BRUEL & KJAER 4220 User guide
, I
·
·.
• 4220
~
INSTRUCTIONS
,
AND
APPLICATION
S
Pistonphone Type
4220
The
Pistonphone
i.
a compact battery-<>perated
preci.ion sound
.ource,
designed
to
enable quick
and accurate overall calibration
of
.ound
measur·
ing equipments. It produces a sound pressure
level
of
124
dB
at
the
microphone diaphragm,
at
a frequency
of
250
HL
Narum.
Denmark
12'
80 05
00
.
$'
BRUKJA,
Copenh_
.
Tele.
: 5316
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Audio Fraquency Re.pon
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Audio Frequ4tncy Va<uurn-Tube '
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Automatic A. F.Reapen
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AutomatloVibr.tion.Exciter'
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· Level Record.ra
Contents
Page
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
..
3
Description
..........................................................
6
General
...
. .
............
.
...........
.
....
..
...
.
................
6
Principle
of
Operation
......................
.
..
..
.........
.
.......
7
Pistons, Non-Linear Distortion
....................................
8
Coupler
and
Adaptors
..........
..
. .
..
....
.
..
............
...
......
9
Motor and
Frequency
Regulation
..................................
11
Batteries
..................................
,.
. .
........
. .
.......
. 12
Barometer. Ambient
Pressure
Correction
..
.
...............
.....
...
13
Operation
.......
.
..
....
..
. .
......
............•.
.. .. ..
.
....•.....
....
14
General
..
.........................
..
..
.
..
.
..........
...
....
...
..
14
Ambient
Pressure
and
Volume Corrections
.............
......
.
...
.
16
Use
of
External
DC
Power
Supply
...........
.
....................
16
Pistonphone's Dismantling
............................
..
........
..
17
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
..
18
Calibration Checks
on
B
&.
K Sound Measuring
Equipment
..........
18
Level Calibration
of
Tape
Recordings
..................
...
......
. . .
21
Specifications
.....
.
................
..
....
...
..
...
.
...............
..
. . 22
Part
Numben
.......
...
.
...
..
.
............
.
..
...
......
.
.......
......
.
23
Introduction
The Pistonphone Type
4220
is designed for the absolute calibration
9'
pre
·
cision sound measuring systems.
A sound level measuring equipment basically consists
of
a microphone,
followed by suitable amplifier stages
and
an
indicating instrument.
The
amplitude and frequency characteristics
of
the
electronic
components
are
generally accurately defined,
but
the
exact determination
of
the
sensitivity
of
the
equipment in relation to sound pressure is less easy to obtain.
The
recommendations
of
the IEC
for
Precision Sound Level Meters (Technical
Commission no.
29)
requires:
"The
complete
apparatus
shall
be
calibrated
in absolute values
at
any specified frequency between
200
and
1000
Hz.
The
accuracy
of
the calibration
at
this frequency•• including
errors
due
io
the
free field measurement.
and
those
due
to
the
actual
electroacoustic
measure·
ments. shall always be beUer
than
± 1 dB." , ;
Different methods
of
sound level
calibration
have been
adapted
by
Brilel
cl
Kjer.
I.e.: the Reciprocity Calibration, the Electrostatic Actuatqr,
the
Noise
Source.
and
the Pistonphone. Each method
has
a
particular
fie,ld
of
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
of
small dimensions.
For
small condenser micro·
phones. such as the B
cl
K microphones.
there
is
no
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
of
re~iprocity
of
reversible
transducers.
It
is carried
out
by
permutation
between
three
microphones
and
the measurement
of
voltage ratios only, with
the
knowledge
of
a few physical
constants. This laboratory method enables absolute pressure calibration
of
microphones to be
made
with excellent accuracy. On
the
other
hand.
It
is
a
relatively tlme.consuming operation.
The Electroltatic Actuator-) produces a fictitious acoustic
pressure
on
the
diaphragm
of a condenser microphone by electrostatic attraction.
It
is partl.
cularly useful
and
very reliable as a
variable
frequency exciter
for
the
automatic recording
of
frequency response curves. However.,
the
inaccuracy
in the determination
of
the
equivalent sound pressure hinders the electrostatic
actuator
from 'being used as an absolute calibration means.
-)
See tbe Instruction ManuII for tbe B
a:
K Mlcropbone Calibration Apparatus Type
4142.
The Noise Source Type 4240 mechanically produces
an
approximate
"white"
random noise. The advantage
of
the
"white"
noise is
that
it
calibrates over a
wi
dp.
range
of
audio frequencies, i.e. in conditions approaching
an
actual
noise measurement.
The
noise source is a
small
independent
device, well-
adapted for field
calibration
check
of
sound
measuring
~quipment,
and
requires no electric
power
for
operation
.
The
accuracy
of
the
noise source
is, however, limited to ±
1.5
dB.
