Bruel & kjaer 2211 User guide

Noise Limit Indicator

Type

2211

Type 2211 Noise

limit

Indicator

is

a noise

and

vibration analyzer designed to keep control

on

the noise or vibration produced by units leaving

a production line.

BRUEL&KJ~R

N..Hum, D

enmark

. fE

800500

• ;;>

BRUKJA,

Copenhagen

. Telex: 5316

BB

2211

Noise

Limit

Indicator

Type

2211

Reprint

february

1966

Contents

Introduction

to

the

Instrument

...........

.

..

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

..

...

3

Introduction

to

Noise

and

Vibl'ation

Measurements

on

products

.....

.

..

4

'Videband Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . f)

Weighted

Measurements

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

.....

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

5

Frequency

Analysis

of

Noise.

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

..

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

..

6

Measurements

in

Sel

ected

Freq

u

ency

Bands

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

7

Capabilities

of

the

Noise

Limit

Indicator

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

.......

..

.

..

....

8

Which

data

must

be

known

before

the

Instrument

is

used

...

.

......

...

.

10

What

s

hould

be

measured,

Noise

or

Vibration

. . . .

........

.....

. . . . . . .

..

13

Practical

arrangements

when

"selling

up"

the

Instrument

.

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

14

Vibration

testing

.

..

.

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

.

......

..

........

.

....

.......

15

Noise

testing

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

.

......

. . .

........

..

...

..

....

.

...

16

Use

of

the

Noise

Limit

IndicatOl'

....

.......

.....

.

..........

..

19

Pre-adjustments

.....

.

.........

..

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

. .

..

..........

19

Adjustment

for

u

se

with

Microphone

Type

4

131-32

.......

.

......

19

Adjus

tm

e

nt

with

lh"

Mic

rophone

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

. . 25

Vibration

Measurement

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

.

....

.

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

. .

25

Adjustment

of

the

"Sens

itivit

y

Increase"

....

..

.

...

..

.

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

28

Operators

Instructi

on .

...

.

...

...........

.......

.

.........

..

......

31

Use

of

both

Input

s

sim

ult

aneous

ly

..

. .

......

. . .

..

...

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

32

Remote

Control

...

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

..

.......

...

........

..

.........

34

Technical

Description

..

.

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

.

.....

..

.

....

...

......

........

36

Input

Amplifiers

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

..

36

Filters

and

Output

Amplifiers

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

..

. .

....

.

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

. . 37

Rectifier

and

Relay

Circuit

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

.

.......

..

...

. . .

......

39

Control

Panel

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

. . .

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

.

.....

..

. 40

Meter

Circuit

....

.....

...

..

.......

...

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

....

. .

....

.

....

41

Power

Supply

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

41

Specification

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

.

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

..

....

.

..

. . .

...

43

Introduction

The

Noise

Limit

Indicator

is

an

instrument

designed

to

provide

a

rapid

a

nd

accurate

evalulltion

of

the

quality

of

products

on

a

production

line

with

respect

to

noise

and

vibration.

This

is

achieved

by

measuring

the

nois

e

and

vibration

of

every

unit

leaving

the

production

line

and

c

omparing

the

result

with

the

noise

and

vibration

level

obtained

from

the

"standard"

unit.

The

instrument

has

be

en

designed

to

offer

the

utmost

simplicity

in

operation.

After

setting-

up,

an

unskilled

worker

ca

n

easily

interpr

et

the

indi

ca

ting

panel

and

thenby

the

acce

ptance

or

rejection

of

the

unit

manufactur

ed.

Rejections

are

indicated

by

red

indicati,ng

lamps.

In

this

way

the

possibility

')f

confusion

or

errors

from

meter

readings

is

completely

eliminated

.

fhe

instrument

measur

es

noise

and

vibration

in

several

frequency

bands

(up

to 12

bands

)

simultaneously.

The

operator's

job

is

simplified

to sH

ch

a

de

gree

that

he

only

presses

a

button

and

notes

if

any

lamp

lights

up

.

If

These

units

ore

bought

seperat

e

ty

~rurL

ng

..tbf.

Jlgig

llmjt lrlIIiootor

I

.--Cathode

FoUoWlr

2613

1

I I

1

Microphone

4131

I

1 I

I I

1

..--

---

--,

ISet of 33

'Il

octave filters

I

l~i

~

5

~~

C~.5

~~

C~,5

I

50"

63

..

80

..

I

100"

125

··

160"

I

200

-

~:

::

~;~:

:

