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.

BHUEL&KJJEH

NaHum, Denmark .

eyf;'

80

05 00 .

~

BRUKJA,

Copenhagen

. Telex:

5316

BB

2211

Noise Limit Indicator

Type

2211

Reprint

April

1963

Contents

Introduction

to

the

Instrument

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

Introduction

to

Noise

and

Vibration

Measurements

on

products

. . . . . . . . 4

'Vid

eband

Measurements

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Weighted

Measurements

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Frequency

Analysis

of

Noise

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

Measurements

in

Selected

Frequency

Bands

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

Capabilities

of

the

Noise

Limit

Indicator

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

Which

data

must

be

known

be

·

fore

the

Instrument

is

used

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

10

Wb.at

should

be

measured,

Noise

or

Vibration

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

13

Practical

arrangements

when

"setting

up"

the

Instrument

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

14

·

vibration

testing

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Noise

testing

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Use

of

the

Noise

Limit

lndicatol'

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

19

Pre-adjustments

.

.....

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

....

.....

..

19

Adjustment

for

use

with

Microphone

Type

4131

-32

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

19

Adjustment

with

1

h"

Microphone

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

25

Vibration

Measurement

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

25

Adjustment

of

the

"Sensitivity

Increase"

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

28

Operators

Instruction

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

31

Use

of

both

Inputs

sim

ult

aneous

ly

...

..

.......

....

.......

..

32

Remote

Control

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

34

Technical

Description

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Input

Amplifiers

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Filters

and

Output

Amplifiers

........

...

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

..

......

...

37

Rectifier

and

Relay

Circuit

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Contro

l

Panel

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

l\feter

Circuit

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

41

Power

Supply

..........

....

..

....

...

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

....

.

......

.

41

Specification

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Introduction

The

Noise

Limit

Indicator

is

an

instrument

designed

to

provide

a

rapid

and

accurate

evaluation

of

the

quality

of

products

on

a

production

line

with

respect

to

noise

and

vibration.

This

is

achieved

by

measuring

the

noise

and

vibration

of

every

unit

leaving

the

production

line

and

comparing the

result

with

the

noise

and

vibration

level

obtained

from

the

"standard"

unit.

The

instrument

has

been

designed

to

offer

the

utmost

simplicity

in

operation.

After

setting-up,

an

unskilled

worker

can

easily

interpret

the

indicating

panel

and

thereby

the

acceptance

or

rejection

of

the

unit

manufactured.

Rejections

are

indicated

by

red

indicating

lamps.

In

this

way

the

possibility

of

confusion

or

errors

from

meter

readings

is

completely

eliminated.

The

instrument

measures

noise

and

vibration

in

several

frequency

bands

(up

to 12

bands

)

simultaneously.

The

operator

's

job

is

simplified

to

such

a

degree

that

he

only

presses

a

button

and

notes

if

any

lamp

lights

up.

If

These

units

ore

bought seperotely

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.

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.

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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.

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

,

i.e.

the

microphone

or

vibration

pick-up,

need

fo

be

near

the

production

line.

The

main

unit

containing

the

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,

to

'

gether

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

instrum

e

nt

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

uni

Is

in

frequency

bands.

Noise

and

vibration

in

most

respects

act

in

a

similar

manner

, thu,;;

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:

a)

wideband

measurements,

i.e.

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,

i.e.

with

instruments

thai

measure

noise

in

certain

pre-selected

frequency

bands

only.

a)

\Videband

Measurements·

Wh

en

making

wid

eba

nd

nwasurements

of

noise

levels,

an

in

s

trum

e

nt

is used

that

res

ponds

e

qually

to

noi

'Se

of

all

fr

e

qu

e

ncies

within

the

audible

rang

e,

which

is

from

20 c/s

to

20ooo

c/s

approximately.

This

m

ea

ns

that

the

measuring

instrum

e

nt

gives

the

same

indication

for

all

sounds

of

e

qual

physical

"strength",

whether

the

sound

is

of

low

, me

dium

or

high

pitch

.

vVh

en m

eas

uring

noi

se

co

ntaining

several

fr

e

qu

e

nci

es,

the

wid

e

band

measurin~

instrument

will

defl

ect

to

a

value

which

is

proportional

to th

e

sum

of

energy

co

ntents

in

all

th

e

diff

ere

nt

noise

frequencies

represented.

