Bruel & kjaer 4920 User guide

Outdoor Microphone System

Type 4920

The

Outdoor

Microphone

System

extends

the

use

of

the

B & K

Condenser

Microphone

into per-

manent

outdoor

installations

where

the

micro-

phone

is

situated

at

a

considerable

distance

from

the

indicating

instruments.

BHUEL

&

K.Y&R

Ncerum,

Denmark

.

~

80

05

00

•

,;:;."

BRUKJA,

Copenhagen

. Telex:

5316

BB

4920

The

Outdoor

Microphone

System

Type 4920

AUGUST

1964

Contents

1.

Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

General

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

The

Amplifier

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Calibration

Oscillator

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

The

Microphone

and

Cathode

Follower

. . . . . . . . . . . . . . . . . . . . . . . . . . 8

The

Wind

Screen

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

The

Rain

Cover

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13

Adjustment

of

the

Amplifier

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

13

Calibration

Oscillator

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Polarization

Voltage

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

13

3. Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Airport

Noise

Monitoring

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

14

Analyses

of

Noise

Signals

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

15

4.

Specification

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.

Description

General.

The

Outdoor

Microphone

System

Type

4920

has

been

designed

to

enable

outdoor

acoustical

measurements

to

be

made

at

a

considerable

distance

from

the

indicating

instruments.

Fig.

1.1

.

The

Outdoor

Microphone

System

Type

4920.

This

'

~ystem

consists

of:

1. A

preamplifier

and

a

conde

'

nser

microphone

fitted

with

wind

screen,

electrostatic

actuator

and

rain

cover,

and

2.

A

water-

proof

cabinet,

which

encloses

an

amplifier,

a

power

supply

and

a

calibration

oscillator.

This

set-up

will

convert

sound

pressures

into

electrical

signals,

and

due

to

the

low

output

impedance

of

the

amplifier,

these

signals

can

be

transmitted

over

a

lon

g

distance

by

means

of

a

cable,

for

instance

a

telephone

cable

or

a

twisted

pair

of

wires.

An

easy

system

check-out

and

calibratlon

can

be

made

when

the

built-in

electrostatic

actuator

is

switched

into

operation.

Fig.

1.1

shows

the

complete

Outdoor

Microphone

System,

and

its

principle

of

opera~ion

is

given

in

the

form

of

a

block

diagram

in

Fig.

1.2.

3

Wind screen

Rain

Cover

and

electro-

static

Actuator

Microphone

and Cathode

Follower

Waterproof cabinet

~

----

--

-

-----

-,

I

~

I

I Output

1 Calibrator

on

I

.I

I

L

____

___________

j

tMafo

Fig. 1.2. Bl

ock

diagram

of

the

Outdoor

Microphone

System

Type

4920.

Amplifiei'.

In

this

equipment

a

two

stage

R-C

coup

led

transistorised

amplifier

is

used

in

which

the

transistors

are

in

gr

ounded

emitter

connection.

A

maximum

gain

of

30 dB

can

be

obtained,

and

a

built-in

screwdriver

operated

potentio-

meter

a

ll

ows

gain

variations

of

up

to

31.5

times

the

microphone

signal.

Normally

the

signal

from

the

microphone

and

cathode

follower

is

fed

to

the

screwdriver

operated

potentiometer

through

a

capacitor,

it

is

possible,

however,

to

take

the

signal

directly

from

the

cathode

follower

just

after

lhe

capacitor

at

the

terminal

on

the

printed

circuit

marked

DIR.

OUTP

.

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Fig. 1.3.

Frequency

characteristic

of

the

amplifier

loaded

with

a 200 Q resistor.

4

To

obtain

a

linear

frequency

response,

see

Fig.

1.3, a

heavy,

negative

feed-

back

has

been

introduced.

However,

when

a

long

transmitting

cable

is

con-

nected

to

the

amplifier

output

it

may

be

necessary

to

correct

for

a

drop-off

in

the

response

at

higher

frequencies

to

obtain

the

desired

linear

cha-

racteristic.

This

correction

is

made

by

connecting

a

capacitor

from

the

~

emitter

of

the

first

stage

to

ground

(on

the

printed

circuit

the

mounting

holes

for

this

capacitor

are

marked

C 30).

The

effect

of

the

correction

is

illustrated

in

Fig.

1.4. At

the

output

of

the

amplifier

a

transformer

is

used

..