I

~~~

: :

1000" 1200··

I 1600 -2000

··

2500··

1 3150

.•

400!}·

500!)' I

I 6300

··

8000

··

10000·· I

IJ:

__

.:....._~.

__

_

..

='J

162202

QQQQQQ

00

...

"::...,,.,.

•

••••

~

..

~

...

::::"'!..

..

-

......

Control

Panel

Fig. 1.

The

Noise

Limit

Indicator

shown

with

the

necessary extra items for

the

measurement

of

noise

and

vibration.

Two

input

channels

and

twelve

output

channels

with

filters can be plugged in.

3

a

lamp

lights,

the

operator

labels

the

unit

just

tested

with

a

number

corresponding

to

that

of

the

lamp,

and

directs

the

rejected

unit

to

the

repair

shop.

The

number

of

the

indicating

lamp

informs

the

repair

shop

of

the

frequency

at

which

the

presset

limit

has

been

exceeded,

thereby

often

giving

the

shop

a

hint

as

to

which

problem

has

to

be

corrected.

The

total

operation

time

for

the

assessment

by

the

apparatus

is

extremely

short.

When

set-up

practically,

the

testing

for

noise

and

vibration

can

normally

be

carried

out

in

less

than

ten

seconds

for

each

unit.

The

apparatus

is

devided

into

units

so

that

only

the

transducing

elements,

Le.

the

microphone

or

vibratioll

pick-up,

need

to

be

near

the

production

line.

The

main

unit

containing

thl'

amplifiers

and

filters

can

be

placed

some

distance

from

the

production

line

and

the

control

panel

with

indicating

lamps

may

be

placed

wherever

it

is

found

most

convenient

for

the

operator.

As

shown

in

Fig. 1

the

main

assembly

containing

power

supply,

meter

circuit,

input

amplifiers

and

output

amplifiers

is,

together

with

the

control

panel,

supplied

as

the

basic

assembly

to

which

can

be

added

a

condenser

microphone,

·a

vibration

pick-up,

and

as

many

filters

as

are

found

necessary

for

the

measurements

in

question.

Microphone

and

vibration

pick-ups

arc

readily

connected

by

plugs

on

the

rear

of

the

instrument,

and

output

am

··

plifiers

and

filters

are

made

as

plug-in

units

which

are

easily

interchange-

able

.

The

electrical

and

acoustic

specification

of

the

instrument

is

fully

described

on

page

43--44.

INTRODUCTION

TO

NOISE

AND

VIBRATION

MEASUREMENTS

ON

PRODUCTS

The

measurement

of

noise

and

vibration

may

be

carried

out

in

several

ways

.

depending

upon

the

purpose

of

the

measurement.

Some

of

the

most

commonly

used

methods

are

discussed

below

in

order

to give a

summary

of

the

reasons

for

measuring

noise

from

production

line

units

in

frequency

bands

. Noise

and

vibration

in

most

respects

act

in

a

sim

·

ilar

manner,

thus

the

term

"noise"

will

be

used,

and

vibration

will

only

be

mentioned

where

noise

and

vibration

act

in

a

different

way.

Some

of

the

most

common

methods

of

measuring

noise

levels

are:

1.

a)

wideband

measurements,

Le.

by

means

of

instruments

with

a

uniform

response

to

all

audible

frequencies.

b)

noise

measurement

with

instruments

that

have

"weighted"

characteristics.

c)

frequency

analysis

of

noise.

d)

measurement

in

selected

frequency

bands,

Le

.

with

instruments

thai

measure

noise

in

certain

pre-selected

frequency

bands

only.

a)

'Videband

Measurements·

When

making

wideband

measurements

of

noise

levels,

an

instrument

is used

that

res

ponds

equally

to

noise

of

all

frequencies

within

the

audible

rang

e,

which

is

from

20 cis

to

20000 cis

approximately.

This

means

that

the

measuring

instrument

gives

the

same

indication

for

all

sounds

of

equal

physical

"s

trength"

,

whether

the

sound

is

of

low,

medium

or

high

pitch.

When

measuring

noise

containing

several

fr

e

quencies,

th

e

wideband

measurin~

instrument

will

deflect

to

a

value

which

is

proportional

to

the

sum

of

e

nergy

contents

in

all

the

different

noise

frequencies

represented.

See Fig.

2.

This

type

of

measurement

is

used

where

it is

desired

to

measure

the

overall

sound pressure level

at

a

certain

place. (S.P.L.

in

db is

usually

with

ref

e

renc~

to

the

accepted

hearing

threshold:

0.0002 ,ubar).

1("5.

Frequency

Microphone

Fig.

2.

Wideband

measurement

of

noise.

The

meter will give

an

indication

which

is

slightly above

the

top

of

th

e highest peak

noted

in

the freque

ncy

spectrum. Noise

of

all frequencies passes unattenuated to the meter.

b)

"We

ighted"

Measurements.

We

ighted

measurements

are

m

eas

ur

e

ments

taken

with

instruments

which

have

response

curves

that,

to

a

certain

deg

ree

, follow

those

of

the

human

ear.

The

human

ear

appears

to

be

most

sensitive

to

me

dium

frequencie-s

and

is

therefore

not

so

irritated

by

low

or

high

pitched

noise.

The

frequen

cy

response

of

"weighted"

measuring

instruments

is

then

shaped