See Fig.

2.

Thi

s

typ

e

of

m

eas

urem

e

nt

is used

wher

e

it

is des

ired

to

measur

e

th

e

overall

sound

pressur

e level

at

a

certain

place.

(S

.P.L.

in

db

is

usually

with

ref

ere

ncP.

to

the

accepted

hearing

threshold:

o.0002 t-tbar).

ir·r:::;.

Frequency

Microphone \

Fig.

2.

Wideband

measurement

of

nois

e.

The

meter

will

give an

indication

which

is

slightly

above

the

top

of

the

highest

peak

noted

in

the

frequency

spectrum.

Noise

of

all frequencies passes

unattenuated

to

the

meter.

b)

"\Veigh

ted"

Measurements.

\Veighted

m

eas

ur

eme

nt

s

are

measur

eme

nts

taken

with

instrum

e

nts

which

hav

e

respons

e c

urv

es

that

,

to

a

certain

degree,

follow

thos

e

of

th

e

human

ea

r.

The

human

ea

r

appears

to

be

most

sensitive

to

medium

frequencies

and

is

th

er

efo

re

not

so

irritat

ed

by

low

or

high

pitched

nois

e.

The

frequency

r

espo

nse

of

"weig

ht

ed"

measuring

in

s

trum

e

nts

is

then

shaped

in

such

a

way

as

to

approximate

th

e

se

nsitivity

curves

of

th

e

human

ea

r.

This

is

normally

done

by

ins

e

rtion

of

one

or

more

filt

e

rs

in

a

wideband

amplifier.

Thr

ee

different

filter

chara

~

t

e ristics

have

been

int

e

rnationally

standardized

for

thi

s

purpose.

•)

An

instrument

with

a

wideband

response

may

also

be

referred

to

as

one

with

a

response

which

is

linear,

overall,

or

flat.

5

In

the

measurement

of

noise

containing

several

frequencies,

the

instrument

measures

the

nois

e

as

th

e

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

lev

e

l)

and

is

stated

in

db

SL (=

db

de

. 2 X lo

-4

.ubar),

curve

A, B

or

C.

Frequency

Microphone

Frequency Weighted

Level

-

-0

Frequency

162207

Fig.

3.

Weighted

measurements.

As

.

~hown

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

pa.ss

unattenuated.

That

means

that

the

medium

frequencies

have

a

predominant

influence

on

the

meter

indication.

c)

Frequency

Analysis

of

Noise.

Frequ

e

ncy

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

Hl

bands,

eac

h

with

the

width

of

one

octave;

a 1

h

octave

band

analyzer

divides

6

~r·es.

l,

Microphone

Frequency

Noise

band

Level

~

------

/

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I I

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c

0

~

L...C

.!!

.

!:!

QJ"C

L.E

Frequency

1622.05

Fig. 4.

Analysing

noise

by

means

of

a series

of

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

depei..dant.

the

range

into

30

bands,

each

1

h

octave

wide.

An

example

of

such

an

analyzer

is

the

B & K

Type

2112

mentioned

on

page

11.

With

an

analyzer

of

this

type

the

noise

band

lev

el

can

be

measured

at

any

arbitrary

frequency

without

any

disturbing

influence

from

other

(and

perhaps

"s

tronger

")

sounds

arriving

at

the

microphone

simultaneously.

In

this

way

it

is

possible

to

investigate

which

frequencies

are

predominant,

measure

their

"strength"

and

character

and

thereby

arrive

at

a

method

for

treatment

of

the

noise

problem.

Bands

narrower

than

1

h

octave

are

not

normally

used

in

noise

measurements;

however

,

in

the

investigation

of

vibration,

an

ana

lyz

er

with

filters

having

a

width

of

less

than

1

h

octave

would

often

be

employed.

The

B & K

Type

2107 is

an

examp

le

of

such

an

analyzer.

d) Measurements

in

Selected

Frequency

Bands.