The principle of the Pistonphone is
that
a
sound
pressure is produced
in
the
coupler cavity by a piston moving in
and
out. This sound
pressure
is
proportional to the variation
of
the ·coupler volume, i.e. to
the
stroke and
area
of
the piston, and the
error
in
the
sound pressure level depends in
particular on the accuracy in the measurement
of
the stroke.
Fig.
1. The
·old"
pistonphone'
j to
the
left
an
old mechanically driven
pistonphone, and to
the
right
an
electromagnetically driven pistonphone.
The pislonphone was first developed
as
a normally driven piston (crank +
connecting rod), see Fig.
1.
With
this
arrangement
is was practically im-
possible to
carry
out
a calibration
at
a sufficiently high frequency.
To
obtain
2
00
Hz, the motor should
run
at
12000 r.p.m.,
and
even
at
much
lower speeds
than this, vibrations occurred in the bearings
and
in
the connecting·rod, with
an eventual wobbling
of
the
motor
spindle, which produced
an
unacceptable
e
rror
in the determination
of
the stroke.
In
addition, the distortion
of
the
piston movement caused by the finite length
of
the
connecting rod
had
to be
taken into account,
and
due to
the
variations
of
the torque
on
the
motor
axle the frequency stability was also insufficient.
For these reasons, the mechanical drive
has
been superseded by
an
electro-
magnetic drive
of
the type used
in
loudspeakers, enabling audio frequencies
to
be easily obtained.
The
piston
stroke
has
to
be
measured optically. This
can be made with a certain accuracy by
means
of
a good niicroscope. How-
ever, the use of a microscope
and
the
difficulty
in
obtaining a high sound
pressure level limit the use
of
this type
of
piston
phone
to
laboratory
work
and
demonstration•.
The
PlltODphoDe Type
4220
is a mechanical device
of
a new design which.
by using two symmetrically mounted pistons, with a floating retaining spring
arrangement, eliminates shaft eccentricity,
and
back-lash
of
bearings from
any influence on the stroke
of
the pistons,
and
thus
avoids the
drawbacks
of
the old pistonphone. This small unit produces a pure tone
of
250
Hz,
with
u sound pressure level of
124
dB
(RMS)
reo
2 X 10-4 ,ubur. allowing correct
calibration
to
be
·made even in very noisy surroundings.
The
Pistonphone.4220
features outstanding simplicity
and
rapidity
of
operation
and
hall a
calibration
accuracy
of
± 0.2 dB, which is
of
the
same
order
as
the
precision
usually
obtained by the'reciprocity method.
It
can
be used
anywhere
where
a well-
defined sound source is required,
and
is especially suited
for
'
the
direct
calibration
of
precision sound measuring equipment in the field,
6
Descri
ption
General.
The Pistonphone Type 422u (Fig. 2) is oi cylindrical
shape
with small
dimensi;ns
(comparable with
the
dimensions
of
a
"flash
light"),
and
is
battery
operated.
It
is consequently compact
and
portable,
always
ready
for
use, having no
mains
connections,
and
can
be
utilised
under
the most
difficult ambient conditions. Once the microphone is fitted to the coupler the
operation consists simply
of
starting
the
pistonphone
and
adjusting the
sensitivity
of
the components in the sound measuring equipment until the
deflection
on
the
indicating
instrument
corresponds
to the S.P.L.
(RMS
Sound Pressure Level) produced by the pistonphone.
Fig. 2.
Photograph
0/
the
Piltonphone.
Each pistonphone is individually calibrated
and
the exact value
of
calibration,
which
can
deviate a few
tenth
of
dB
from
the
nominal
124 dB, is indicated
on the calibration
chart
which is supplied with the
instrument
upon delivery.
The calibration is given
at
normal
atmospheric pressure (760
mm
Hg). The
maximum
error
of
calibration
is less
than
± 0.2 dB.
Fig. 3. Assembly
drawing
0/ the
Pi.tonphone.
6
The necessary corrcction for variation In
atmospheric
pressure II
read
directly
in
dB
off
a
barometer
supplied
with
the
pistonphone.
In
the
ordinary
range
(0°-6Q°C),
temperature has practically
no
cffect
on
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
of
operation
of
the
Pistonphone
Type
4220 is
shown
in
Figures
4
and
'
5.
Thc two pistons
are
symmetrically driven by
means
of
a
cam
disc,
mounted
on
the
shaft
of
a miniaturc clectric
motor.
The
cam,
which
is
made
of specially selected tempered steel, is
machined
to
a high degree
of
accuracy,
according to the law r = a +b sin 4 a (Fig. 5).
*)
When
rotating,
the
cam
will
give
the
pistons a sinusoidal movement
at
a
frequency
equal
to
four
times
the speed of rotation. Conscqucntly
thc
cavily volume is varied sinusoidally
and
the RMS sound pressure produced will be
:-
where:
2
A.S
P = r X
P.
