Bruel & kjaer 4215 User guide

Artificial Voice Type 4215

This

instrument

is

an

acoustically

well-defined

sound

source,

which

may

be

used

for

free-field

measurements

as

wel

l as

for

measurements

under

c

onstant

sound

pressure

conditions.

BHUEL&KJ&R

Nif!rum,

Denmark

•

Phone

80

05 00

Telegrams:

BRUKJA,

Copenhagen

BB

421!5

ARTIFICIAL

VOICE

TYPE

4215.

August

1959

Description

General

Description

Operation

Contents

General

.

.........

..

........

.

.........

.

..

.

.....

.

......

...

..

. . .

Recording

th

e

Frequen

cy

Response

of

Mic

rophones

.

..

. .

.. ..

.

...

.

Determin

ati

on

of

Mi

c

rophone

Se

nsitivity

...........

...

.......

..

.

Specification

3

4

10

11

13

15

Description.

General.

The

Artificial

Voice

Type

4215 is

designed

for

use

in

laboratories

and

pro-

duction

lines

where

a

well

defined

sound

source

is

required,

and

covers

the

frequency

and

pressure

range

that

is

normally

covered

by

the

human

voice.

The

apparatus

is

of

cy

lindrical

shape,

and

it

differs

from

the

artificial

voices

normally

used

for

telefon

measurement

by

its

provision

for

automatic

regulation

of

th

e

sound

level.

It

consists

of

a

small

loudspeaker

mounted

in

a

metal

housing

,

provided

with

a

co

upler

and

with

sockets

for

the

mounting

of

a

regulating

microphon

e

with

Cathode

Follower.

Damping

material

Coupler

\

Gold

covered

contact

Socket

for

regulating

microphone

Guard

ring

Loudspeaker

connect

Gold

covered

contact

l.:j1l~~~-~~f-

Guard ring

contact

for

follower

Fig.

1.

Sectional

view

of

the

Artificial

Voice.

3

Description.

The

sound

producing

element

of

the

Artificial

Voice

is a

small,

dynamic

type

loudspeaker,

the

diameter

of

which

is 90

mm.

It

is

mounted

in

an

aluminum

housing

with

rigid

walls

and

of

cylindrical

shape,

as

shown

in

Fig.

1.

In

front

of

the

loudspeaker

is

placed

a

special

coupler

(mouth

open-

ing)

which

is

made

removeable,

in

order

to

make

it

possible

for

the

user

to

insert

couplers

for

particular

purposes.

The

coupler

is

fastened

by

means

of

a

union

ring,

which

can

also

be

used

lo

fasten

a

lightning

ring,

thus

ensuring

an

airtight

connection

to

the

microphone

under

t

est.

The

special

coupler

supplied

with

the

Artificial

Voice

h

as

an

opening

dia-

meter

of

38

mm.

It

may

be

used

for

measurements

under

constant

sound

pressure

conditions

(provided

th

at

th

e

microphone

can

be

mounted

directly

at

the

opening)

as

well

as

for

free

field

measurements.

Microphone under

test

Regulating

microphone

type

4132

Fig. 2.

Artificial

Voice

with

regulating

microphone

and

test

object

mounted

for

pressure

calibration.

In

the

opening

is

placed

a

socket

for

mounting

of

a

regulating

microphone,

i.e.

the

standard

Condenser

Microphone

Cartridge

Type

4132. As

seen

from

Fig.

2,

lhe

front

of

the

microphone

cartridge

is

exactly

in

level

with

the

opening

of

the

coupler,

whereby

is

obtained

a

sound

pressure

regulation

in

the

very

front

of

the

Artificial

Voice.

By

employing

the

B & K

standard

microphone

the

sound

pressure

can

be

kept

constant

to

within

2

db

in

the

frequency

range

100

cfs

to

7

kc

/s.

The

socket

w'

hich

is

placed

in

th

e

mouth

opening

is

connected

to

another

socket

placed

on

the

side

of

the

housing.

To

thi

s

socket

a

Ca

thod

e

Follower

4

Type

2612

or

2613

can

be

fastened.

By

means

of

a

screened

cable

the

cathode

circuit

potential

of

the

cathode

follower

is

connected

to

the

guard

ring

seen

on

Fig.

