BRUEL & KJAER 4220 Guía del usuario

Pistonphone Type

4220

The

Pistonphone

compact

battery-operated

precision

sound

source,

designed

enable

quick

and

accurate

overall

calibration

sound

measur-

ing

equ

pments.

produces

sound

pressure

level

124 dB

the

microphone

diaphragm,

frequency

250

Hz.

BRUEL&K.'-'AEA

NA:.RUM,

DENMARK.

Teleph.:

00.

Cable

BRUKJA

. Telex:

5316.

COPENHAGEN

4220-

0300053

Piston

phone

Type 4220

Reprint

october

1967

Contents

Page

Introduction

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

Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

General

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

Principle

Operalion

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

Pistons,

Non-Linear

Distortion

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

Coupler

and

Adaptors

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

Motor

and

Frequency

Regulation

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

Batteries

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

Barometer.

Ambient

Pressure

Correction

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

Operation

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

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

...

......

..........

General

........

. .

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

. .

......

. .

Ambient

Pressure

and

Volume

Corrections

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

Use

External

Power

Supply

....

...

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

Pistonphone's

Dismantling

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

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

Calibration

Checks

B & K

Sound

Measuring

Equipment

. . . . . . . . . .

Level

Calibration

Tape

Recordings

...

....

, . . . . . . . . . . . . . . . . . . . . .

Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Part

Numben

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

Introduction

The

Pistonphone

Type

4220 is

designed

for

the

absolute

calibration

pre-

cision

sound

measuring

systems.

Sound

level

measuring

equipment

consists

basically

microphone,

followed

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

relation

sound

pressure

is less

easy

obtain.

The

recommendations

the

IEC

for

Precision

Sound

Level

Meters

(Technical

Commission

no.

29)

requires:

"The

complete

apparatus

shall

calibrated

absolute

values

any

specified

frequency

between

200

and

looo

Hz.

The

accuracy

the

calibration

this

frequency,

including

errors

due

the

free

field

measurement,

and

those

due

the

actual

eleclroacoustic

measure-

ments,

shall

always

better

than

± 1

dB."

Different

methods

sound

level

calibration

have

been

adapted

Briiel

Kjrer, i.e.:

the

Recipro~ity

Calibration,

the

Electrostatic

Actuator,

the

Noise

Source,

and

the

Pistonphone.

Each

method

has

particular

field

applica-

tion.

They

are

all

carried

out

pressure

calibrations

(fictitiously

the

case

the

actuator),

the

calibrating

sound

source

being

tightly

coupled

the

microphone

coupler

small

dimensions.

For

small

condenser

micro-

phones,

such

the

B & K

microphones,

there

appreciable

difference

under

looo

Hz,

between

the

pressure

and

the

free

field sensitivities.

The

Reciprocity technique*),

which

the

method

retained

the

American

Standard

ASA Z 24-4 (1949), is

based

upon

the

law

reciprocity

reversible

transducers.

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

made

with

excellent

accuracy.

the

other

hand,

is a

relatively

time-consuming

operation.

The

Electrostatic Actuator*)

produces

fictitious

acoustic

pressure

the

diaphragm

condenser

microphone

electrostatic

attraction.

parti-

cularly

useful

and

very

reliable

variable

frequency

exciter

for

the

automatic

recording

frequency

response

curves. However, the

inaccuracy

the

determination

the

equivalent

sound

pressure

prevents

the

electro-

static

actuator

from

bein

used

absolute

calibration

means.

See

the

Instruction Manual

for

the

B & K Microphone Calibration

Apparatu~

Type

4142.

The

Noise

Source

Type

4240

mechanically

produces

approximate

"white"

random

noise.

The

advantage

the

"white"

noise

that

calibrates

over

wide

range

audio

frequencies,

i.e.

conditions

approaching

actual

noise

measurement.

The

noise

source

is a

small

independent

device, well-

adapted

for

field

calibration

check

sound

measuring

equipment,

and

requires

electric

power

for

operation.

