BRUEL & KJAER 4220 Guía del usuario

, I

·.

• 4220

INSTRUCTIONS

AND

APPLICATION

Pistonphone Type

4220

The

Pistonphone

a compact battery-<>perated

preci.ion sound

.ource,

designed

enable quick

and accurate overall calibration

.ound

measur·

ing equipments. It produces a sound pressure

level

124

the

microphone diaphragm,

a frequency

250

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· Level Record.ra

Contents

Page

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Description

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

General

...

. .

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

...........

....

...

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

Principle

Operation

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

.........

.......

Pistons, Non-Linear Distortion

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

Coupler

and

Adaptors

..........

. .

....

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

...

......

Motor and

Frequency

Regulation

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

Batteries

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

. .

........

. .

.......

. 12

Barometer. Ambient

Pressure

Correction

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

.....

...

Operation

.......

....

. .

......

............•.

.. .. ..

....•.....

....

General

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

..........

...

....

...

Ambient

Pressure

and

Volume Corrections

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

......

...

Use

External

Power

Supply

...........

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

Pistonphone's Dismantling

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

........

Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Calibration Checks

K Sound Measuring

Equipment

..........

Level Calibration

Tape

Recordings

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

...

......

. . .

Specifications

.....

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

....

...

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

. . 22

Part

Numben

.......

...

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

...

......

.......

......

Introduction

The Pistonphone Type

4220

is designed for the absolute calibration

pre

cision sound measuring systems.

A sound level measuring equipment basically consists

a microphone,

followed by suitable amplifier stages

and

indicating instrument.

The

amplitude and frequency characteristics

the

electronic

components

are

generally accurately defined,

but

the

exact determination

the

sensitivity

the

equipment in relation to sound pressure is less easy to obtain.

The

recommendations

the IEC

for

Precision Sound Level Meters (Technical

Commission no.

29)

requires:

"The

complete

apparatus

shall

calibrated

in absolute values

any specified frequency between

200

and

1000

Hz.

The

accuracy

the calibration

this frequency•• including

errors

due

the

free field measurement.

and

those

due

the

actual

electroacoustic

measure·

ments. shall always be beUer

than

± 1 dB." , ;

Different methods

sound level

calibration

have been

adapted

Brilel

Kjer.

I.e.: the Reciprocity Calibration, the Electrostatic Actuatqr,

the

Noise

Source.

and

the Pistonphone. Each method

has

particular

fie,ld

appllca·

tion.

They

are all

carried

out as

pressure

calibrations

(fictitiously in

the

case

of the actuator), the calibrating

sound

source being tightly coupled to the

microphone by a coupler

small dimensions.

For

small condenser micro·

phones. such as the B

K microphones.

there

appreciable

difference

under

1000

Hz,

between the pressure

and

the free field sensitivities.

The Reciprocity technique-I, which is the

method

retained In

the

American

Standard

ASA

Z 24·4 (1949). is based

upon

the

law

re~iprocity

reversible

transducers.

is carried

out

permutation

between

three

microphones

and

the measurement

voltage ratios only, with

the

knowledge

a few physical

constants. This laboratory method enables absolute pressure calibration

microphones to be

made

with excellent accuracy. On

the

other

hand.

relatively tlme.consuming operation.

The Electroltatic Actuator-) produces a fictitious acoustic

pressure

the

diaphragm

of a condenser microphone by electrostatic attraction.

is partl.

cularly useful

and

very reliable as a

variable

frequency exciter

for

the

automatic recording

frequency response curves. However.,

the

inaccuracy

in the determination

the

equivalent sound pressure hinders the electrostatic

actuator

from 'being used as an absolute calibration means.

See tbe Instruction ManuII for tbe B

K Mlcropbone Calibration Apparatus Type

4142.

The Noise Source Type 4240 mechanically produces

approximate

"white"

random noise. The advantage

the

"white"

noise is

that

calibrates over a

dp.

range

audio frequencies, i.e. in conditions approaching

actual

noise measurement.