in

such

a

way

as to

approximate

the

sensitivity

curves

of

the

human

ear.

This

is

normally

done

by

insertion

of

one

or

mor

e

filters

in

a

wideband

amp

lifier.

Three

differ

e

nt

filter

chara;teristics

have

been

int

e

rnationally

standardized

for

this

pllrpose

.

• )

An

instrument

with

a

wideband

response

may also be

referred

to as one

with

a

response

wbich

is

linear,

overall,

or

lIat.

5

In

the

measurement

of

noise

containing

several

frequencies

,

the

instrument

measures

the

noise

as

the

sum

of

the

energy

contents

after

the

frequency

weighting

has

taken

place,

see

Fig.

3.

Weighted

measurements

are

mainly

used

in

cases

where

it

is

desired

to

investigate

noise

interference

of

working

conditions

etc.

The result

read

off

an

instrument

with

weighted

characteristics

is

the

sound

level

(different

from

sound

pressure level)

and

is

stated

in

db

SL

(=

db

de. 2 X 10-4

,ubar),

curve

A,

B

or

C.

Frequency

Microphone

Frequency Weighted

Level

c

o

:;:;

'-0

QJu

~'O

LE

Frequency

16

220

7

Fig.

3. Weighted measurements. As

shown

the noise spectrum

is

shaped by

means

of

a filter so

that

parts

of

the noise

is

depressed

and

only the

medium

frequencies are allowed

to

pass unattenuated.

That

means

that

the

medium

frequencies have a predominant influence on

the

meter

indication.

c)

Frequency

Analysis

of

Noise.

Frequency

analysis

is

carried

out

by

instruments

that

are

capable

of

selecting

and

measuring

specific

noise

frequency

bands

only,

although

several

other

noise

frequencies

may

be

present.

The

frequencies

can

be

selected

by

means

of

a

series

of

filters

which

are

successively

inserted

in

a

wideband

measuring

instrument,

and

which

make

the

instrument

sensitive

to

only

a

certain

frequency

band

at

a

time

,

see

Fig.

4.

Usually

in

noise

measurement

the

audible

frequency

range

is

divided

into

a

number

of

bands

of

equal

relative

bandwidth.

Thus

,

an

octave

band

analyzer

divides

the

audible

range

into

10

bands

,

each

with

the

width

of

one

octave;

a 1

,6

octave

band

analyzer

divides

6

,

Mi

c

rophone

Filter characteristic

:-"'J"r-

v--

.....

(-y

....

y-v-.,--

I I I I I

II

I \

I n 1\ I I

1\

/1

II

~I ~I

\

,

,I

\

J\

I 1 1I

1\

II

1I I

/ /

\,

I

I,

/I

' ' I I \ 1 \ \

/ I XI

I,

'V

\I

1/

I \

/

Frequency

Naise

bond

Level

/

Filter

~

--

- -

I I

I \ §

I \ L..

:g

I \

!!

.

~

/ \

OJ

"0

L-

__

-'-

,

_----'

,

_____

..

::E

S

Frequency

f62205

Fig. 4. Analysing noise

by

means

of

a series

01

narrow band filters that

can be inserted successively

in

the amplifier. Only the noise having a

frequency

that

falls within

the

range

of

the

inserted filter

is

passed to

the

meter.

The

meter

indication

in

this

way

becomes frequency deper.dant.

the

range

into

30

bands,

each

Ih

octave

wide. An

example

of

such

an

a

nalyzer

is

the

B & K

Type

2112

mentioned

on

page

11.

With

an

analyzer

of

this

typ

e

th

e noise band level

can

be

measured

at

any

arbitrary

frequen

cy

without

any

disturbing

influence

from

other

(and

pe

rhaps

"stronger

")

sounds

ar

riving

at

th

e

microphon

e

simultaneously.

In

this

way

it is possible to

investigate

which

fr

e

qu

encies

are

predominant,

measur

e

their

"strength"

and

c

haracter

and

thereb

y a

rrive

at

a

method

for

treatment

of

the

noise

problem

.

Bands

narrow

er

than

lh octave

are

not

normally

used

in

nois

e

measurements;

how

ever,

in

th

e inves

tigation

of

vibration,

an

analyzer

with

filters having

a

width

of

less

than

lh octave

would

often

be employed.

The

B & K

Type

2107 is

an

example

of

such

an

analyzer

.

d)

Measurements

in

Seleeted

Frequeney

Bands.

Once

laboratory

investigations

hav

e

been

carried

out

upon

objects

from

a

production

lin

e (or

on

the

prototyp

es)

and

th

e

noise

and

vibration

generated

by

the

obj

ects is

analysed

e.g.

by

means

of

an

analyzer

as

me

ntioned

above,

it

can

be

calculated

which

of

th

e

nois

e

frequencies

must

be checked in

production.

Ofte

n,

atte

ntion

ma

y

be

confined

to

certain

important

frequ

encies

only,

and

other

bands

ignor

ed. A special

instrum

e

nt

which

measures

these

particular

noise

frequ

encies (see Fig.

5)

can

then

be

used

to

check

the

units

7

from

the

production

line.

Such

an

instrument

is

the

Noise

Limit

Indicator

Type

2211.

It

measures

noise

or

vibration

in

a

number

of

pre-selected

frequency

bands

simultaneously.

The

bandwidth

can

be

chos

en

to

be

1 octave

or

lh octave,

depending

on

the

conditions.

, Microphone Filter. characteristics

lDJ0J\n.

Frequency c

.2

F

ilt

ers

/

('iill[\

[I[!1

Frequency

162206