X
---=
dynes/cm2
,
Vy2
r
...
C,IC. = ratio
of
specific
heats
Cor
the
gas in the cavity
(r
=
1.402 for air,
at
20·C
and
1 atm.).
P
....
atmospheric pressure, expressed in
dync/cm
2•
A, -
area
of
one of the pistons.
S
..
pcak amplitude of motion
of
one
piston
from
the
mean
position.
V ... volumc
of
cavity with
the
pistons
in
mean
position + the
equivalent volume of
the
microphone. P
The
sound
pressure level in
dB
is
consequently:
S.P.L. = 20 log p;-. with
P.
= 2 X
10-
4 .ubar = 0.0002 dynes/cm2 =
threshold
of hearing.
It
is assumed
In
the above equation
that
A. X S
«V,
and
that
the com·
pression is adiabatic. These conditions
are
satisfied entirely in the case
of
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
at
20 Hz, **)
It
is possible ,to define, wilh great precision, the
different
quantities involved
in
the
e9uation. ,The piston
stroke
S, which
was
difficult to
measure
in
the
case
of
,im electromagnetically driven pistonphone,
Is
simply here,
one
fourth
of
the
difference between
the
maximum
and
the
minimum
diameters
of
the
cam. Accurate mcasurement
of
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
Is
availahle on request. Willi
this disc the reference frequency is
125
Hz
and the distortion less than 1.5
%.
U)
Experiments on the Pistonphone
Type
4220
have
shown
that
th~
'
compression
can he
considered as adiabatic down to a frequency of 1
Hz
(the
error
being
approx
. 0,3
dB
at 1
Hz
).
symmetrically located with respect to
the
cam,
any
mechanical
deviation
due
to
the
mounting
or
to a wobbling
of
the
motor
is
compensated
in
the
total
movement
of
the
pistons,
and
has
no
influence
on
the S.P.L.
produced
.
R.ta
ini~
SI
.pring
Fig.
4.
Cross-section showing the principle
o(
operation.
The frontal surface A,
of
the pistons
and
the volume V
of
the
cavity
are
abo
easy to measure accurately,
and
as
the diameter
of
the
pistons
is great with
regard to the
stroke
the influence of the space between
the
pistons and the
walls
is
negligible. (The tightness
of
the pistons is satisfactory, without any
lubricating material, down to
about
20
Hz) .
Finally a possible
error
on
the ambient pressure Po is
far
under
the required
limit, as ten
degrees
on
the scale
of
the
barometer
corresponds
to 0.04 dB.
These features make
it
possible to calculate the S.P.L. produced by
the
Pistonphone Type 4220
at
the
standard
frequency of 250 Hz to within less
than
± 0.2
dB
(±
2 %).
r • • +
~
Il
n'«
"';g.
5. Sketch
o(
the
.inu.oidallil
.haped
cam.
Pistons, Non-Linear Distortion.
The design of the piston drive in the
Pistonphone
4220
makes
an
allowance
for very low non-linear distortion.
The
distortion is produced,
not
by Imper.
8
feelion
of
the specially shaped
cam
which
can
be considered
as
of
a
rather
perfect sinusoidal shape, but by
the
wear
of
the
piston
tips sliding
on
the
cam. During experiments it has been noticed
that
the
tiPI
, if
no
special
care
has been taken,
can
become
worn
on
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
of
the
retaining
spring
Is
carefully,
adjusted
to
maintain
even contact
of
the
piston
tips
on
the
cam,
without
excessive
pressure. By means
of
these precautions,
the
distortion
factor
of
the
Piston-
phone
is 'maintained lower t
han
3 %
at
250 Hz.
The
frequency spectrum
of
the signal produced by the plstonphone
Is
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.
_.o.
......
.'
·
·
.-. ·
·
--·
10
100
1000 10000
C/I
Fig.
6.
Ty[.ical frequency ,pectrum
01
the Pistonphone recorded by mearts
0/
the B
If
K Spectrum Recorder
Type
3312. The ,pectrum
,hows
a second
harmonic
0/
1.8
%,
and some noise cau.ed
in
the coupler
by
the motor,
around
7000
Hz.
COI:pler
aDd
Ad.pton.
The coupler cavity
of
the
Pistonphone
4220
has
been designed witl1
~!men
sions which
are
small with respect
to
the
wave-length
of
th~
sopn4.
hi
tbe
freq<Jency range involved.
The
coupler
bas
an
opeqlng
diameter
of
23.8 mm,
fitting microphones with a
standard
diameter
of
0.986,Inch
(al
specified
in
A.S.A.
Z 24-8-1949) such
as
the Types B
&.
K 4181
and
4182.
W.
E. 640
AA
,
MR
103. ,
In.addition. three
adaptors
are
supplied
with
the
Pistonphone.
enabling
the
opening
diameter
to be reduced
without
changing
the
total
volume
of
the
cavily.
for
use
with
lh.