The

noise source is a

small

independent

device, well-

adapted for field

calibration

check

sound

measuring

~quipment,

and

requires no electric

power

for

operation

The

accuracy

the

noise source

is, however, limited to ±

1.5

dB.

The principle of the Pistonphone is

that

sound

pressure is produced

the

coupler cavity by a piston moving in

and

out. This sound

pressure

proportional to the variation

the ·coupler volume, i.e. to

the

stroke and

area

the piston, and the

error

the

sound pressure level depends in

particular on the accuracy in the measurement

the stroke.

Fig.

1. The

·old"

pistonphone'

j to

the

left

old mechanically driven

pistonphone, and to

the

right

electromagnetically driven pistonphone.

The pislonphone was first developed

a normally driven piston (crank +

connecting rod), see Fig.

With

this

arrangement

is was practically im-

possible to

carry

out

a calibration

a sufficiently high frequency.

obtain

Hz, the motor should

run

12000 r.p.m.,

and

even

much

lower speeds

than this, vibrations occurred in the bearings

and

the connecting·rod, with

an eventual wobbling

the

motor

spindle, which produced

unacceptable

rror

in the determination

the stroke.

addition, the distortion

the

piston movement caused by the finite length

the

connecting rod

had

to be

taken into account,

and

due to

the

variations

the torque

the

motor

axle the frequency stability was also insufficient.

For these reasons, the mechanical drive

has

been superseded by

electro-

magnetic drive

the type used

loudspeakers, enabling audio frequencies

be easily obtained.

The

piston

stroke

has

measured optically. This

can be made with a certain accuracy by

means

a good niicroscope. How-

ever, the use of a microscope

and

the

difficulty

obtaining a high sound

pressure level limit the use

this type

piston

phone

laboratory

work

and

demonstration•.

The

PlltODphoDe Type

4220

is a mechanical device

a new design which.

by using two symmetrically mounted pistons, with a floating retaining spring

arrangement, eliminates shaft eccentricity,

and

back-lash

bearings from

any influence on the stroke

the pistons,

and

thus

avoids the

drawbacks

the old pistonphone. This small unit produces a pure tone

250

Hz,

with

u sound pressure level of

124

(RMS)

reo

2 X 10-4 ,ubur. allowing correct

calibration

·made even in very noisy surroundings.

The

Pistonphone.4220

features outstanding simplicity

and

rapidity

operation

and

hall a

calibration

accuracy

± 0.2 dB, which is

the

same

order

the

precision

usually

obtained by the'reciprocity method.

can

be used

anywhere

where

a well-

defined sound source is required,

and

is especially suited

for

the

direct

calibration

precision sound measuring equipment in the field,

Descri

ption

General.

The Pistonphone Type 422u (Fig. 2) is oi cylindrical

shape

with small

dimensi;ns

(comparable with

the

dimensions

"flash

light"),

and

battery

operated.

is consequently compact

and

portable,

always

ready

for

use, having no

mains

connections,

and

can

utilised

under

the most

difficult ambient conditions. Once the microphone is fitted to the coupler the

operation consists simply

starting

the

pistonphone

and

adjusting the

sensitivity

the components in the sound measuring equipment until the

deflection

the

indicating

instrument

corresponds

to the S.P.L.

(RMS

Sound Pressure Level) produced by the pistonphone.

Fig. 2.

Photograph

the

Piltonphone.

Each pistonphone is individually calibrated

and

the exact value

calibration,

which

can

deviate a few

tenth

from

the

nominal

124 dB, is indicated

on the calibration

chart

which is supplied with the

instrument

upon delivery.

The calibration is given

normal

atmospheric pressure (760

Hg). The

maximum

error

calibration

is less

than

± 0.2 dB.

Fig. 3. Assembly

drawing

0/ the

Pi.tonphone.

The necessary corrcction for variation In

atmospheric

pressure II

read

directly

off

barometer

supplied

with

the

pistonphone.

the

ordinary

range

(0°-6Q°C),

temperature has practically

cffect

the

pistonphone'l

cali·

bration

The

Pistonphone Type

4220

is delivered in a wooden case with

the

barometer

and

different adaptors.