Fig.

5.

This figure

shows

the

measuring principle

of

the

Noise

Limit

Indicator. Several filters are inserted.

The

meter is

substituted

by

red

lamps,

so

that

there is

an

indicator for e a c h filter.

Thereby

the

excess

of

the

noise band limits

of

several filters can

be

indicated simultaneously.

The

;ilter attenuation can be adjusted for each filter individually.

The

noise

frequency

bands

and

the

levels

set

as

the

maximum

limit

for

each

of

the

bands

selected

should

be

chosen

as a

result

of

careful

l

aborato

ry

investigation

of

the

object

to

be

examined.

It

may

also

be

desirable

to

keep

a

check

on

the

overall

noise

level

of

the

object

by

means

of

wideband

and

weighted

measurements

.

The

Noise

Limit

Indicator

is

capab

le

of

checking

a

number

of

pre-selected frequency band levels

as

well as

the

overall noise

level, both wideband and weighted, simultaneously.

CAPABILITIES OF

THE

NOISE LIMIT INDICATOR

The

equipment

can

be

used

for

a wide

range

of

tests,

since

a

number

of

measurements

can

be

carried

out

simultaneously

with

the

Noise Level

Indicator

.

Some

af

the

most

common

ly used

measurements

that

can

be

made

with

the

instrument

are:-

8

1)

Simple

"go-no

go"

testing

for

noise

or

vibration

or

both

simultaneously

(see Fig. 6a).

0

a)

0

Standard

0

b)

0

DO

D O

Extra

Standard

0

c)

0

0 0 =

=

0 = 0 =

= =

E

xtra

Standard

0

00

d)

000

0000

Closs 1 2 3

Noise

Limit

Indicator 0

Rejects

Noise

Limit

Indicator

Noise

Limit

Indicator

Noise

Limit

D

0

Rejects

t622()f

D§

Rejects

Indicator 0

Rejects

(62200

Fig. 6. Different measuring possibilities

with

the

Noise

Limit

Indicator.

a)

Simple testing dividing the objects

from

a production line

in

"rejects"

and

"standards"

with

respect to noise

and

vibration.

b)

Same

as a),

but

additionally dividing

the

approved objects

in

two classes,

"standard"

and

"extra".

c)

If

only

one check

of

the noise or vibration

of

a unit is required, the two

inputs

of

the

instrument

may

be used

to

serve

two

production lines or two

kinds

of

products

on

a line, e.g.

two

types

of

motors, each

with

its own

noise frequencies

and

limits.

d)

When

investigating

the

noise or vibration level

of

a series

of

units, the

instrument

may

be used to divide

the

units

in

3, 4 or 5 classes as regards

noise or vibration.

9

2)

Testing

of

units

as

in

1

but

additionally

dividing

the

"go"

objects

in

two

classes:

"Standard"

and

"

Extra

" (Fig. 6b).

3)

Dividing

units

into

three

classes,

"extra",

"standard"

and

"

rejects

"

with

respect

to

noise

only,

checking

two

types

of

units.

Alternatively

tests

on

one

type

of

units

from

two

directional

points.

(Fig. 6c).

4)

Dividing

units

into

three

classes

as

above

with

respect

to

vibration

only,

checking

on

two

types

of

units,

or

on

one

type

of

unit

at

two

positions.

5)

Dividing

one

type

of

units

into

3, 4

or

5 classes

with

respect

to

noise

or

vibration

(only

one

microphone

or

accelerometer

is

needed)

(Fig. 6d).

Several

other

combinations

are

possible

depending

upon

the

purpose.

Each

of

the

two

input

amplifiers

can

be

connected

to

a

microphone

or

accelero·

meter,

so

as

to

employ

either

two

microphones,

two

accelerometers

or

one

microphone

and

one

accelerometer

.

Which

data

must

be

known

before

the

Instrument

is

used.

As

the

Noise

Limit

Indicator

is

not

an

analysing

instrument,

certain

in

·

vestigations

must

be

carried

out

before

it

can

be

used. In

ord

er

to

correctly

adjust

and

use

the

Noise

Limit

Indicator

the

following

factors

must

be

pre-

determined:

1.

The

centre

frequencies

and

the

width

of

the

frequency

ballJd·s

that

must

be

checked.

II.

The

maximum

noise

and/or

vibration

limit

for

each

band.

III.

The

ambient

conditions

under

which

the

measurement

should

be

carried

out.

In

certain

cases,

for

instance

in

the

production

of

motors

,

gear

boxes

or

other

items

which

are

manufactured

to

custom

er's

specification,

these

factors

may

be

given

by

the

customer

as

part

of

his

specification.

It

is

then

extr

emely

important

that

item

III

above

is

carefully

specified.

Under

"ambient

con-

ditions"

it

is

necess

a

ry

to

state

:

1.

Ambient

noise

or

vibration

level.

2.

Properti

es

of

the

measuring

room

or

chamber,

i.e.

hard

walls, non-

reflecting

walls,

dim

e

nsions

etc.

3.

Exact

location

of

object

and

microphone

within

the

room

.

4.

Mechanical

conditions

of

object

such

as:

a)

loaded

or

idle

running.

b)

method

of

susp

e

nsion,

i.e.

under

simulated

w

u~

king

co

nditions

or

on

special

test

stand

,

in

the

latter

case

the

stand

must

be described.

5.