1

whereby

the

input

to

ground

capacity

of

the

cathode

follower

is

reduced

to

a

minimum.

The

centre,

spring

loaded

contact

in

both

sockets

are

covered

with

gold

to

ensure

the

best

possible

electrical

connection

and

low

noise

conditions.

When

used

wit.h

the

control

microphone

the

apparatus

is

essentially

a

co

nstant

sound

pressure

source*)

with

an

extremely

low

internal

impedance,

equivalent

to a

constant

voltage

generator

in

electrical

circuitry.

The

sound

pressure

is

constant

in

the

mouth

opening

and

along

the

axis

of

symmetry

of

the

artificial

voice

when

no

reflections

are

present.

The

use

of

the

Artificial

Voice

under

closed

condition

does

not

offer

other

problems

than

those

encountered

through

wave

'

motions

in

the

coupler

volume,

and

those

due

to l

eakage

to

the

outside.

The

coupler

volume

should

consequently

be

kept

small

and

too

much

leakage

may

be

avoided

through

a

light

and

simple

packing

between

the

L

est

object

and

the

Artificial

Voice

.

The

packing

is

easily

obtained

by

the

use

of

a

foamed

ruhber

pad

(see

Fig.

2}

.

The

two

small

holders

on

s

id

e

of

the

housing

normally

offer

a

convenient

support

for

a

spring

arrangement

used

to

keep

the

test

object

in

place

if

this

is

desireable.

To

provide

frequency

independent

regulation

at

higher

frequencies

the

sensitive

part

of

the

test

object

should

be

placed

as

close

to

the

regulating

microphone

as

possible.

Thereby

the

same

sound

pressure

level

is

present

at

both

microphones.

At a

frequency

of

5ooo

c/s

f.

ex.

the

wavelength

2

in

air

will

be

approximately

6.8

em.

The

distance

between

the

diaphragm

of

the

regulating

microphone

and

the

sensi

tive

part

of

the

test

object

should

now

be

<<

2

/4

"'

l.7

em.

The

pressure

response

curve

shown

in

Fig. 3 is

obtained

with

a

distance

of

o.6

em

between

the

two

diaphragms.

a9u.9,

o

~e

1

£,

0

o

~

o o o o o o o o o o,

9.R.O

•.

Oo o o o o o o o o o

o

,g.,

~,q,,o

o o o o o

o

,~~.

o

.,

.........

CIIiil-=

•

=·

.....

Artificial

Vnic• Type 4

215

Condenser

Mjcrophonp

Type

4132

:::~

=~--k=

D

- •

..50dh.

- ·

...l1.:..1.:..5

--;"~~~

·~~

"'

oooo

"'

oo

~

ooo

oo

~o

oo

""o

o"&'8~o

~

oo

0

o

0

~DoD

f!f9276

Fig. 3.

Typical

pressure

response

curve

of

the

Artificial

Voice

with

regulating

microphone

emp

l

oyed.

*)

A

constant

sound

pressure

source

is

here

defined

as

a

source

which

, by

means

of

a

utomatic

regulation,

keeps

constant

th

e

sound

pressure

in

front

of

itself

when

the

frequency

and

the

lo

ad

is

varied.

5

The

Artificial

Voice

may

also

be

used

under

closed

conditions,

but

without

regulation.

The

pressure

response

curve

without

regulation

is

shown

in

Fig.

4.

0~000000000000000000000000000000000000000000000

BrUe~)c.r

......

:

lrt•l

•

I.,

lrhl

•

I.,

::::

:

-"""'

·

..

.........lllO..d.b._,_.,_.,

_J__

"-

IJIL

,

~

__2Q__

......

H..l

--

0

- .

.5ll..d.b.

.....

~

w-

.,_.

,

.1Cll_

.,.

,

___E_E_

o

o o o o o o o o o o o

"'

o o o o

"'

o o

rr'o

o

'tf'

o o o

'8'

o o 'g' o

o

~

o o o

IDOOO

0 0 0

10

D

0 0 0

'8

0 0 0 0 0 0 0 0

Fig. 4.

Pressure

response

curve

of

4215

without

regulation

(regulating

micro-

phone

inserted

but

not

us

ed

for

regulation)

.

Free

field

measurements

can

furthermore

be

very

conveniently

car

ried

out

by

means

of

the

Artificial

Voice.