Principle

of Operation.

The

principle

operation

the

Pistonphone

Type

4220 is

shown

Figures

and

Thc two pistons

are

symmetrically driven by

means

cam

disc,

mounted

the

shaft

a miniaturc clectric

motor.

The

cam,

which

made

of specially selected tempered steel, is

machined

a high degree

accuracy,

according to the law r = a +b sin 4 a (Fig. 5).

When

rotating,

the

cam

will

give

the

pistons a sinusoidal movement

frequency

equal

four

times

the speed of rotation. Conscqucntly

thc

cavily volume is varied sinusoidally

and

the RMS sound pressure produced will be

where:

A.S

P = r X

---=

dynes/cm2

Vy2

...

C,IC. = ratio

specific

heats

Cor

the

gas in the cavity

1.402 for air,

20·C

and

1 atm.).

....

atmospheric pressure, expressed in

dync/cm

2•

A, -

area

one of the pistons.

pcak amplitude of motion

one

piston

from

the

mean

position.

V ... volumc

cavity with

the

pistons

mean

position + the

equivalent volume of

the

microphone. P

The

sound

pressure level in

consequently:

S.P.L. = 20 log p;-. with

= 2 X

10-

4 .ubar = 0.0002 dynes/cm2 =

threshold

of hearing.

is assumed

the above equation

that

A. X S

«V,

and

that

the com·

pression is adiabatic. These conditions

are

satisfied entirely in the case

the

Pistonphone Type 4220, where A, X

S/V

<0.0002,

and

where

the

heat

con·

duction correction would be less

than

0.05 dB

20 Hz, **)

is possible ,to define, wilh great precision, the

different

quantities involved

the

e9uation. ,The piston

stroke

S, which

was

difficult to

measure

the

case

,im electromagnetically driven pistonphone,

simply here,

one

fourth

the

difference between

the

maximum

and

the

minimum

diameters

the

cam. Accurate mcasurement

this difference,

though

relatively small, offers

no problem by using n precision micrometer. In addition, as

the

pistons

are

A cam disc correspondlnll to the law r

8 + h sin 2 a

availahle on request. Willi

this disc the reference frequency is

125

and the distortion less than 1.5

Experiments on the Pistonphone

Type

4220

have

shown

that

th~

compression

can he

considered as adiabatic down to a frequency of 1

(the

error

being

approx

. 0,3

at 1

symmetrically located with respect to

the

cam,

any

mechanical

deviation

due

the

mounting

to a wobbling

the

motor

compensated

the

total

movement

the

pistons,

and

has

influence

the S.P.L.

produced

R.ta

ini~

.pring

Fig.

Cross-section showing the principle

operation.

The frontal surface A,

the pistons

and

the volume V

the

cavity

are

abo

easy to measure accurately,

and

the diameter

the

pistons

is great with

regard to the

stroke

the influence of the space between

the

pistons and the

walls

negligible. (The tightness

the pistons is satisfactory, without any

lubricating material, down to

about

Hz) .

Finally a possible

error

the ambient pressure Po is

far

under

the required

limit, as ten

degrees

the scale

the

barometer

corresponds

to 0.04 dB.

These features make

possible to calculate the S.P.L. produced by

the

Pistonphone Type 4220

the

standard

frequency of 250 Hz to within less

than

± 0.2

(±

2 %).

r • • +

n'«

"';g.

5. Sketch

the

.inu.oidallil

.haped

cam.

Pistons, Non-Linear Distortion.

The design of the piston drive in the

Pistonphone

4220

makes

allowance

for very low non-linear distortion.

The

distortion is produced,

not

by Imper.

feelion

the specially shaped

cam

which

can

be considered

rather

perfect sinusoidal shape, but by

the

wear

the

piston

tips sliding

the

cam. During experiments it has been noticed

that

the

tiPI

, if

special

care

has been taken,

can

become

worn

one side,

and

In due course slightly

distort the movement of the pistons.

The pistons are made of a special

synthetic

material,

presenting a

particularly

low friction coefficient with steel.