1,4
and
%
inch
diameter
microphones.
A special
adaptor
is available
on
request
for
fitting
the
old
B & K
Microphone
Type
4111
with
diameter
.36
mm·).
The
adaptors
are
easily
mounted
onto
.the
pistonphone
by
pres~ing
them
into
the
coupler
opening
and
into
each
other.
See Fig.
7.
The
coupler
opening
and
the
adaptors
are
fitted
with
a
rubber
ring
whjch
ensures -
an
airtight
connection
to
the
microphone
under
test.
The
coupler
cavity is also
fitted
with
a
capillary
tube
for
static
pressure
equalization
with the pressure
outside
.
a. b.
Fig
.
7.
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
of
the cavitu
is
the same in both case•.
The
pistonphone
can
be
adapted
to almosl 'lny
type
of
microphone.
as a
suitable
adaptor
can
easily be
made.
which
ha,
a
larger
or
smaller
diameter.
The use
of
special
adaptors
does
not
off
•.r
any
other
problems
than
those
of
external leakages.
**)
However.
the
dlmo.:nsions
should
remain
small
in com·
parison
with
~
in
order
to avoid
any
disturbanct'
due
to
wave
motions
in the
cavity.
(~
in
air
is
approximately
34 cm (13")
at
250 Hz.
and
10
cm
(4")
at
800 Hz).
and
a
ratio
of
1 to 3.5 between the
total
depth
and
the
diameter
of
Ihe coupler is
recommended
in
order
to
obtain
the
best possible frequency
response.
From
the
pistonphone
formula
it
is seen
that
the
S.P.L.
produced
by
the
Pistonphone is inversely
proportional
to
the
volume V
of
the
cavity.
The
nominal S.P.
L.
of
the
Pislonphone
corresponds
to
the
situation
where
a
B& K microphone.
with
the
normal
protecting
grid. is inserted.
In
this
case
Ihe lolal volume
of
the
cavity is 19.6
cm
s•
including
the
equivalent
volume
of
the microphone: 0.15
cm
S
for
B & K Types
4131-4132.
For
any
change
in
Ihe total volume
V.
a
corresponding
correction
must
be
made
on
the
given
S.P.L.
of
Ihe
Pistonphone.
See "Volume
Correction"
in
the
following
chapter
.
•) B
a:
K part number:
DB
0386.
U)
When usinll the Plstonphone with some types or microphones. a sllllh! and slow varl.
ation
In
the readinll
or
the Indicatin
II
Instruments may be noted. This Is caused by poor
equalization between the pressure Inside these microphones and the·external pressure.
10
Motor aDd FrequeDey ReplaHoD. .
The
Pistonphone
is equipped with a precision
DC
motor.
'In
order
to ensure
the frequency stability, the rotating
speed
is regulated to within 1 %
by
a
centrifugal switch
and
a
current
regulating
transistor.
See Fig. 8.
This
arrangement
completely eliminates
any
distortion
coming
from
the
cam
speed. In addition, such
an
accurate definition
of
the
frequency
Is
required
for calibration
of
non-linear sound level meters (e.g.
in
the
case
of
a sound
level meter having the standardized "weighted" frequency response
A,
or
B,
it
'is
known
that the response
at
250
Hz
is 8.6 dB
or
1.4 dB, respectively, lower
than
the response
at
1000
Hz)
.
CheCk
t'
Measure
r.xr-------~--JV~~---1------------~
Off
t
<J
Fig
. 8.
Diagram
01
the
frequency
regulator.
Centrifugal
switch
The control switch ("flash-light" type)
has
three
positions: "Off-Measure·
Check".
In
the "Measure" position
the
current
from
the
batteries is fed to the
molar
through the regulating system,
which
maintains
a
rotating
speed
of
3750 rpm,
i.e.
a frequency of
250
Hz.
In
position "Check" the batteries
are
directly connected to the motor,
and
the
frequency will be
somewhat
higher,
e.g.
350-400
Hz
for fresh batteries. By pushing the switch back
and
forth
between "Measure"
and
"Check" positions a quick
check
on
the
batteries
i.
obtained.
If
no frequency variation is noted,
It
is time to replace the
batterie
•.
(The batteries could still give some
more
hours
use,
but
the
frequency
of
the produced sound would be lower
than
250 Hz).
The frequency of the Pistonphone
may
be varied up to
800
Hz
by using
an
edernal
DC
power supply instead
of
the batteries,
the
control
switc"
being
in position "Check".
The
motor
has
a
shunt
characteristic
and
Its
rotating
speed
Is
approximately
proportional
to
the
connected voltage.
The
upper
limit
of
the motor rotating speed, corresponding to a frequency of
800
Hz, is
attained
for a voltage
of
approximately
24
Volts.