When

the

regulating

microphone

is

em-

ployed

,

the

frequency

response

of

the

Artificial

Voice is

flat

within

the

limits

given

by

the

pressure

response

of

the

regulating

microphone.

The

output

pressure

will

,

however,

be

inversely

proportional

t'O

the

variation

in

sensitivity

of

the

regulating

microphone.

The

free

field

correction

for

the

0~00000000000000000~000000000000000000000000000

OrUe~)CIIr

......

.:~~~~~t~''l"l'

~~~~·l•

~~~~~~~~~~I•~••~'

'l'i•l•~~~~~:as.,

15

.,

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-

4215

•

::

:::

frep-fip!d

response

bistancp

tp

measuring

mjcrnpbnop

-a5 m

,_

....,,

__60_db_

,_.,_.

,

~c

.._

....

,"~

_

....

,

__6__

0

- •

.5.ll.d.b..

..

.

-..lk..2..::.59

w_._.

,

...l.!l..D_.,.

,..........L..A. o

> o o o o o o o o o o o o o o

c!'

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rf'

o o o o o o o

'go

o o

~

o o o-& o oo

'8

JS

-o o o o' o'

~

0o

fSfJR78

Fig.

5.

Free

field

response

of

Type

4215

with

regulating

microphone

employed.

00000000000000000000000000000000000000000000000

llrOe~)CIIr

...

~~~~~~~~

1

":•'1'1'

1

••~~~~~~~~~~

1

'1'"1'

~~~~-~·~~~~~:IS

D • •

....__,a.,-.

.......

Artificial

Voice

Type

4215

Fig.

6.

Free

field

response

without

regulation

(microphone

inserted,

but

not

employed).

6

Ylicrophone

Cartridge

Type

4132

with

pwtection

grid

for

·180°

incidence

is

less

than

1

db

up

to

9

kc

/s,

thus

giving

a

free

field

response

of

the

Artificial

Voice

as

shown

in

Fig.

5. (

Further

information

about

the

free

field

correction

of

the

microphone

is

found

in

the

instruction

manual

Type

4131-32

and

in

the

B. & K.

Technical

Reviews

No. 1

and

2 1959.

In

Fig.

6

the

free

field

response

is

shown

for

the

Artificial

Voice

used

without

regulation.

\Vhen

measuring

c

lose

talking

microphones

such

as

those

used

in

telephone

21U

a.

Fig.

7.

Measuring

arrangements

used

to

obtain

the

curves

shown

in

Fig. 5

and

6

respectively.

7

systems

and

megaphones,

the

distance

between

the

mouth

opening

and

the

microphone

is

so

small

that

interaction

between

the

nearfields

around

the

test

object

and

the

Artificial

Voice

will

influence

the

frequency

response.

The

influence

on

the

sound

pressure

in

the

mouth

opening

caused

by

a

cylindrical

obstacle

with

a

diam

eter

of

40

mm

and

placed

in

a

distance

of

4

em

from

the

opening

does

not

excee

d ± 2

db

at

frequencies

be

low

5ooo

cjs.

(This

counts

with

regulation

as

well

as

without

regulation

).

a9a.W

1

£,

oo

-43

o

o~~o~o~o~o~oio~o~o~,9.~.,0~.~.0.

1

.o~o~o~o~o~o~o~ololololo!,?l

..

,l~!'ol

..

lolol

o

l

o

iolo~o

:0..

~

.o.

--

4115 • •

........

Sound

fje!

d L

em

tOto

• • •

from

mouth

openjng

~S928f

Fig.

8.

Curve

showing

the

influence

of

a 24

mm

microphone

with

flat

front

placed

in

a

distance

of

4

em

from

the

mouth

opening.

C

urve

II

shows

the

frequency

response

as

taken

with

th

e 24

mm

microphone

and

curve

I

shows

the

frequency

response

as

obtained

with

a

microphone

which

has

negligible

effect

on

the

sound

field.

In

Fig.

8 is

shown

the

influence

of

a 24

mm

microphone

with

flat

front

placed

in

a

distance

of

4

em

from

the

mouth

opening.

Fig.

9 s

hows

the

variation

in

fr

e

quen

cy

respon

se

with

di

s

tan

ce

as

m

eas

ured

with

the

above

mentioned

24

mm

mi

c

rophon

e (B. &

K.