Once

laboratory

investigations

have

been

carried

out

upon

objects

from

a

production

lin

e

(or

on

the

prototypes)

and

the

noise

and

vibration

generated

by

the

objects

is

analysed

e.g.

by

means

of

an

ana

lyz

er

as

mentioned

above

,

it

can

be

calculated

which

of

the

noise

frequencies

must

be

checked

in

production.

Often,

attention

may

be

confined

to

certain

important

frequencies

ortly,

and

other

bands

ignored.

A

special

instrument

which

measures

these

particular

noise

frequencies

(

see

Fig.

5)

can

then

be

used

to

cheek

the

units

7

from

the

production

lin

e.

Such

an

instrument

is

the

Noise

Limit

Indicator

Type

2211.

It

measures

noise

or

vibration

in

a

number

of

pre-selected

frequency

bands

simultaneous

ly.

The

bandwidth

can

be

chosen

to

be

1

octave

or

1

h

octave,

depending

on

the

cond

itions.

lr

5

ES.

Microphone Frequency

Filter

characteristics

IIJ\£v\u.

Filters

/

Frequency c

0

rfiDI1

11r:

=I

Frequency

16

220

6

Fig. 5.

This

figure

shows

the

measuring

principle

of

the

Noise

Limit

Indicator.

Several

filters

are

ins

erted.

The

meter

is

substituted

by

reel

lamps,

so

that

tl1ere is

an

indicator

for

each

filter.

Thereby

the

excess

of

the

noise

band

limits

of

several

filters

can

be

indicated

simultaneously.

The

filter

attenuation

can

be

adjusted

for

each

filter

incliviclually.

The

noise

frequency

bands

and

the

levels

set

as

the

maximum

limit

for

each

of

the

bands

sel

ected

should

be

chosen

as

a

result

of

careful

·

laboratory

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

capable

of

checking

a

number

of

pre-selected

frequency

band

levels

as

well

as

the

overall

noise

level

,

both

wideband

and

weighted

,

simultaneously.

CAP

ABIT

..

ITIES

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

commonly

used

measurements

that

can

be

made

with

the

instrument

are:

-

R

1)

Simple

"go-no

go"

testing

for

noise

or

vihration

or

both

simultaneously

(

see

Fig.

6a).

0

a)

0

Standard

0

b)

0

DO

Extra

Standard

D

I)

D

D D

c:::::::J

c:::J

D

c:::::::J

D c:::J

c:::J c:::J

Extra

Standard

0

00

d)

000

0000

Closs 1 3

Noise

Limit

Indicator 0

Rejects

Noise

Limit

D

Indicator D

Noise

Limit

Rejects

t622ot

Rejects

Indicator 0

Rejects

162200

Fig.

6.

Different

measuring

possibilities

witil

tile

Noise

Limit

Indicator.

a)

Simple

testing

dividing

tile

objects

from

a

production

line

in

((rejects"

and

<<

standards

"

witil

respect

to

noise

and

vibration.

b)

Same

as

a)

,

but

additionally

dividing

tile

approved

obj

ects in

two

classes,

<<

standard

"

and

<<e

xtra

".

c)

If

only

one

check

of

tile

noise

or

vibration

of

a

unit

is

required

, tile

two

inputs

of

tile

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

th

e

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:

"S

tandard"

and

"Ex

tra

" (Fig.

6b).

3)

Dividing

units

into

three

classes

,

"extra",

"sta

ndard"

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

order

to

correctly

adjust

and

use

the

Noise

Limit

Indicator

the

following

factors

must

be

pre-

determined

:

I.

The

centre

frequencies

and

the

width

of

the

frequency

bands

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

customer's

specification,

these

factors

may

be

given

by

the

customer

as

part

of

his

specification.

It

is

then

extremely

important

that

item

III

above

is

carefully

specified.

Under

"a

mbient

con-

ditions"

it

is

necessary

to

state:

1.

Ambient

noise

or

vibration

level.

2.

Properties

of

the

measuring

room

or

chamber,

i.e.

hard

walls,

non-

reflecting

walls,

dimensions

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

suspension,

i.e.

under

simulated

wt,rking

conditions

or

on

special

test

stand,

in

the

lattor

case

the

stand

must

be

described.

5.

For

vibration

measurement

,

the

method

of

fastening

the

vibration

pick-up

as

well

as

the

exact

point

where

it

must

be

placed.