The

accuracy

the

noise

source

is,

however,

limited

± 1.5 dB.

The

principle

the

Pistonphone

that

sound

pressure

produced

the

coupler

cavity

piston

moving

and

out.

This

sound

pressure

proportional

the

variation

the

coupler

volume, i.e.

the

stroke

and

area

the

piston,

and

the

error

the

sound

pressure

level

depends

particular

the

accuracy

the

measurement

the

stroke.

Fig.

The

"old"

pistonphones;

the

left

an old

mechanically

driven

pistonphone,

and

the

right an electromagnetically

driven

pistonphone.

The

piston

phone

was

first

developed

normally

driven

piston

(crank

connecting

rod}, see Fig. 1.

With

this

arrangement

was

practically

im-

possible

carry

out

calibration

sufficiently

high

frequency

obtain

200 Hz,

the

motor

should

run

12ooo r.p.m.,

and

even

much

lower

speeds

than

this,

vibrations

occurred

the

bearings

and

the

connecting

rod,

with

eventual

wobbling

the

motor

spindle,

which

produced

unacceptable

error

the

determination

the

stroke.

addition,

the

distortion

the

piston

movement

caused

the

finite

length

the

connecting

rod

had

taken

into

account,

and

due

the

variations

the

torque

the

motor

axle

the

frequency

stability

was

also

insufficient.

For

these

reasons,

the

mechanica

drive

has

been

superseded

electro-

magnetic

drive

the

type

used

loudspeakers,

enabling

audio

frequencies

easily

obtained.

The

piston

stroke

has

measured

optically.

This

can

made

with

certain

accuracy

means

good

microscope.

How-

ever,

the

use

microscope

and

the

difficulty

obtaining

high

sound

pressure

level

limit

the

use

this

type

pistonphone

laboratory

work

and

demonstrations.

The

Pistonphoue

Type

4220 is a

mechanical

device

new

design

which.

using

two

symmetrically

mounted

pistons,

with

floating

retaining

spring

arrangement,

eliminates

shaft

eccentricity,

and

back

lash

bearings

from

any

influence

the

stroke

the

pistons,

and

thus

avoids

the

drawbacks

the

old

pistonphone.

This

small

unit

produces

pure

tone

250 Hz,

with

sound

pressure

level

124 dB (RMS) re. 2 X 10-4 .ubar,

allowing

correct

calibration

made

even

very

noisy

surroundings.

The

Pistonphone

4220

features

outstanding

simplicity

and

rapidity

operation

and

has

calibration

accuracy

± o.2 dB,

which

the

same

order

the

precision

usually

obtained

the

reciprocity

method.

can

used

anywhere

where

a well-

defined

sound

source

required,

and

is especially

suited

for

the

direct

calibration

precision

sound

measuring

equipment

the

field.

Description

General.

The

Pistonphone

Type

4220 (Fig.

cylindrical

shape

with

small

dimensions

(comparable

with

the

dimensions

"flash

light"),

and

battery

operated.

consequently

compact

and

portable,

always

ready

for

use,

having

mains

connections,

and

can

utilised

under

the

most

difficult

ambient

conditions.

Once

the

microphone

fitted

the

coupler

the

operation

consists

simply

starting

the

pistonphone

and

adjusting

the

sensitivity

the

components

the

sound

measuring

equipment

until

the

deflection

the

indicating

instrument

corresponds

the

S.P.L. (RMS

Sound

Pressure

Level)

produced

the

pistonphone.

Fig.

holograph

the

Pistonphone.

Each

pistonphone

individually

calibrated

and

the

exact

value

calibration,

which

can

deviate

few

tenth

from

the

nominal

124 dB, is

indicated

the

calibration

chart

which

supplied

with

the

instrument

upon

delivery.

The

calibration

is given

normal

atmospheric

pressure

(760

Hg).

The

maximum

error

calibration

is less

than

± o.2 dB.

(66(22

Fig. 9.

Assembly

drawing

the

Pistonphone.