Type

4131 ).

The

curve

shown

in

Fig.

10

indicates

the

decrease

in

so

und

pr

ess

ure

level

oooooooooooooooooooooooooooooooooooooooooooc

BrUe~Jatr

....

lrlel

6

ll•r

lr

..

l

a.

ll•r

db

-"'-"'

------.

Frequency

response

120

of

4215

measured

with

4131 -

24mm

em

Condenser

microphone

vmgn"J\

regulation

-----------'

00

_________

90

z..

....,,

__so__

--

·- - -

'-

.....

'"

2..Q__

._

...

,- --

- ._2.5__

"'""~

w;'~

~

';;'~~~o

o o o

"'

o o

~

o

'8'

o o o

'8'

o

~

o

~

o o

o

~

o o

""'&

'8

~

/)""

o c

fS9282

Fig.

9.

The

variation

of

the

frequency

response

with

distance

when

measur

-

ing

the

nearfield

with

the

above

mentioned

24

mm

microphone.

8

-.........

......_

~

........

~

"""

~~

~

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

~~--

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

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

..........

2 3 4 6 8

10

20

30

40

60

80

100

200

300

400

600

1000

Distance from mouth opening

in

em

------11~

1~9273

Fig. 10.

The

variation

in

sound

pressure

as a

function

of

distance.

Measured

with

a

microphone

which

has

negligible

effect

on

the

sound

field.

130 db

f;p~%?a3

..

·~v~~r

~'::

=

~.:

-

.,

,..,.5.0J1.b_._l.!l:12:58

at

114

db

"'

·

~---_E.E__

•

.~.

--;;::.

i:">

o o o o o o o

~

o o o o o

'&

o o

"'&

o o

•o

o o

c!"

o o

~o

o

~

o

o o

~

o o

~

o o""" o"'

I:J

··

-o

"''\

o o o

n o o o o o o o o o o o o o o o o o o " o n o o o o o o o o o o o o o o o n

n,o

o o o o o o o o

'1..

•

.!_!!1••+·.

'il

.~

""""-

.....

1

..

--.!tlfi_.

..

Third

harman

« •

"'

distortion

gt

114

db

god

gt

at

130

db

f.ll-froQ2Ulici

r

at

114

db

. -

Typical curves

~'::a..~.:

-

..

,..,.5JLd.tL

....

1.9:12:58

""

·

~---.E.E_

0 • '

••

"

p o o o o o o o

~

o o o o o

'&

o o

"'&

o o

""

o o o o o o

8"'

o o

~

o

.:,

o

~

o o

~

o o

"'3"

o

15-

·-o

01

tl

o o o

fSPZU

Fig.

11.

Distortion

generated

in

the

Artificial

Voice

at

sound

pressures

of

1

mbar

and

o.1

mbar.

a)

Fundamental

+

2nd

harmonic

distortion.

b)

Fundamental

+

3rd

harmonic

distortion.

for

increa

s

ing

distance

to

the

mouth

opening.

The

measurement

is

carried

out

at

looo

cjs

and

with

negligible

effect

on

the

sound

field.

Fig.

11

shows

the

distortion

generated

in

the

Artificial

Voice

at

high

sound

pressures

compared

to

the

fundamental

signal.

9

Operation.

General.

As

mentioned

under

"

Description"

the

Artificial

Voice

may

be

used

for

measurements

under

pressure

conditions

as

well

as

for

free

field

measure-

ments.

Typical

applications

for

the

Artificial

Voice

used

under

pressur

e

conditions

are

the

testing

of

microphones

for

oxygen

masks

,

testing

of

silencers,

exhaust

pipes

,

air

inlets

etc.

Applications

for

free

field

measurem

e

nts

are

testing

of

normally

handheld

microphones

and

semiprofessional

microphones

for

tape

recorders,

inter-

communication

sys

t

ems,

public

address

systems,

dictating

machines,

free-

field

labor

atory

t

esting

of

hearing

aids

and

microphones

for

the

100-7ooo

cjs

frequency

range.

The

Artificial

Voice

Type

4215

is

intended

for

use

in

conjunction

with

lhe

Automatic

Frequency

Response

Recorder

Type

3302,

which

consists

of

a

B.F.O

.