It
should
never be exceeded
and
must be used only for
short
periods
of
time. (See also the Note p. 17).
The
lower limit depends
on
the leakage time
constant
in the cO\lpler cavity
and
is
about 20 Hz when no special care is
taken
to ensure
the~ightness
(down
to 2 Hz
or
even less with a simple film
of
grease).
In
position "Check" ,the
stability of the rotating speed, i.e.
of
the frequency, is directly related to the
stability
of
the voltage between the
motor
terminals,
and
especially
at
low
frequencies the
DC
source
should
have a low
internal
impedance
(less
than
2 ohms). Between the given limits
of
the
frequency,
the
S.P.L.
produced
l>y
the
Pislonphone
is
conslant
and
independent
of
the frequency.
*)
Batteries.
The Pistonphone
is
supplied with long-life
mercury
cell batteries (Mallory
Iype
RM
-5(R)) upon delivery. The battery
compartment
holds 7 cells givin's
a lolal voltage
of
approximately 8 volts.
Though
the cost
of
mercury
cell '
batteries is higher
than
the cost
of
ordinary
ones, the adoption
of
this type
of battery
is
the cheapest
in
the long
run.
The
reason is
that
in
normal
use,
such
o,s
laboratory
standard,
the
Pistonphone
is utilized
only
a few
minutes
per day, and
perhaps
even less.
Under
these conditions,
the
batteries
life is
limited by
their
storage life.
Therefore
the
use
of
mercury
cells, which
feature a long storage life, is recommended. According to
the
manufacturer,
Ihe batteries
RM
-3(R), supplied
with
the
Pistonphone
can
be,
stored
for
more
than
6 years. This
duration
corresponds
to a utilization
of
the
pistonphone
of one
or
two minutes
per
day.
a.
Fig
. 9. Sketch
of
the battery compartment
(a) with Mallory RM-3(R) cell.
(b)
with Mallory RM-4(R) cells
Under circumstances where extended use is
made
of
the Pistonphone, larger
mercury cell
batteries
may
be
advantageous. These
are
Mallory type
RM·
4(R) with a diameter
of
50
mm
(the
diameter
of
the
RM.
SIR) units is
25
*) The noise from the motor has a resonance peak
at
bigb frequency
(7-8
kHz), but
Is
always more than
40
dB
below
tbe
signal. However, when
running
the
motor
at
ill
highest speed (i.e.
800
Hz)
tbe
noise resonance
peak
can
produce
a,
small rise of about
0.2
dB
of
the
total S.P.L.
in
the
coupler
cavity. '
12
I
mm). They can be filted
In
the
battery
compartment
by removing
the
Inner
plastic lining tube. (Fig.
9).
The service life,
with
these batteries, is exlendfld
approximately
50
%.
Burometer
UZ
0001
for Ambient
I'resaure
'Correction.
The calibration
of
the Pistonphone is given
for
normal
atmospheric pressure
conditions:
P.
=
760
mm
Hg.
For
importadt
barometric
or
altimetric changes
in the ambient pressure, a correction must be made. A barometer,
graduat~d
both
in
dB of correction
and
in
mm
Hg
of
pressure
is supplied with
tI~e
Pistonphone for this purpose.
The
barometer
range is
from
795
to
695
mm
IJg
and
is divided into two 360° scales.
The
outer
graduation
of
the
scales
is in dB, and
the
inner, in mm Hg (Fig. 10).
The
outer
scale, graduated from
795
to
675
mm
Hg, indicates
the
dB
cqr-
rection
for
the usual barometric variations
and
can
be used
at
altitudes
1JP
10
600
m (1800 feet) above sea level.
The
inner
scale, graduated
from
716
,to
595
mm
Hg
is intended to be used at
altit~des
from
600
10
2000
m (1800 -
6000
feet).
Fig. to. The'
Preuure
Correction Barometer UZ
OOOt.
From
time to time, and especially
after
transport
or
after
Important
changes
in altitude, it is recommended to check
the
calibration
of
the
barometer.
The
pointer
of
Ihe barometer can be
adjusted
by
means
of
a screw
at
the
rear
0,'
the barometer.
For
altimelric 'variation
of
the ambient pressure beyond
2000
m
and
up
to
18000
m
(66000
feet) i.e. down to
60
mm
Hg,
the
correction curve given in
Fig.
11
can be used.
A conversion
chart
giving the equivalence between
the
pressure expressed in
different units II shown
in
Fig.
12.
11
Operation
General.
After the
~')und
measuring equipment under test
has
been
warmed
up
and
made
ready
for use
in
the
120--130
dB
range
and
with
"linear"
frequency
response, operation
of
the
Pistonphone is as
follows:-
(1)
Fit
the
microphone
of
the
equipment
under
test to
the
coupler
opening.
The
Pistonphone can be held in one
hand,
in
any
position.