The

cam

and

pistons

are

lubricated with

high quality oil. Also, the tension

the

retaining

spring

carefully,

adjusted

maintain

even contact

the

piston

tips

the

cam,

without

excessive

pressure. By means

these precautions,

the

distortion

factor

the

Piston-

phone

is 'maintained lower t

han

3 %

250 Hz.

The

frequency spectrum

the signal produced by the plstonphone

shown

in Fig. 6.

o 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 U 0 0 0 0 '0 0 0 0 0 0

_.o.

......

.-. ·

--·

100

1000 10000

C/I

Fig.

Ty[.ical frequency ,pectrum

the Pistonphone recorded by mearts

the B

K Spectrum Recorder

Type

3312. The ,pectrum

,hows

a second

harmonic

1.8

and some noise cau.ed

the coupler

the motor,

around

7000

Hz.

COI:pler

aDd

Ad.pton.

The coupler cavity

the

Pistonphone

4220

has

been designed witl1

~!men

sions which

are

small with respect

the

wave-length

th~

sopn4.

tbe

freq<Jency range involved.

The

coupler

bas

opeqlng

diameter

23.8 mm,

fitting microphones with a

standard

diameter

0.986,Inch

(al

specified

A.S.A.

Z 24-8-1949) such

the Types B

K 4181

and

4182.

E. 640

103. ,

In.addition. three

adaptors

are

supplied

with

the

Pistonphone.

enabling

the

opening

diameter

to be reduced

without

changing

the

total

volume

the

cavily.

for

use

with

lh.

1,4

and

inch

diameter

microphones.

A special

adaptor

is available

request

for

fitting

the

old

B & K

Microphone

Type

4111

with

diameter

.36

mm·).

The

adaptors

are

easily

mounted

onto

.the

pistonphone

pres~ing

them

into

the

coupler

opening

and

into

each

other.

See Fig.

The

coupler

opening

and

the

adaptors

are

fitted

with

rubber

ring

whjch

ensures -

airtight

connection

the

microphone

under

test.

The

coupler

cavity is also

fitted

with

capillary

tube

for

static

pressure

equalization

with the pressure

outside

a. b.

Fig

Mounting the dillerent

Bit

K microphones on the Piltonphone.

(a)

·one

inch" Microphone Tupe 4181 or 4182.

(b)

·half

inch" Microphont! T"pe

4tl8

or 4184.

The total volume

the cavitu

the same in both case•.

The

pistonphone

can

adapted

to almosl 'lny

type

microphone.

as a

suitable

adaptor

can

easily be

made.

which

ha,

larger

smaller

diameter.

The use

special

adaptors

does

not

off

•.r

any

other

problems

than

those

external leakages.

**)

However.

the

dlmo.:nsions

should

remain

small

in com·

parison

with

order

to avoid

any

disturbanct'

due

wave

motions

in the

cavity.

air

approximately

34 cm (13")

250 Hz.

and

(4")

800 Hz).

and

ratio

1 to 3.5 between the

total

depth

and

the

diameter

Ihe coupler is

recommended

order

obtain

the

best possible frequency

response.

From

the

pistonphone

formula

is seen

that

the

S.P.L.

produced

the

Pistonphone is inversely

proportional

the

volume V

the

cavity.

The

nominal S.P.

the

Pislonphone

corresponds

the

situation

where

B& K microphone.

with

the

normal

protecting

grid. is inserted.

this

case

Ihe lolal volume

the

cavity is 19.6

s•

including

the

equivalent

volume

the microphone: 0.15

for

B & K Types

4131-4132.

For

any

change

Ihe total volume

corresponding

correction

must

made

the

given

S.P.L.

Ihe

Pistonphone.

See "Volume

Correction"

the

following

chapter

•) B

K part number:

0386.

When usinll the Plstonphone with some types or microphones. a sllllh! and slow varl.

ation

the readinll

the Indicatin

Instruments may be noted. This Is caused by poor

equalization between the pressure Inside these microphones and the·external pressure.

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