Type

1014

and

a

Level

Recorder

Type

2304.

An

arrangement

is

Fig. 12.

Measuring

arrangement

for

recording

the

frequency

response

of

sma

ll

microphones.

10

shown

in

Fig.

11

where

the

Microphone

Amplifier

Type

2604*) is

used

as

a

"Compressor

Amplifier",

which,

when

properly

adjusted,

enables

the

sound

pressure

level

at

the

opening

of

the

Artificial

Voice

to

be

read

off

directly

from

the

indicating

meter.

For

amplification

of

the

test

signal

another

Micro-

phone

Amplifier

Type

2604 is

used,

featuring

the

advantage

of

a

rapid

and

easy

sensitivity

calculation

of

the

microphone

under

test

(see

page

13).

Recording the

Frequency

Response

of

Microphones.

The

measuring

set-up

shown

in

Fig.

12 is

arranged

for

measurements

under

constant

sound

pressure

conditions,

but

may

just

as

well

be

used

for

free

field

measurements.

The

microphone

under

test

can

then

be

mounted

a

short

or

longer

distance

away

from

the

Artificial

Voice,

depending

on

the

applica-

tion.

The

sound

pressure

at

the

diaphragm

of

the

test

object

will

decrease

with

the

distance

according

to

the

curve

shown

in

Fig.

10

and

the

amplification

of

the

amplifiers

must

consequently

be

increased.

The

operation

procedure

given

below

can

be

used

in

both

cases.

Before

the

measurement

is

carried

out,

the

instrument

should

be

adjusted

as

follows:

Type

1014

(The

B.F.O.

of

Type

3302):

Adjust

the

frequency

scale

zero

point

as

described

in

the

instruction

manual

for

Type

1014.

Then

set

the

control

knobs

as

follows:

"Compressor

speed":

"100".

"Output

Voltage":

"0".

"Compressor"

Input

Terminal:

Connected

to

the

output

of

Type

2604 (I).

"Compressor

Voltage":

Approximately

5

on

the

control

knob

marking.

"Load"

Terminals:

Connected

to

the

loudspeaker

terminals

of

Type

4215.

"Impedance"

switch:

6

ohms.

Frequency

Dial:

1ooo

cjs.

The

"Magnetic

Clutch",

"Modulation

Frequency",

"Modulation

Swing"

and

the

"Attenuator"

control

knobs

should

be

set

to

their

"off"

or

"zero"

positions.

The

remaining

control

knobs

should

not

be

touched

after

the

zero

setting

is

carried

out.

Type

2304

(The

Level

Recorder

of

Type

3302):

Insert

the

potentiometer

(50 db)

and

the

recording

paper

QP

3614,

QP

3615

or

QP

3616,

and

set

the

control

knobs

as

follows:

"Lower

Limiting

Frequency":

"20

c/s".

"Potentiometer

db-range":

"50".

"Writing

Speed":

"100

mm/sec".

*)The

Microphone

Amplifiers

Type

2604

may

be

substituted

by

Type

2603

without

any

change

in

the

operation

procedUJ c.

11

"Amplifier":

"On".

"Record/Slider

free":

"Record".

Gear

Box

wheel:

"B".

Input

Potentiometer

Terminal:

Connected

to

the

output

terminal

of

Type

2604

(II).

Zero

Level

Adjustment:

Fully

counterclockwise.

Input

potentiometer

control

knob:

Fully

clockwise

.

Type

2604 (I):

Adjust

the

screw-driver

operated

potentiometer

marked

"

Sensitivity,

Condenser

Microphone"

as

described

in

the

Instruction

Manual

Type

2604

with

respect

to

the

sensitivity

of

the

regulating

microphone.

Then

set

the

control

knobs

as

follows:

"

Input

Switch":

"Condenser

Microphone".

"

Input

Potentiometer

" :

Any

position.

"Co

ndenser

Microphone

"

Input

Terminal:

Connected

to

the

regulating

mi

c

rophone

of

the

Artificial

Voice.

"

Range

Multiplier":

"0

db

(x1)".

"Me

ter

Switch":

"R.M.S.

(Fast)".

"

Power"

Switch:

"On".

"Meter

Range":

"

100

db

(1

V)".