*)
(2)
Start the Pistonphone by pushing the gliding switch,
and
check the
batteries by pushing the switch forwards
and
backwards between the
"Measure"
and
"Check" positions. A definite variation in frequency
indicates
that
the batteries
are
in good condition.
(3)
The Pistonphone's switch being in "Measure" position, adjust, with the
free hand, the sensitivity
of
the
sound measuring equipment until the
db
+2
-2
_4
- 6
-8
-10
.12
-14
-
16
-
18
-20
-
22
Change in ISound Pressure Leval
versus change in ambient pressure
for
Pistonphone
Type
4220
.-'-
f- O
o.s
-1
760
750
740
730 720
71Q
7lO
690
680
670
mmH,g
~
-mi
l~m~Om3~~0
ti-
~ltAbo,"
800 700
o 0.5 1
o
0.6
600
2
1.2
3
U
500
la
llVll
400 300 200
456789101112
2.5
3,1
3,7
4,3
5,0
5.6
6,2
7.5
100
Om
mHg
15
20
km
Abo,"
9.3
120'
MIlH
SI.
LI.II
"0'r28
Fig,
11. Correction curve lor operation
at
low pressure,
or
high alHtude"
I)
A slight variation
In
the noise produced
by
the motor can
be
noticed when holding the
pistonphone in different positions, This has no relation to the frequency and the S.P.L.
produced.
14
Indication coincides with the S.P.L.
(RMS
Sound
Pressure
Level)
pro·
duced
by
the Plstonphone. See Applications,
for
the
case
of
B
&.
K equip.
ment.
(4)
Stop
the
Plstonphone as soon as
the
calibration
has
taken place
In
order
to conserve
the
batteries.
Ambient
Pressure
and
Volume Corrections.
The S.P.L. produced by the Pistonphone is equal to
the
calibration value
indicated
on
the Calibration Chart. However, a correction
must
be
made
to
this value, if the ambient pressure is
not
the
normal
atmospheric pressure
(760
mm
Hg =
1013
millibar =
29.9
inches Hg),
and
also, If
the
total
volume
of
the coupling cavity, including the equivalent volume
of
the
microphone,
is
not the same as in the reference case,
where
a B
&.
K condenser microphone
with
normal
protecting grid
Is
Inserted.
The ambient pressure correction is immediately
read
off
the
barometer
supplied .with the pistonphone, as described above.
The volume correction Is, according
to
the
piston
phone
formula,
if
V'
Is
the
total volume
of
the coupler cavity:
20
log10
~,
dB,
to
be
added
to
the
calibra.
,
lion value which corresponds to the volume V =
19
.6
cm
s. • .
For
example, in
the
case
of
a
pistonphone
calibrated
to
123
.9 dB,
and
assuming the
pointer
of
the
barometer
Indicating -0.2
dB
and
the
plstpn·
vhone fitted with a special coupler to a certain microphone so
that
the
total
volume is V' =
24.1
c.c., the S.P.L.
produced
will
be
in dB:.
S.P.L. -123.9 -0.2 +
20
log
~::~
... 123.9 -0.2
-1.8
-=
121.9
Volume corrections for use with different types
of
standard
microphones
directly fitted to the Pistonphone
are
given in
the
following table:
Type
Without
Protecting
grid
With
~rotecting
Grid
WE640AA
+ 0.3 dB + 0.
42
dB
MR
103
+ 0.3 dB + 0.42 dB
+0.3 dB
B
&.
K
4131,
4182
(with coupler
adapting
0
ring
DB
0111)
16
MIU,barl
HilUmettrl
of
mercury
Inch.,
of
mercury
Kg/em'
1 millimeter
01
""""''J''t3332
millibar-a.03t4
inch"
01
mo"'ury.l
Tarr
.• O.
OOI35t
k8/CfTI~
10~m
.
. .
,
','
,.
\'
.•.
;?
T'
, V ., Y
,.
',,
• , " , ,
zp
., .,
Ip
',,
" ?
,.
....
+:+:+:+
:t:+:
+
;
+:+:+:+:+:+:+:+:+:+:+:+:+:+-~
,,1
st.
~
~
lie
Wi
Jb'
Iii
ttl
Itt
iiI
II-i'
~
••
Ij~·
~"~jj'
"
IoH.
~~~~~~.~'~I~I~'~'-T:~~~~~~M"~·~·~~-t~~~'~'~'~U~I.~.~.~.-
,~,
Fig. 12. Chart for conversion
of
barometer readings in different units.
Use
ofExteroal
DC
Power
Supply_
The Pistonphone is
normally
used
at
a fixed frequency. However, in
certain
instances, usc
can
be
made
of
the
fact
that
the
S.P.L. is independent
or
the
frequency, to check
the
response
of
a sound measuring
system
.