6.

Other

special

requirements

such

as

temperature,

prior

treatment

, etc.

etc.

In

other

words,

it

is

necessary

to

lay

down

a

specification

which

is

detailed

e

nough

to

ensure

that

the

test

conditions

relevant

to

requirements

I

and

II

can

be

reproduced

.

10

In

most

cases

the

requirements

for

noise

or

vibration

limits

are

not

available

in

the

form

of

well-defined

specifications.

The

usual

quality

reference

is a

standard

unit

which

meets

the

customers'

requirements

and

which

has

to

be

reproduced

within

certain

tolerances.

The

problem

then

is

to

carry

out

measurements

on

this

object

in

order

to

obtain

the

information

required

in

1

and

II.

The

ambient

conditions

for

the

preliminary

measurements

should

be

similar

to

those

under

which

the

unit

was

approved.

Later,

in

production,

the

ambient

conditions

may

be

different,

so

it

is

then

necessary

to

investigate

what

changes

in

the

noise

field

the

change

of

ambient

condition

has

in-

troduced.

In

order

to

investigate

the

noise

and

vibration

generated

by

a

standard

unit,

an

arrangement

as

shown

in

Fig.

7a

can

be

employed.

a)

Microphone level

Recorder

I6221Z

cable

b)

Accelerometer level Recorder

162198

Fig. 7.

a)

Analysis

of

noise

by

means

of

an

A.F.

Spectrometer

Type

2112

and

a

Level

Recorder

Type

2305.

b)

For

Analysis

of

vibration

,

the

microphone

is

interchanged

with

an

accelero-

meter.

This

"set-up"

consists

of

a

microphone,

a 1

/a

octave

band

analyzer

and

a

level

recorder.

The

microphone

is

placed

near

the

test

object

so

that

it

readily

picks

up

the

generated

noise,

which

is

then

fed

to

the

analyzer

in

the

form

of

an

electrical

signal.

The

analyzer,

which

is

synchronized

with

the

level

recorder

paper

drive

,is

now

adjusted

to

scan

the

frequency

range

from

16

cis

to

31

kc

/s.

The

noise

generated

by

the

test

object

is

measured

and

recorded

in

1

/a

octave

bands

and

presented

as

a

spectrogram

(noise

11

Sound Pressure Level

1/3 octave· Sound Pressure

Level

~

"

Lin"

SOO

cts I Sound Level

curve

"

A

'~~.::.______

-

~

M-orif19~iK•

'-

-

.

Noise

from

db

dl>

l--

f--

1--

gegr-bm<

jn

on

eLectric

b

anddrill

Djstgnce

Scm

1

"'

13

.---::

o

~

ct

=

gv

-:::-

e

--

"'

-

"

I

=J--

a

Ml)CS.is_

1-

-

100

,

--

H-

1-

H----

20000

q ,

•DOOOA

I C

U...

1000

10000

(t&ft/2111)A t C

lA

1619.(.1

(a)

Fig.

8.

Noise

spectrogram

as

obtained

witil

tile

set-up

shown

in

Fig.

7.

1

h

octat~e

band

analysis

.

Sound

Pressure

Level.

111

octave

Sound Pressure

Level

~

Sound Level curve

"A'~

::.:;:

~

"

L

i~

'