The

necessary

correction

for

variation

atmospheric

pressure

read

directly

off

barometer

supplied

with

the

pistonphone.

the

ordinary

range

(0°-60°C),

temperature

has

practically

effect

the

pistonphone's

cali-

bration.

The

Pistonphone

Type

4220 is delivered

wooden

case

with

the

barometer

and

different

adaptors.

Principle

Operation.

The

principle

operation

the

Pistonphone

Type

4220 is

shown

Figures

and

The

two

pistons

are

symmetrically

driven

means

cam

disc,

mounted

the

shaft

miniature

electric

motor.

The

cam,

which

made

specially selected

tempered

steel, is

machined

high

degree

accuracy,

according

the

law

r = a +b

sin

4 a (Fig. 5).

When

rotating,

the

cam

will

give

the

pistons

sinusoidal

movement

frequency

equal

four

times

the

speed

rotation.

Consequently

the

cavity

volume

varied

sinusoidally

and

the

RMS

sound

pressure

produced

will

be:-

where:

2 APS

P = rX

-dynes/cm2

Vv2

r =

Cp/C-r

ratio

specific

heats

for

the

gas

the

cavity

1.402

for

air,

20°C

and

atm.).

=atmospheric

pressure,

expressed

dyne/cm2.

::a

area

one

the

pistons.

::a

peak

amplitude

motion

one

piston

from

the

mean

position.

V = volume

cavity

with

the

pistons

mean

position

the

equivalent

volume

the

microphone.

The

sound

pressure

level

consequently:

S.P.L. = 20 log

~·

with

= 2 X

-4

flbar

= o.0002 dynes/cm2

=threshold

hearing.

assumed

the

above

equation

that

X S <<

and

that

the

com-

pression

adiabatic.

These

conditions

are

satisfied

entirely

the

case

the

Pistonphone

Type

4220,

where

X S/V <o.0002,

and

where

the

heat

con-

duction

correction

would

less

than

o.05"dB

20Hz.

)

is possible

define,

with

great

precision,

the

different

quantities

involved

the

equation.

The

piston

stroke

which

was

difficult

measure

the

case

electromagnetically

driven

pistonphone,

simply

here,

one

fourth

the

difference

between

the

maximum

and

the

minimum

diameters

the

cam. Accurate

measurement

this

difference,

though

relatively

small,

offers

problem

using

precision

micrometer.

addition,

the

pistons

are

A cam disc

corresponding

the

law

r = a +b sin 2 a is available on request.

With

this

disc

the

reference

frequency is

125

and

the

distortion

less

than

1.5

Experiments

the

Piston

phone

Type

4220

have

shown

that

the

compression

can

considered as

adiabatic

down

to a frequency of 1

(the

error

being

approx.

o.3

1Hz).

symmetrically

located

with

respect

the

cam,

any

mechanical

deviation

due

the

mounting

wobbling

the

motor

compensated

the

total

movement

the

pistons,

and

has

influence

the

S.P.L.

produced

Retaini

~ g

spring

Fig.

Cross-section

showing

the

principle

operation.

The

frontal

surface

the

pistons

and

the

volume

the

cavity

are

also

asy

measure

accurately,

and

the

diameter

the

pistons

great

with

regard

the

stroke

the

influence

the

space

between

the

pistons

and

the

walls is negligible.

(The

tightness

the

pistons

satisfactory,

without

any

lubricating

material,

down

about

20 Hz).

Finally

possible

error

the

ambient

pressure

Po is

far

below

the

required

limit,

ten

degrees

the

scale

the

barometer

corresponds

o.04 dB.

These

features

make

possible

calculate

the

S.P.L.

produced

the

Pistonphone

Type

4220

the

standard

frequency

250 Hz

within

less

than±

o.2 dB

(±

%).

b sin 4

/<'ig.

Sketch

the

sinusoidally shaped cam.

Pistons,

Non-Linear

Distortion.

The

design

the

piston

drive

the

Pistonphone

4220

makes

allowance

for very low

non-linear

distortion.

The

distortion

produced,

not

imper-

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