The frequency
of
the Pistonphone
can
be varied within the range 30
Hz-
800
Hz
by applying
an
external
DC
voltage varying from 1.5 to
24
volts
approximately. The necessary connections should be
made
to the rings which
are
seen when the
battery
compartment
is unscrewed. (The -is connected to
the outer ring,
and
the + to the center ring,
marked
"BATT +"
).
The Piston-
phone switch is set in "Check" position.
When
approaching 800 Hz (at
800
Hz
the motor
runs
at
12000
r.p.m.), the
Pistonphone
must
only be used
for
short
periods
of
time,
and
as
it could be dangerous for
the
motor
to
exceed the maximum speed, the voltage corresponding to the maximum
frequency should be progressively attained, for example
by
means
of
a
potentiometer as
shown
in Fig.
13
.
At
the lowest frequencies,
around
20
Hz,
it
is necessary to have a
DC
source with low internal resistance (less
than
2 ohms) in
order
to ensure stability
of
the
rotating
speed
of
the
motor.
+
----,
24V-
Fig.
13.
Connection
of
external
DC
power
.upply.
An
easy, but
rather
rough way
of
obtaining low frequencies, consists
of
rapidly moving
the
switch
back
and
forth
between
the
"Off"
and
"Measure"
positions.
When
calibrating a sound level
meter
equipped with
an
indicating
instrument featuring a convenient dynamic charllcteristic, a quick check
of
the low frequency response
of
the sound level
m~ter
can
be obtained
in
this
way by observing the deflection
on
the
·instrument,
and
an
abnormal
loss
or sensitivity
at
the low frequencies
can
rapidly
be detected. Inversely,
an
18
abnormal
leakage in the coupler
of
the
Pistonpbone
can
be
detected
when
calibrating
an
instrument tbe linearity
of
whicb is ascertained
at
low fre·
quencies.
DllmantllDl of the Plltoapllone.
To
take
tbe
Pistonpbone
apart,
proceed
as
follows (see also Fig.
2):-
(1)
Remove the coupler by unscrewing
it
from
· tbe Pistonphone.
(2)
Remove the piston retaining spring
and
take
out
the pistons.
(3)
Unscrew the two set·screws
from
the
body
of
tbe
Pistonphone.
(4)
Unscrew the motor housing from
the
black
motor
mounting piece.
(6)
Gently push out the pistons.
To reassembJe the
Pistonphone:-
(1)
If
necessary,
the
cam
disc
and
the
pistons
should
be cleaned
in
a
detergent.
(2)
Screw the motor housing onto
the
motor
unit.
(3)
If
necessary, apply a drop
of
bigh quality oil
to
the
cam
dise (e.g. Esao
"Univis 40"
or
Shell "Microtlme Type A Ultra Light").
(4)
Replace the two set·screws.
(6)
Insert the pistons
and
replace
the
retaining spring. Finally replace
the
coupler.
Note:
The
pressure
of
the retaining
spring
should
be
20
g.
approximately.
A too low pressure would cause distortion,
and
a too high
pressure
produces
an
excessive wear
on
the
piston tips.
If
it
is
intended
to
operate
the
Piston·
phone
at
800
Hz
it
may be necessary to slightly increase the spring pressure
in
order
to ensure constant contact
of
the
piston tips
on
"
the
cam disc, in
spite
of
the
high rotating speed. However,
the
spring pressure
must
be de·
creased again as soon as returning
to
the
normal
rotating
speed.
Spare
springs
and
pistons
can
be
ordered
separately.
See
the
parts
list.
Caution: Never touch
the
piston.
while operating.
17
Applications
The direct, absolute pressure calibration
of
sound level meters provided
by
the Pistonphone is
more
reliable
and
accurate
than
calibration
from
a built-in
reference voltage and the knowledge of the microphone sensitivity (K factor) .
Calibration
of
the
Preelslon
Sound
Level Meter
Type
2203.
The Pistonphone is weil
adapted
for use in connection with the Sound Level
Meter Type 2203 which is also portable
and
battery
operated (Fig. 14). The
operation is as follows:
(1)
Switch on, by pulling out the
central
knob. The indicating lamp
starts
to flash.
(2)
Turn
the
transparent
knob fully clockwise.
(3)
Turn
the black
knob
until
"120 dB" is
in
the red circle.
The
S.P.L. is
read
on the sound level meter
as
the deflection
on
the meter scale
(-
10
to
+10
dB)
plus
the
dB
number
in
the
red
circle.
(4) After a
short
wait, check the batteries by setting the
central
knob
to
"Batt.". The deflection should lie between the two limits
of
the "Battery"
scale.
(5)
Set the control
knob
to "Lin." (Fast). 30 seconds
after
switching on, the
Sound Level Meter is
ready
for calibration.
(6)
Push
the Pistonphone
onto
the microphone
of
the
Sound Level Meter
and
operate as described in "Operation", p.
14.
(7)
The indication
of
the
Sound
Level Meter
should
be
equal
to
the
S.P.L.
produced by the Pistonphone.