For

vibration

measurement,

the

method

of

fastening

the

vibration

pick-up

as

well

as

the

exac

t

point

where

it

must

be

placed

.

6.

Other

special

requirements

such

as

temperature,

prior

treatment,

etc.

In

other

words,

it

is

necessary

to

lay

down

a

specification

which

is detailed

enough

to

ensure

that

the

test

conditions

relevant

to

requirem

e

nts

I

and

II

can

be

reproduced.

10

In

most

cases

the

requirements

for

noise

or

vibration

limits

are

not

available

in

th

e

form

of

well-defined

specifications.

The

usual

quality

referenc

e is a

standard

unit

which

meets

th

e

customers'

requirements

and

which

has

to

be

reproduced

within

ce

rtain

tole

ranc

es.

The

problem

th

en is to

ca

rry

out

m

eas

urem

e

nts

on

thi

s

object

in

order

to

obtain

th

e

information

required

in

I

and

II.

The

ambient

cond

itions

for

the

pr

e

liminar

y m

eas

ur

eme

nts

should

be

similar

to

those

und

er

which

th

e

unit

was

a

pproved

. Later, in

production

,

th

e

ambient

conditions

m

ay

be

different,

so it is

then

necessary

to investigate

what

changes

in

the

nois

e field

the

change

of

ambient

condition

has

in-

troduced.

In

order

to

investigate

th

e

noise

and

vibration

generated

by

a

standard

unit

,

an

arrangement

as

shown

in

Fi

g. 7a

can

be

em

ploy

ed.

a)

b)

Test

object

Microphone

Accelerometer

2112

I1U

Fig.

7.

cable

Level

Recorder

!422'l

cable

Level

Recorder

162198

a)

Analysis

of

noise

by

means

of

an

A.F.

Spectrometer

Type

2112

and

a Level

Recorder

Type

2305.

b)

For

Analysis

of

vibration,

th

e

microphone

is interchanged

with

an

accelero-

meter.

This

"set-up"

consists

of

a

microphone,

a

1h

octave

band

analyzer

and

a

level

recorder.

The

microphone

is

placed

near

the

test

obje

ct

so

that

it

r

ea

dily

picks

up

the

generated

nois

e,

which

is

then

fed

to th

e

analyzer

in

the

form

of

an

electrical

signal.

The

analyzer,

which

is

synchronized

with

the

level

record

er

pap

er

drive

,is

now

adjusted

to

scan

the

frequency

range

from

16 cJs to

31

kc

/s.

The

noise

generated

by

the

test

object

is

measured

and

record

ed

in

1h

octave

bands

and

presented

as

a

spectrogram

(noise

11

Sound

Pressure

Level.

113

octave

Sound

Pressure

LeveL

---,.

"L'

..

500qs

I

Sound

Level

curve

"

A

::'

~

::":

'~

:'"

..

~

00000000000

0

000000

000

0 0

O

!~!

O

0 0 0 0 0 0 0 0 0 ' 0 0 0

O

~~

I b0

I,D:!!.!

I

.......

50

25

Mol..

....

at·

..........

. .

......

_--

Noise from

....

&,eae1-e~1~

i

Jn

bgoddri"

Distgnce

Scm

113

octgve

anaLysis

II

~

100

1000

10000

/

6f

9

·H

(0)

Fig.

8.

Noise spectrogram as obtained

with

the

set-up

shown

in

Fig.

7.

1h

octal'e band analysis.

Sound

Pressure level.

1/1 ode'll!

Sound

Pressure

level

~

500

CIS

I

Sound

Levelcurve·:A.'

~

"li

~

a 0

[J

0 0

CI

a

~

0 0 0

[]

0 0 0 0 0 a 0

[J

· O 0 a 10

~

o !o 0 0 0

00

a 0 0 0 0 0 0 0

~

b0

I'D:.!.!.!I

. '

+!OZI

Mol

'....

'-

'

~ _..... .

)1

I i

lCl7t

""'-~-

-

Noise from

....

I)!!gr_bo!C jn

~

Tff'--OOlO

Distance

5 em

1/1

oeiave

analysjs

~

100

1000

'O/9f2.

(

b)

Fig.

9. Noise spectrogram obtained

with

the

set-up

in

Fig. 7,

but

with

the

Spectrometer

Type

2112 set for octave analysis.

curve)

as

shown

'

in

Fig.

8.

From

this

curve

the

1h

octave

noise

level

in

any

frequency

band

within

the

audible

range

can

be

read

directly,

so

that

a

determination

can

be

made

of

the

noise

limits

to

which

the

Noise

Limit

Indicator

should

be

adjusted.

The

B & K

Analyzer

Type

2112

mentioned

above

may

also

be

used

for

the

analyses

with

octave

bands.

The

recorded

results

will

then

appear

as

shown

in

Fig

. 9.

12

This

arrangement

can

be

used

for

the

measurement

uf

vibration

as

well, if

the

microphone

is

int

e

rchanged

with

an

acceleromet

e

r,

e.g.

one

of

the

B & K

Types 4312-

13

-14-

15

.

To

investigate

carefully

the

noise

or

vibration

generated

from

an

object

several

individual

spectrograms

or

sometimes

several

series

of

spectrograms

must

be

obtained.

The

microphon

e

should

be

placed

in

several

positions

in

relation

to

the

test

object,

and

spectrograms

should

be

recorded

for

each

position.

Th

e

records

ar

e

then

compared

in

order

to

find

the

position

where

the

microphone

obtains

the

highest

sensitivity

for

those

noise

frequencies

that

are

considered

most

significant.

This

multiple

measurement

technique

is e

ven

more

necessary

in

vibration

tests.

The

accelerometer

should

be

placed

experimentally

at

several

points

on

the

test

object

in

order

to

find

a

position

where

it is

sensitive

to

vibrations

of

the

frequencies

considered