"

Weighting

Network":

"Linear

10-

200ooo

c/s".

Type

2604 (II) :

Adjust

the

sensitivity

of

the

amplifier

as

described

under

"o

peration"

in

the

instruction

manual

for

Type

2604.

Then

set

the

control

knobs

as

follows:

"Input

Switch":

"Direct".

"Input

Potentiometer":

Any

position

.

"Range

Multiplier":

"0

db

(x

1)

".

"Meter

Switch":

"R.M.S.

(Fast)".

"

Power"

Switch:

"On".

"Me

ter

Range

":

"100

db

(1

V)".

"

\Veighting

Network":

"

Linear

".

When

the

instruments

are

adjusted

as

described

above,

the

actual

measure-

ment

can

take

place.

1.

Place

the

microphon

e

to

be

tested

in

front

of

the

acoustical

coupler

of

the

Artificial

Voice.

The

microphone

should

fit

airtight

to

the

coupler,

eve

ntually

mounted

by

means

of

a

pi

ece

of

foam

rubber,

when

pr

ess

ure

conditions

are

desired

.

2.

Connect

the

output

leads

of

the

microphone

under

test

to

the

input

jack

of

the

Microphone

Amplifier

Type

2604

(II).

3.

Turn

the

Output

Voltage

"

knob

of

th

e

B.F.O.

to

maximum.

4.

Rotate

the

"Compressor

Voltage

"

knob

of

Type

1014

until

the

meter

pointer

of

the

Microphone

Amplifier

Type

2604

(I)

deflects

to

the

10

db

mark

on

the

db

scale.

12

A

sound

pressure

level

of

110

db

re

2 X 10-4

,ubar

is

then

obtained

in

front

of

the

microphone.

(

If

a

sound

pressure

level

of

f.

inst.

90

db

is

required,

the

"Me

ter

Range"

s

witch

of

Type

2604 (I) is

set

to

80

db

SPL.

The

sound

pressure

level

is

then

80

db

+

10

db

(

read

off

from

the

scale)

=

90

db.

When

low

sound

pr

ess

ure

levels

are

desired

,

the

loudspeaker

should

be

co

nnected

to

the

Attenuator

Output

of

th

e

B.F.O.

,

in

order

to

mak

e

th

e

compressor

circuit

work

·

in

th

e

middl

e

of

its

working

range

,

see

also

the

instruction

manual

for

type

1014

}.

Th

e

mov

e

ment

of

th

e

met

er

pointer

of

101-t-

should

preferably

fall

within

-10

db

to +

10

db

on

the

met

er

sca

le

of

th

e o

sc

illator

wh

en

th

e

frequ

e

nc

y

sca

le is

scanned.

5.

Adjust

the

range

switches

on

the

Type

2604 (II)

until

a

suitable

deflec-

tion

of

the

Level

Recorder

stylus

is

obtained

(

f.

ex.

around

40

db

).

6.

Scan

Lhe

frequency

rang

e

of

Type

1014

to

make

sure

that

the

stylus

deflection

of

the

Level

R

eco

rder

is

well

within

the

width

of

the

record-

ing

paper

for

all

frequencies.

Otherwis

e

re-

a

djust

the

amplification

of

Type

2604 (II)

or

adjust

Lh

e "

Input

Potentiometer"

knob

on

the

Level

H.

eco

rder.

7.

Set

lh

e

fr

e

qu

e

ncy

dial

of

th

e

B.F.O.

to 20 c/ s.

8.

Set

the

"Mag

netic C:lutch" to "

On

".

9.

Adjust

th

e

Re

co

rding

Pap

er

until

th

e

writing

arm

indi

ca

t

es

20

cjs,

by

turning

the

finger

wh

eel

placed

to

th

e

right

of

th

e

gear

box.

10.

Pu

sh

Lh

e

gear

lever

mark

ed

3

mm

/sec.

11.

Start

th

e

motor

by

mean

s

of

the

toggl

e

sw

itch

mark

ed

"Mo

lor

".

The

frequency

rang

e is

now

automatically

scanned,

and

the

fr

e

quen

cy

re

spo

n

se

curve

of

th

e

mi

c

rophone

und

er t

es

t is r

eco

rd

ed

on

th

e

fre-

qu

e

ncy

ca

librated

paper.

