ways

before

impinging

on

the

microphone,

all

incidences

have

to

be

considered

simultaneously

creating

what

is

called

random

incidence

or

diffuse

field

conditions.

The

International

Electrotechnical

Commission

(T.C. 29, W.G. 8)

has

edicted

a

practical

rule

for

the

cal-

culation

of

the

random

incidence

response

as

the

root

mean

square

of

the

free-field

responses

for

different

orientations,

taking

into

account

the

area

of

the

corresponding

surface

elements.

See p. 16.

According

to

this

rule

the

diffuse

field

corrections

are

found

to

be

much

smaller

than

the

free-

field

corrections

in

the

case

of

the

B&K

microphones.

For

the

one-inch

microphones

the

difference

between

the

pressure

response

and

the

random

incidence

response

is

less

than

4

db

up

to

around

20

kc

/s.

A

microphone

with

flat

pressure

frequency

characteristic

should

consequently

be

preferred

for

indoor

measurements.

However,

by

mounting

specially

designed

devices

instead

of

the

protective

grid

(see

Accessories),

the

respnose

may

be

made

practically

independent

of

the

angle

of

incidence

in

an

extended

frequency

range.

The

free-field

and

diffuse-field

responses

are

then

identical,

and

the

same

microphone

will

suit

for

both

outdoor

and

indoor

measure-

ments.

Description

of

the Condenser Microphone Cartrldges

Type

4131

and

4132.

Two

slightly

different

one-inch

microphone

carlrid

.ges

are

available:

(

1)

the

Microphone

Cartridge

Type

4131

designed

for

free--field

measure-

mel)ls

and

featuring

a

normal

incidence

free-field

frequency

characteristic

whiph

is'

flat

up

to 18

kc

/s

(with

protecting

grid

mounted}.

(2)

the

Miciophone

Cartridge

Type

4132

presenting

a

pressure

frequency

characteristic

which

is

flat

up

to 7

kc

/

s.

The

two

cartridges

have

basically

the

same

mechanical

construction

and

differ

only

by

the

damping

of

the

diaphragm

resonance.

The

resonance

is

over-

damped

in

the

case

of

Type

4131,

in

such

a

way

that

the

normal

incidence

free-field

pressure

increase

is

compensated

by

the

decrease

of

pressure

sen-

sitivity

up

to

18

kc

/

s.

In

the

case

of

Type

4132

the

resonance

is

approximately

critic~lly

damped

in

order

to

obtain

the

flattest

possible

pressure

freq~cy

response.

The

resonance

frequency

,

which

is

approximately

the

same

for

the

~tridges,

is

around

7.5

kc

/s.

The

damping

is

controlled

by

means

of

appropriate

holes

in

the

back

plate

and

by

adjusting

·

the

tension

of

the

diaphragm.

7

Towards

the

low

frequencies

the

response

of

the

cartridges

is

only

affected

by

the

influence

of

the

pressure

equalizing

arrangement.

This

arrangement

consists

of

a

capillary

leakage

hole

through

which

the

equalization

of

the

static

air

pressure

on

both

sides

of

the

diaphragm

is

obtained

at

a

suitable

rate.

The

influence

of

ambient

pressure

(or

altitude)

variations

on

the

microphone

sensitivity

may

be

minimized

by

proper

design

of

the

pressure

equalization

hole.

(For

adjustment

of

the

pressure

equalization

see

p. 24).

In

the

case

of

the

one

inch

cartridges,

the

equalization

hole

is

situated

in

front

of

the

grid-

and

coupler

mounting-thread

in

order

to

enable

the

pressure

equalization

to

be

obtained

also

in

the

case

of

closed

cavity

measurements.

The

time

constant

of

the

pressure

equalization

of

Types

4131-4132 is o.05

second.

This

corresponds

to

a - 3

db

cut-off

frequency

of

3 c/s

approximately,

and

will

not

influence

the

measurements,

since

the

-3

db

low

frequency

cut-off

of

the

complete

microphone

is

10

cis.

The

mechanical

construction

of

the

cartridges

is

shown

on

Fig.

6.

The

choice

of

the

materials

and

th~

design

of

the

suspensions

of

the

different

parts

are

made

principally

with

a

view

to

obtaining

the

best

possible

long-term

stability

and

temperature

independency

of

the

sensitivity

.

The

cartridges

are

subjected

to

a

complete

artificial

aging

process

during

manufacture.

The

basic

parts

are

made

of

high

nickel

alloy

(K-Monel).

The

insulator

used

is

silicone

treated

qua

ftz

and

for

guaranteeing

a

disturbance-free

connection

to

the

cathode

follower,

gold

contacts

are

used.

The

diaphragm

is

made

of

a 4

fl

thick

nickel

plate

and

is

mounted

at

a

distance

of

22

fl

from

the

back

plate.

This

distance

is

reduced

to

20

fl

when

the

200 V

polarization

voltage

is

applied.

The

polarized

cartridge capacity,

which

is

of

the

order

of

60

pF,

is

measured

individually

for

each

cartridge

(see

the

calibration

chart).

The

insulation

is

verified

as

higher

than

105

M.

Q.

2311mm60

NS

2

I

.e

(0.910" 60

NS

2)

__

_

__

___

_l

_____

___

22.7Smm

E~-

(0.896")

E~

I

':J~a

~~

r

I

~_g

~

I

~

l

~

'

~

b371mm-Dod

I

(0

.

936"

:to

.

oooaJ

I e•r t 161179

23.11

mm

60

NS

2 E

a>

E~

(0

.910" 60

NS

2)

~~

Mg

+

f>imensions

of

the

"one-inch"

microphone

cartridges

Type

4131-4132.

R

•

Brüel & Kjær 4131/32 User guide

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