most

important.

In

some

cases it

may

be

necessary

to

employ

more

than

one

accelerometer

for

production

line testing

in

order

to

obtain

a

distinct

reading

for

all

the

vibration

frequencies of

interest.

For

instanc

e,

a

motor

may

generate

vibrations

at

several

frequencies

,

but

it

may

not

be

possible

to

find

one

point

on

the

motor

chassis

where

all

the

inter

esting

frequ

encies

are

repres

e

nted

with

sufficient

strength

to

give a

dear

measurement.

It

is

th

en

ne

cessary

to

divide

the

m

eas

urement

,

i.

e.

to

plac

e two

vibration

pick-ups

at

different

positions

and

thus

measure

one

group

of

the

desired

fr

e

qu

encies

by

means

of

one

pick

-

up

and

the

remaining

group

with

th

e

other

pick-up. Of

course,

an

identical

method

must

th

en be

us

ed

during

the

production

control.

Where

no

noise

re

quirem

e

nts

are

set

by

the

customer,

but

where

it

is

desired

to

make

a

uniform

product

from

the

points

of

view

of

noise

and

vibration,

the

limits

are

often

set

by

the

average

noise

figure

of

the

produced

units. A sufficie

ntly

larg

e

series

is

measured

and

an

average

noise

figure

determined.

Th

e

limit

is

then

set

within

a

suitable

toleran

ce

from

the

average.

Th

e

use

of

this

limit

set

by

the

average

noise

figure

not

only

ensures

a

uniform

product

but

also

facilitates

the

detec

tion

and

improvement

of

smaller

er

rors

or

i

ginating

in

the

production

system.

It

may

then

be possible to

low

er

the

noise

and

vibration

limit

of

the

products

.

WHAT

SHOULD

BE

MEASURED?

NOISE

or

VIBRATION?

Before

the

apparatus

is

used

it

is

necessary

to

decide

what

to

measure,

noise

or

vibration

,

at

which

frequencies

,

and

how

much

noise

or

vibration

can

be

tolerated

.

Upon

the

question

"Noise

or

vibration"?

noise

in

most

cases seems

to

be

more

important.

Howev

er,

vibration

is

often

easier

to

measure,

because

it

is

easier

to

isolate

the

test

object

from

other

unwanted

and

disturbing

vibrations

that

occur

in

a

factory

than

it

is to

isolate

the

object

from

the

high

level

background

noise

which

normally

occurs

close to

production

13

lines.

As

noise

is

always

linked

to

mechanical

vibration,

it

has

therefore

become

quite

common

to

measure

vibration

in

the

place

of

noise,

or

to

measure

both

where

the

ambient

noise

conditions

permit

it.

The

advantage

of

measuring

vibration

instead

of

noise

is

most

clearly

seen

in

m

eas

urements

where

the

noise

generated

is

of

the

order

of

30-50

db

SPL

.

This

is

the

normal

background

noise

in

most

areas

where

no

industrial

activities

are

taking

place

.

In

such

cases

a

noise

insulating

arrangement

must

be

employed

to

shield

the

test

object

during

measurement.

The

use

of

shields

often

in-

volves

extra

probl

e

ms

and

costs.

If

vibration

measurements

are

sufficient

in

such

circumstances,

the

advantage

has

been

gain

ed

that

the

vibration

pick

-

up

can

be

placed

on

that

part

of

the

machinery

wh

e

re

the

vibrations

are

strongest,

while

a

microphone

would

only

measure

the

overall

noise

in

general. A

higher

"selectivity"

is

thus

obtained

by

using

the

vibration

pick-up

instead

of

the

microphone,

and

thus

it

is

easier

to

separate

the

noise

generated

by

the

test

object

from

the

background

noise

.

When

switching

from

noise

measurement

to

vibration

measurement

, i.e.

when

vibration

is

measured

to

keep

control

on

noise, it is

necessary

to

carry

out

some

laboratory

investigation

on

the

re

lationship

betwe

en

noise

and

vibration

.

Of

course,

noise

from

machinery

is

generated

by

mechanical

vibrations

.

but

the

predominant

noise

frequency

is

not

necessarily

the

same

as

the

pre-

dominant

vibration

frequency.

Often

the

strongest

vibration

has

a

frequency

too

low

to

be

noticed

by

the

human

ear,

while

a less

serious

vibration

peak

produces

a

more

annoying

noise,

because

it

falls in a

frequency

rang

e

where

the

ear

is

more

sensitive.

Even

if

the

frequency

is

in

the

easily

audible

range

a

serious

vibration

may

not

give rise

to

a

high

noise

level

because

the

efficiency

of

noise

radiation

may

be

poor.

It

is

therefore

necessary

to

record

one

or

several

spectrograms

of

both

the

noise

and

the

vibrations

generated

by

a

test

object.

The

ne

xt

step

is to

compare

the

two

sets

of

spectrograms

and

after

this

determine

what

degree

of

vibration

can

be

allowed

in

order

to

make

sure

that

the

noise

level is

kept

under

a

certain

limit.

It

is

also

necessary

to

take

into

consideration

whether

the

object

is going