Determination

of

Microphone

Sensitivity.

Th

e sens

itivit

y

of

th

e

mi

c

rophon

e

und

er l

es

t

can

be

determined

very

con-

veniently

when

the

sound

pressure

lev

el

at

the

opening

of

the

Artifical

Voice

is

adjusted

to

74

db

SPL

which

is e

qual

to

1

,ubar

or

to 134

db

SPL

which

is e

qual

to 1

mbar.

vVith

lh

e

sound

pressure

lev

el

se

t

to

74

db

plu

s

as

many

db

as

th

e

sound

pressure

ha

s de

creased

according

to

Fig.

10,

th

e

range

switches

of

th

e

microphon

e

amplifier

2604 (

II

) is

adjusted

until

a

s

uitabl

e de

flection

is

obtained

on

th

e me

ter

.

Th

e m V r

ea

ding

on

the

mel

er

sca

le

in

conjunction

with

the

"

Meter

Range"

a

nd

"Range

Multiplier"

sw

itch

es

then

directly

indicates

th

e

sensitivity

of

th

e

mi

c

rophon

e

being

t

es

ted

in

m V

per

,ubar.

The

Automatic

Frequency

R

es

pon

::;

e He

co

rd

er

Type

3302

s

hown

in

Fig.

12

can,

if

desired,

be

replac

ed

by

th

e

Audio

Frequency

Response

Tra

ce

r

Typ

e

13

4707,

which

features

a

rapid

visual

check

of

the

microphone

response

curve.

This

might

be

especially

convenient

when

the

measuring

arrangement

is

used

in

production

line

test

systems.

The

operation

procedure

is

similar

to

the

one

described

above,

except

for

the

settings

of

the

Automatic

Frequency

Response

Recorder

Type

3302.

For

the

operation

of

the

A.F.

Response

Tracer

Type

4707,

set

the

control

knobs

as

follows:

14

Adjust

the

frequency

scale

zero

point

as

described

in

the

instruction

manual

Type

4707.

Then

proceed

as

follow

s:

"Co

ndenser

r.p.n1." :

"3",

"6"

or

"12"

r.p

.In.

"Compressor

Speed":

"S

low".

"Co

mpressor

Voltage":

10

on

th

e

control

knob

marking.

"Compressor

Input":

Connected

to

the

output

of

Type

2604

(1).

"A.F.

Output

Voltage":

"

0".

"A.F.

Output"

socket:

Connected

to

the

loudspeaker

terminals

of

Type

4215.

"Attenuator":

"6

ohn1s,

5

volts".

"T

racer

Input"

socket:

Connected

to

the

output

of

Type

2604 (

II

).

"Lower

Limiting

Frequency":

"20

c/s".

"Tracer

Input

Voltage":

Should

be

adjusted

to

give a

suitable

zero

level,

f.

ex.

10

mV.

Turn

the

"AF

Output

Voltage"

knob

fully

clockwise

and

adjust

the

"Co

mpressor

Voltage"

knob

till

a

suitable

output

level

is

obtained.

Specification

Frequency

Response

(with

regulating

microphone):

Linear

to

within

± 2

db

from

100 cj s to

7ooo

cjs

(see

Figs.

3

and

5).

Max.

Sound

Pressure

Level:

115

db

for

free-field

measurements

, 4

em

from

mouth

opening.

Approx.

130

db

for

measurements

under

constant

sound

pressure

conditions.

(

db

re

2 X 10-4

,ubar).

Obtained

when

connected

to

B.F.O.

Type

1014, 6 Q

output

terminals.

Distortion:

Less

than

1 %

at

looo

cjs

at

sound

pressures

lower

than

115

db

re

o.0002 ,ubar.

Mouth

Opening:

Diameter

of

aperture

is 38

mm

(1.5").

May

be

used

for

1neasurements

under

co

nstant

sound

pressur

e

conditions

as

well

as

for

free

field

measurements

.

Power

handling

capacity:

3

Watt.

Dimensions:

Height

10.5

em

(414

"),

Diameter

10.5

em

(4

1

,4

").

Weight:

2

kg

(4

lbs.

).

15

B K

P

rinted

in

Copenhagen,

Denmark

Au

g

ust

195

9

BRUEL & KJAER 4215 User guide

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