If
not, correct
by
means
of
the screw·
driver operated potentiometer
marked
"Adj....
Fig.
14
. Calibration
01
the
Precision
Sound
Level Meter
Type
2203.
18
CaUbration of
Sound
MeaauriD.
Equipment
Conalatln.
of
a
"B
a:
I"
Condenler
Microphone Connected
to
one
of
tbe
FoUowln.
Inltrumentl:
(a) MicrophoDe Amplifier
Type
2803
or
26M
(Fl
••
15a).
(b)
Frequency
Analyzer
Type
2107
(FfI.
15b).
(e) Audio
Frequency
Spectrometer
Type
2111
or
2112
(Fl
••
Ue).
I.
Case
of
the
·one
inch" microphones (Cartridges
Type
4131 or 4132).
For
calibration by means
of
the
Pistonphone,
the
control
knobs
of
these
instruments should be adjusted
as
follows:
(1)
POWER
to "On".
(2)
INPUT
SWITCH to "Condenser Microphone".
(3)
METER RANGE to "120 dB".
(4)
WEIGHTING NETWORK to
"Linear"
or
"Curve C".
(5)
METER SWITCH to "R.M.S. -
Fast".
(6)
RANGE MULTIPLIER to "x 0.3 - 10 dB".
I
The
measured S.P.L. in these conditions is
read
directly
in
dB
reo
2 X
10-~
,ubar,
as
the
sum
of
the "dB SL" setting
of
METER RANGE
switch
+
the
dB
setting
of
RANGE MULTIPLIER +
the
meter
pointer
deflection
in
dB. Cali-
bration
(item
(3)
of
the
general
procedure
p.
14)
consists
of
adjusting
the
indication of the
instruments
to
the
S.P.L.
value
produced
by
the
Piston-
phone, by means
of
the screwdriver
operated
potentiometer
marked
SENSI-
TIVITY CONDENSER MICROPHONE.
With,
for
example, a
Pistonphone
producing
124.1
dB, the deflection
should
be
14.1
dB.
a.
b.
c.
4220
4220
4220
UAOO26
2604
UAOO26
2107
UA
0026
2112
Fig
.
15.
The Pistonphone used
lor
direct S.P.L. calibration
01:-
(a) Microphone Amplifier
2604.
(b) Analyzer
2107
.
(c) Spectrometer
2112.
'USH
2.
Case
01
the "li2-inch"
and"
l/4-inch" microphones (Cartridges
Type.
4133,
4134, 4135, 4136). ·
Due to the lower sensitivity
of
these
microphones,
a correction
has
to
be
made
to the "dB SL" global
reading
obtained
from
the
indications
of
the
19
control knobs
and
the
meter deflection
al
explained above. A
lultable
in·
crease
of
sensitivity
should
be given the amplifier by
means
of
the
aUenuators
in
order
to obtain a convenient deviation
of
the
pointer
during
the
calibration.
For
example, in
the
case
of
the half·inch microphone having a sensitivity
which
is
around 14 '
dB
lower
than
the one·inch microphone, the sensitivity
of
the measuring
instrument
can
be increased
by
20
dB
by
shifting
the
METER BANGE
swi~ch
one step
down
with
respect to
the
adjustment
in·
dicated for the one-iflch microphones.
The
pointer
will
then
deflect
around
20
dB
(full deflection)
when
calibrating
with
the
124
dB
S.P.L.
of
the
Piston·
phone. -
If
now
the
deflection is
adjusted,
by
means
of
the
SENSITIVITY
CONDENSER MICROPHONE
potentiometer
to exact/y 20 dB, the global
reading is 110 dB, while
the
sound
level is 124 dB,
which
means
immediately
Ihat the c
orrection
to be
made
on
the
global
reading
is +14 dB.
With
a
Pistonphone
producing
123.8 dB S.L., the
correction
is exactly + 13.8
dB
etc.
By this procedure,
the
correction
is
determined
with
an
error
which is
equal
to the pistonphone
calibration
error
(less
than
± 0.2
dB)
plus
the
reading
error.
Reference level
on
Sound
Level Recordlng8
and
Spectrogram
..
The recording of the Sound Level as a function
of
time
can
be made by
means
of
a level recorder, such
as
the B
Ii
K High Speed Level Becorder 2305
connected to the output
of
the measuring instrument.
(n addition, the Analyzer Type 2107
and
the Spectrometer Type 2112
are
often
us
ed in connection with the B
Ii
K Level Recorders in
order
to obtain
the
frequency
distribution
of
the
sound
being investigated.
They
are
designed to
be driven synchronously with the pre-calibrated recording
paper
of
the level
4221
4\U
1.lnl
2112
•
~.
~
~------------------~
Fig. 16. Calibration
of
a
.et-up
for frequency ana1llsjJ
of
noi.e
.
20
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