to be

tested

under

conditions

that

are

similar

to

the

ambient

conditions

at

the

pla

ce

where

it will

eventually

be

used,

or

under

other

specified

test

conditions.

It

must

be

carefully

investigated

to

what

degree a

possible

change

in

ambient

conditions

will

influence

the

measurement.

For

instance

, a

careful

examination

should

be

made

of

the

change

in

vibration

of

an

object

when

mounted

on

a

solid

fixture

and

afterwards

tested

on

a

spring-suspended

test

stand

.

PRACTICAL ARRANGEMENTS

WHEN

"SETTING

-

UP"

THE

INSTRUMENT

The

practical

arrangements

that

are

necessary

when

setting

up

the

Noise

Limit

Indicator

are

often

the

most

important

factors

in

the

installation

of

the

equipment.

In

most

cases

wh

e

re

it is

desired

to

measure

noise

or

vibration

14

of

the

units

leaving a

production

line, a

test

stand

for

the

quality control

will

already

be

provided

.

Noise

and

vibration

inspection

can

then

be

included

in

the

normal

procedure,

adding

only

a

fraction

to

the

time. However,

it

may

sometimes

be

necessary

to

modify

the

existing test

stand

slightly, or,

if

no

stand

is

provided

,

to

make

a

light

-

weight

arrangement

for

the

noise

and

vibration

inspection.

The

following gives examples

of

how

the

testing

may

be

arranged

·.

Vibration

Testing.

Vibration testing

has

one

main

disadvantage

compared

to

noise testing:

It

is

necessary

to

pl

ace

the

vibration pick-up

mechanically

in

contact

with

the

test

object

and

in

exactly

the

same

position

for

each

unit

tested.

In

noise

testing

the

object

can

pass

close

by

the

microphone

without

touching it.

There

is

one

way

of

overcoming

this

problem

when

testing

vibration.

If

the

test objects

vibrate

as

a whole,

the

vibration

pick-up

can

be

placed

on

the

test

stand.

The

stand

should

then

be

of

a

fairly

light-we

ight

construction

To Noise

li

mit

Indic

at

or

To

No

ise

Le

vel

Indicator

f622{O

Fig.

10

. Placing the object

on

a light-weight test stand enables vibration

pick-ups to be

mounted

on

the test

stand

. Vibration

in

two

directions can be

checked

in

one operation.

To

Noise

Level

Ind

ic

ator

/-

7

tl

I , '

/ ,'

I "

/ ,

t..

f

,,~?:

To

N

ois

e

level I

nd

icator

Mo

veable

or

m

162213

Fig. 11.

The

accelerometer

may

be fastened

by

means

of

a small magnet

or

with the aid

of

a springloaded arm.

15

and

be

placed

on

springs

or

on

a piece

of

foam

rubber

as

outlined

in

Fig.

10.

The

accelerometers

are

then

placed

on

the

stand

as

in

Fig

. 10.

The

two

channels

of

the

Noise

Limit

Indicator

can

now

be utilized

to

measure

vibrations

in

two

directions,

or,

if

only

one

direction

is

required,

to

serve

two lines

simultaneously.

Unless

the

test

object

is going

to

be

suspended

in

a

similar

way

as

it

will

be

in

the

place

where

it

is to

be

used

,

the

effect

of

the

change

in

suspension

should

be

carefully

examined

and

taken

into

consideration

before

this

test

method

is applied.

If

the

accelerometer

cannot

be

placed

on

a test

stand,

or

in

cases

where

it

is

necessary

to

place

the

accelerometer

at

certain

places

on

the

machinery

in

order

to

obtain

sufficient

"selectivity"

of

the

measurement

,

the

accelero-

meter

can

often

be

fastened

to

the

object

by

means

of

a

small

magnet

,

screwed

onto

the

accelerometer

as

shown

in

Fig.

11

left

. Also

the

use

of

a

moveable

spring-loaded

arm

as

outlined

in

Fig.

11

right

can

often

be

advantageous.

The

common

method

of

hand-held

operation

employing

a test

probe

or

rod

is

shown

in

Fig.

12

.

To

No

ise

Level

Indicator

f62f97

Fig.

12. Conventional hand-held operation

of

the accelerometer

may

also

be

used.

NOTE

:

Whichever

method

is

used

to

fasten

the

accelerometer

it

must

be

ensured

that

the

fastening

arrangement

does

not

introduce

any

extra

resonance

peaks

at

frequencies

that

will

disturb

the

measurement.

Noise

Testing.

Noise testing

has

an

advantage

over

vibration

testing

in

that

it

does

not

involve

any

mechanical

contact

between

the

test

object

and

the

measuring

equipment.

The

microphone

may

thus

be

placed

over

a

production

line

where

the

units

to

be

tested

pass

underneath

.

The

units

must

be

consis

tantly

positioned

at

the

moment

that

the

measurement

is

carried

out

,

but

they

do not

have

to

be

touched

by

the

measuring

equipment.

16

On

the

other

hand,

nois

e

measurement

are

mor

e

dependent

upon

ambient

co

nditions

than

vibration

m

eas

ur

e

ments

.

Wh

en

for

vibr

a

tion

measurem

e

nt

s

th

e test

specimen

can

sometimes

be

isolated

simpl

y

by

m

ea

ns

of

a

pi

ece

of