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---

10000

20

~~~:

Fig. 1.4.

Frequency

response

correction.

a.

Response

of

amplifier

and

a

long

cable

without

correction.

b.

Response

of

amplifier

with

correction.

c.

Response

of

amplifier

and

a long cable

with

correction.

'

.,

,

•

cu.

to

obtain

several

output

impedances.

The

secondary

windings

of

this

trans-

forfl,ler

are

detailed

in

Fig.

1.5

and

by

means

of

two

or

three

shorting

links,

it

is

possible

to

use

any

of

the

following

load

impedances:

200

Q,

200 Q

floating,

200 Q

symmetrical

or

50

Q.

The

01.1tput

terminals

of

the

amplifier,

which

are

located

on

the

printed

circuit

can

be

directly

connected

to a

transmission

line,

the

length

of

which

may

be

of

up

to

10

km.

The

amplifier

is

highly

stabilized

varying

less

than

± o.2

QB

for

a ± 10 %

variation

in

line

voltage.

The

long

term

stability

is

expected

to

be

± o.5

dB

when

the

equipment

is

in

continuous

use.

Furthermore

the

amplifier

is

well

temperature

stabilized

and

will

be

virtually

unaffected

from

-30°

C

to

+60° C

if

power

is

drawn

continuously.

See

Fig.

1.6.

5

200.A.

Osc.on o

Di

r.

out

o

<>:1_

200.n

Sym

Osc.

on

.Dir.

out

200..n.

Floating

Osc.on 0

A

mp.ou

t{

~

f-<>

<:J__

Di

r.

out o

SO.n.

Osc.

on

o

Amp.out

Di

r.out

o

16+880

Fig. 1.5.

Load

impedance

conditions.

Calibr

..

tion

Oscillator.

As

mentioned

earlier,

the

microphone

system

contains

a

device

by

which

it

is

poss

·

ible,

without

any

alterations

being

made

to

the

measuring

set-up,

to

make

an

easy

system

check-out

and

calibration.

A

known

voltage

from

6

a

I

/~

r'-b

-1

-2

-20

-10

0 10 20

-

...

-

1',._---

30 40 50

60

Col.signol

rel.90

dB

SPL.

---f-16~-88~

c

Fig. 1.6.

Typical

amplification

and

calibration voltage variations

with

'

temperature.

a

calibration

oscillator

in

the

amplifier

assembly

is

fed

to

an

electrostatic

actuator

built

into

the

microphone

rain

cover.

This

voltage

produces

a

known

force

which

acts

on

the

microphone

diaphragm

and

is

comparable

to

a

sound

pressure.

The

actuator,

delivered

with

the

microphone

system,

is

adjusted

at

the

factory

so

as

to

produce

an

equivalent

SPL

of

90 ± 1

dB

by

the

injection

of

an

AC

voltage

of

215

V,

this

being

the

output

voltage

from

the

calibration

oscillator.

It

is

possible

to

adjust

this

voltage

by

changing

the

resistor

marked

OSC.

VOLT

ADJ

on

the

printed

circuit.

Transm. cable

Osc.

start

Transm. end

Input

to indicating

or analysing instruments

-25V

Receiving end

16+81:3

Fig. 1.7. Connection for

remote

control

of

the

calibration generator.

The

frequency

of

the

oscillator

is 500

Hz

and

it

may

be

adjusted

by

means

of

the

iron

core

marked

OSC.

FREQ.

ADJ.

As

there

is

no

DC

bais

voltage

between

the

actuator

and

the

diaphragm,

the

frequency

of

the

signal

from

the

microphone

itself

will

thus

be

1ooo

Hz.

A

careful

stabilization

of

the

output

voltage

from

this

oscillator

makes

it

practically

independent

of

ambient

temperature

variations.

See

Fig.

1.6

curve

a.

To

operate

the

calibration

oscillator

a

10-25

V

battery

is

connected

either

between

one

of

the

signal

leads

and

ground

(2

wire

system),

see Fig.

1.

7,

or

between

a

special

lead

from

the

amplifier

assembl

y

and

ground

(3

wire

system).

When

using

the

battery

between

signal

lead

a

nd

ground,

care

must

be

taken

not

to

short

circuit

the

signal

or

the

DC

voltag

e.

If

the

indicating

instruments

have

a

capacitor

in

the

input

circuit,

the

DC

voltage

can

be

supplied

through

a 5 k.Q

resistor,

but

in

the

case

of

a

transformer

input

where

the

transformers

are

not

floating,

a

series

capacitor

must

be

used.

7

Microphone Cartridge

Type

4133

and

the

Cathode

Follower

2615.

The

microphone

used

consists

of

a 1/

2"

microphone

cartridge

and

a

cathode

follower

attached

to

a 10 m (30 foot)

cable

between

the

microphone

and

the

amplifier

box.

This

microphone

cartridge

is

screwed

onto

the

housing

of

the

cathode

follower

forming

a

small

rugged

unit.

Connection

of

the

microphone

must

be

made

directly

to

the

input

socket

on

the

amplifier

assembly

which

matches

the

microphone

connecting

plug

.

Stabilized

'

plate

and

heater

voltages

for

the

cathode

follower

and

polarization

voltage

for

the

cartridge

are

available

at

this

seven-pin

socket.

The

200

volt

polarization

voltage

may

be

adjusted

by

means

of

a

potentiometer

marked

POL.

VOLT.

ADJ.

on

the

amplifier

assembly,

with

the

point

for

measuring

this

voltage

marked

MEAS.

The

Microphone

Cartridge

Type

4133

features

a

normal

incidence

free-

field

frequency

characteristic

which

is

flat

up

to

12.5

kHz

(with

rain

cover

mounted).

Fig. 1.8.

The

Wind

Screen

UA 0082.

Wind

Screen.

The

wind

screen

is

designed

for

mounting

on

the

cathode

follower

equipped

with

a

condenser

microphone

cartridge

and

the

rain

cover.

It

is

covered

with

a

double

layer

of

nylon

cloth,

and

gives

an

effective

reduction,

of

the

order

of

10 dB

or

higher

of

wind

induced

noise

at

lower

wind

velocities.

It

is

possible

to

mount

the

1", 1/

2"

or

1/4"

microphone

inside

the

wind

screen,

but

when

mounting

a 1/

2"

or

a 1/

4"

microphone

and

cathode

follower

a

tightening

ring

and

mounting

tube

must

be

used,

refer

to

Fig.

1.9.

The

8

BRUEL & KJAER 4920 User guide

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