500C/S

I

~~~

""'-

~

o o o o o o o o o o o o o o o o o o o o o o o lo !o lo o o o o o o o o o o o o o

~

~~

b o

..

"=f--- ':::::':::1-

f----

·- -'-

I'-

'

~

-

M-

~~

Noise

from

gegr-box

jn

~

rtF-

..

,

Jll

Q_c.1lri!L__

"'

"

cnal

)'Sls___

.

1-1

-

1

--

100

(b)

.

....

1-

( :'"

''

20000

c/o

•OCOOA

I C

l.lrt.

1000

10000

(l6ll/2fl!)

A.

• c

Lin.

·

1619f

2

Fig.

9.

Noise

spectrogram

obtained

witli

tile

set-up

in

Fig. 7,

but

witil

tile

Spectrometer

Type

2112

set

for octave analysis.

c

urv

e)

as

shown

in

Fig

. 8.

From

thi

s c

urve

th

e 1

h

octave

noise

level

in

any

frequ

e

ncy

band

within

th

e

audible

range

can

be

read

directly,

so

that

a

det

e

rmination

ca

n

be

mad

e

of

th

e

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

res

ults

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

interchanged

with

an

accelerometer,

e.g.

one

of

the

B & K

Types

4308-9

-10-11.

To

investigate

carefully

the

noise

or

vibration

generated

from

an

object

several

individual

spectrograms

or

sometimes

several

series

of

spectrograms

must

be

obtained.

The

microphone

should

be

placed

in

several

positions

in

relation

to

the

test

object,

and

spectrograms

should

be

recorded

for

each

position

.

The

records

are

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

multipl

e

measurement

technique

is

even

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

e

mploy

more

than

one

accelerometer

for

production

line

tes

ting

in

order

to

obtain

a

distinct

reading

for

all

the

vibration

frequencies

of

interest.

For

instance

, 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

interesting

frequencies

are

represented

with

sufficient

strength

to

give a

clear

measurement.

It

is

then

necessary

to

divide

the

measurement,

i.e.

to

place

two

vibration

pick-ups

at

different

positions

and

thus

measure

one

group

of

the

desired

frequencies

by

means

of

one

pick-up

and

the

remaining

group

with

the

other

pick-up.

Of

course

,

an

identical

me

thod

must

then

be

used

during

the

production

control.

Where

no

noise

requirements

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

sufficiently

large

series

is

measured

and

an

average

noise

figure

determined.

The

limit

is

then

set

within

a

suitable

tolerance

from

the

average.

The

use

of

this

limit

set

by

the

average

noise

figure

not

only

ensures

a

uniform

product

but

also

facilitates

the

detection

and

improvement

of

smaller

errors

originating

in

the

production

system.

It

may

then

be

possible

to

lower

the

noise

and

vibration

limit

of

the

products.

\VHAT

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.

However

,

vibration

is

often

easier

to

measure

,

because

it

is

easier

to

isolate

the

test

object

from

other

unwanted

and

disturbing

vibrations

lhat

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

measurements

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

problems

and

costs.

If

vibration

measurements

are

sufficient

in

such

circumstances,

the

advantage

has

been

gained

that

the

vibration

pick-up

can

be

placed

on

that

part

of

the

machinery

where

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

relationship

between

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

range

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

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

Lak

e

into

consideration

whether

the

object

is

going

to

be

tested

under

conditions

that

are

similar

to

the

ambient

conditions

at

the

place

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

where

it

is

desired

to

measure

noise

or

vibration

14

•

of

the

units

leaving

:.

a pr:Oducti<m l·

ine,.{t

:tesbr

stand

for

_

.the

1

qu.alitY,

control

will

already

be

provided.

· ·f.l.i

r,

::!

. ,

Noise

and

vibration

ins

:pe'ction -can

then

·

b'e

:

i

'

nclttd

e~

in

th

e

normal

pr

'

ocedttre,

adding

·

only

· a

fraction

to

the'p'r·

evious

·

'

te~t

time.

H'O\vev.er, ·

it

·

inay

sotileth:hes

be

necessary

to

modify

the

existing

test

stand

sli-gh1lyr

:r

o'l'

;

:r

jf

L

no

stand

t.-

is

.1}rovlided, to ·

make

·a llgM-vve'ig'ht:

artangemeiiV

'

t\

~

i

r

•'

th~

ndise

a!ndr·

vibr~tioh

'ihspecti'o'I1.

The

.

foH:Owitig

;g

iv

~s

~xa

m

·

ples

·:

of

how

·

the

t~sfihg

·'

may

be

·

a:Frat1gerd.

Vibrati~n

Testing

..

::

,.

..

. .

r.

•:,

)'

.,.,

...

( 1:r· .

rr

.

r·

Vibration

te

tl

ting

.

has

..one

mai,n

.,

di~acl,v,~tage

;

COIJitpaf~d

to :

noise

testing:

It

.

~s

nt'cessary

1to

place

tl'\e

'

.v~br~tion

.

picl~-up

,.,

mechq.nk~lly

,

i.t;t

.

eontact

.w,itl}

th

e

test

object

and

il}

,·

e~a,ctly

.

tl)

~

S~lll,f1

,·.

POSi~iQI1

for

each

uni~

tes

te<;l.

,

.In

npi~e

testing

.

the

object

can

pass

.

cl~SG

:

by

.,

tJ;te

·

mjcrqphone

.\Vi.thout 1

touching

it.

·

There

is

one

w·

ay

of

overc6h1ing

thi

s..p·

roblem

when

. tes-ting

vibr

.

ation.

H

the

·

test

.

objects

vibrate

as

a

whole,

the

'

vibration

·

piek

·

up

·r

can

.c

be

plac~d

on

the

test

stand

.

The

stand

should

then

be

of

·a

faitly

r;

light

..

weight

construction

'

r

..

To

Noise

Limit

. _Indicator

To

Noise

Level

Indicator

!62

2

fO

Fig.

10.

Placirrg

the

'

object

on

a

light-weight

test

·

stand

· -enables

vibration

pick-ups

to

be

mo

'

unted

on

the

test

stand.

Vibration

in

two

directions can be

.

,,1

..

'

,,

\\1,)·

' i

·,·

; , I

....

, ,

c~~~k(~

,

i~

.

o,r~e

,

,_

?Prratiqn

.

To

Noise Level

Indicator

\.)

1'/_\l

· i r · !

..,

r

,_

;

-'-'-/,-'""'~--'---

To

Noise

Level

lnd

icator

"'

'

1~?213

'(.

'.Pig.

·

11

~

The

acce1ehYineteT

,·

in~y

'be'

fasteiied

''

by

' triedtis

of

'a i/iniill

magnet

or

with

the

.;

aid

tdf

{i

i

fl

sp-ringloaded

r

•

arm

~

!

,,

, , r

and

be

placed

on

springs

or

on

a

piece

of

foam

rubber

as

ouHined

in

Fig.

10.

The

accelerometers

are

then

placed

on

th

e

stand

as

in

Fig.

10.

The

twv

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

simi

l

~r

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

Noise

Level

Indicator

f62f97

Fiy.

1'2.

Conventional

hand-held

opewtion

of

the

accelerometer

may

also be

used.

NOTE:

Whichever

method

is

used

to

fasien

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

n1icrophone

may

thus

be

placed

over

a

production

line

where

the

units

to

be

tested

pass

underneath.

The

units

must

be

consistantly

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,

noise

measurement

are

more

dependent

upon

ambient

conditions

than

vibration

measurements.

When

for

vibration

measurements

the

test

specimen

can

sometimes

be

isolated

simp

ly

by

means

of

a

piece

of

foam

rubber

beneath

the

specimen,

it is

often

necessary

to

"screen"

the

object

when

sound

measurements

are

carried

out.

Unfortunately

,

the

screening

for

ambient

sounds

encircles

the

obj

ect,

making

measurements

difficult.

Testing

products

that

are

making

a

loud

noise

,

such

as

internal

combustion

engines

,

To

Noise

Limit

lndicator

/

I

/

To

Noise

Limit

Indicator

I

/

162196

Fig. 13.

Where

the

noise

intensity

generated

by

the

test

object

exceeds

the

ambient

noise

to

a fair degree,

ti1e

microphone

can be

placed

in a

convenient

distance

from

the

object

as

shown

left.

If

the

ambient

noise

comes

too

close

to

the

noise

to

be

measured

, a

light-weight

screen

may

be

necessary

as

shown

to

the

right.

To

Noise

Limit

Indicator

Fig. 14.

Noise

testing

in

heavy

screen.

The

room

has

reasonably

small

outside

dimensions

and

walls

of

heavy

damping

material.

This

room

is

not

suited

for

directional

testing,

as

it

produces

a

diffuse

sound

field

at

lower

frequencies.

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

microphon

e

can

be

placed

in

the

screening

as

outlined

in

Fig.

13.

The

problem

of

screening

mainly

arises

when

it

is

desired

to

measure

wunds,

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

have

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.

Damping 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

.

It

is

advisable

in

many

cases

to

consult

an

acoustical

expert

for

the

solution

to

the

problem

of

test

room

constructions.

lR

BRUEL & KJAER 2211 User guide

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