foam

rubber

beneath

the

spec

im

en,

it is

often

necessary

to

"

screen"

th

e

object

when

sound

m

eas

ur

eme

nts

are

carried

out.

Unfortunately,

th

e

screening

for

ambient

sounds

e

ncircl

es

the

object,

maki

ng

measurements

difficult. Testing

products

that

are

making

a l

oud

nois

e,

suc

h

as

internal

comb

ust

ion

engines.

To

Noise

Limit

Indicator

Light

-weight

screening

To

Noise

limit

Indicator

/ Microphone

=

~-::"

l

r------,

/'

/

/ /

,/

!62196

Fi

g.

13. Where

th

e noise inte

nsity

generated

by

the

test object exceeds the

ambie

nt

noise to a fair degre

e,

the

microphone can be placed

in

a convenient

distance from

th

e object as s

hown

left.

If

th

e

ambient

noise comes too close

to the noise to be measure

d,

a light-weight screen

may

be

necessary as shown

to

the

right.

To

Noise Limit

Indicator

Fig. 14. Noise testing

in

heavy

screen.

Th

e

room

ha

s reasonab

ly

small outside

dimensions

and

walls

of

heavy

damping material. This

room

is

not

suited for

directional testing, as it produces a diffuse so

und

fi

eld

at

lower frequenci

es.

17

of

course

creates

no

problems,

because

the

test

object

produces

more

noise

than

is

normally

expected

from

the

ambient

noise

sources.

For

such

cases

a

microphone

can

be

used

to

measure

noise

without

any

screening,

provided

it is

placed

fairly

close

to

the

object.

Objects

making

noise

in

the

medium

loudness

range

can

normally

be

tested

under

a

light

screen,

where

the

microphone

can

be

placed

in

the

screening

as

outlined

in

Fig. 13.

The

problem

of

screening

mainly

arises

when

it

is

desired

to

measure

sounds,

the

loudness

of

which

are

low

as

a

whisper,

and

where

ambient

noise is

conspicuous.

Several

methods

of

screening

may

be

employed.

The

object

may

be

placed

in

a

small

hard-walled

chamber

in

order

to

produce

a diffuse

sound

field

for

the

measurement

(see Fig. 14),

or

it

can

be

placed

in a

larger

silent

(heavily

absorbing)

room

to

make

it

possible

to

obtain

directional

noise

patterns

(see Fig. 15).

The

common

requirement

for

both

rooms

is

that

they

must

hav

e a

"shell"

of

heavy

material

to

attenuate

the

disturbing

sound

from

outside

.

This

"shell"

can

be

built

as a

brick

wall,

single

or

double,

or

a

wooden

wall

with

sand

filling,

the

thickness

depending

upon

the

desired

attenuation

.

Oomping

material

To

Noise

Limit

Indicator

Fig.

15. Noise testing

in

room

with

absorbing walls. Directional testing

is

possible,

but

the

chamber

must

be largely dimensioned.

In

all cases

it

is

necessary

to

find

out,

by

means

of

laboratory

investigations,

what

effect

the

protection

against

disturbing

noise

has

on

the

sound

field

of

the

test

object,

compared

to

its

sound

field

condition

when

actually

in

use.

lt

is

advisable

in

many

cases

to

consult

an

acoustical

expert

for

the

solution

to

the

problem

of

test

room

constructions

.

18

USE

OF

THE

NOISE

LIMIT

INDICATOR

Pre-adjustments.

As

already

mentioned

on

page

10,

it

is

necessary

to

pred

etermi

ne

the

nois

e

frequencies

to

be meas

ur

ed, as well as

the

noise

levels

to

be tol

era

ted.

When

the

width

and

th

e

centre

frequencies

of

the

required

number

of

frequency

bands

as

well

as

the

noise

or

vibration

level

for

each

band

have

been

laid

down,

and

the

practical

arrangements

made,

the

instrument

should

be

adjusted.

This

adjustment

should

be

made

by

som

eo

ne

who

has

a

fundamental

idea

of

e

lectrical

circuits

or of

noise

measurements

in

general.

It

should

preferably

be

carried

out

in

accordance

with

th

e following

procedure.

Adjustment

for

Measurement

with

Microphones

Type

4131-4132.

The

use

of

the

Noise

Limit

Indicator

in

its

simplest

form

will be described

first.

That

is,

when

the

instrum

ent is

coupled

to

only

on

e

microphone,

B & K

Type

4131

or

4132. (Usi

ng

other

microphones,

see

page

24)

.

It

is

assumed

162283

Fig. 16.

An

Output

Amplifier

withdrawn

from

the

chassis.

The

screws intended

for fastening

the

filter chassis are indicated at

the

rear.

that

the

object

has

to

be

tested

for

noise

by

means

of

one

microphone

and

that

the

frequencies

and

noise

limits

are

given.

1.

The

microphone

is

placed

in

its

correct

fixed

position

in

relation

to

the

test

object.

Then

the

cathode-follower

cable is

connected

to

the

input

socket

I

on

the

rear

panel.

In

addition,

the

control

panel

is

connected

to

the

main

chassis

by

the

muIticore

cable,

the

power

cable is connected

to

the

power

line

and

power

switched

on.

19

BRUEL & KJAER 2211 User guide

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